Antimicrobial Therapy in Veterinary Medicine

246 Section II. Classes of Antimicrobial Agents

organisms and multidrug-resistant staphylococcal ate. There is increasing interest in the use of amikacin
infections. In human medicine, it is often combined
with anti-pseudomonal penicillins in the treatment of for treatment MRSA and MRSP infections (Frank and
P. aeruginosa infections in neutropenic patients.
Loeffler, 2012; Papich, 2012)
Horses. Amikacin is approved for use in the United
States and Canada for the intrauterine treatment of Bibliography
bacterial endometritis of mares and should be reserved
for P. aeruginosa and K. pneumoniae infections as Adland-Davenport P, et al. 1990. Pharmacokinetics of amika-
activity against Streptococcus zooepidemicus is poor. cin in critically ill neonatal foals treated for presumed or
Pharmacokinetic studies support the use of 2 g intrau- confirmed sepsis. Equine Vet J 22:18.
terine infusions once daily rather than IM treatment
(Orsini et al., 1996). Bucki EP, et al. 2004. Pharmacokinetics of once-daily amikacin
in healthy foals and therapeutic drug monitoring in hospi-
Amikacin is used in neonatal foals in the treatment talized equine neonates. J Vet Intern Med 18:728.
of septicemia or pneumonia. Magdesian and others
(2004) found that a once-daily dose of 21 mg/kg in Butt TD, et al. 2001. Comparison of 2 techniques for regional
foals did not cause nephrotoxicity and suggested that antibiotic delivery to the equine forelimb: intraosseous
once-daily dosing might be more efficacious than perfusion vs. intravenous perfusion. Can Vet J 42:617.
divided daily dosing, for reasons discussed earlier. As
efficacy correlates to the Cmax:MIC ratio, an initial Dabareiner RM, et al. 2003. Injection of corticosteroids,
dosage of 25 mg/kg q 24 h is suggested for foals to hyaluronate, and amikacin into the navicular bursa in
achieve peak concentrations of > 40 μg/ml (Bucki horses with signs of navicular area pain unresponsive to
et al., 2004). other treatments: 25 cases (1999–2002). J Am Vet Med
Assoc 223:1469.
Amikacin is also used in the treatment of musculo-
skeletal infections caused by Staphylococcus spp. and Davis JA, et al. 2011. Anatomical distribution and genetic
Gram-negative bacteria. Due to the expense of sys- relatedness of antimicrobial-resistant Escherichia coli from
temic therapy, it is often administered by intra-articu- healthy companion animals. J Appl Microbiol 110:597.
lar injection, or by regional intravenous or intraosseous
perfusion to the distal limbs. Such local administra- Frank LA, Loeffler A. 2012. Meticillin-resistant Staphylococcus
tion results in high amikacin concentrations in joints pseudintermedius: clinical challenge and treatment options.
and tendon sheaths and avoids systemic toxicity (Butt Vet Dermatol 23:283.
et al., 2001; Kelmer et al., 2012; Parra-Sanchez et al.,
2006; Taintor et  al., 2006). When performing intra- Giguère S, et al. 2012. In vitro synergy, pharmacodynamics,
articular injections with corticosteriods or chondro- and postantibiotic effect of 11 antimicrobial agents against
protective drugs (e.g., hyaluronate), because of the Rhodococcus equi. Vet Microbiol 160:207.
catastrophic consequences of sepsis, a small amount of
amikacin is frequently added to the therapy (Dabareiner Green SL, Conlon PD. 1993. Clinical pharmacokinetics
et al., 2003). of  amikacin in hypoxic premature foals. Equine Vet J
25:276.
Dogs and Cats. Amikacin is approved for parenteral
use in dogs in the United States. It is also used in cats. Green SL, et al. 1992. Effects of hypoxia and azotaemia on the
Indications include serious Gram-negative infections pharmacokinetics of amikacin in neonatal foals. Equine
(pyelonephritis, skin or soft tissue infections) caused by Vet J 24:475.
otherwise resistant Enterobacteriaceae or P. aeruginosa,
for which alternate drugs are not available or appropri- Kelmer G, et al. 2013. Evaluation of regional limb perfusion
with amikacin using the saphenous, cephalic, and palmar
digital veins in standing horses. J Vet Pharmacol Ther
36:236.

Lei T, et al. 2010. Antimicrobial resistance in Escherichia coli
isolates from food animals, animal food products and
companion animals in China. Vet Microbiol 146:85.

Magdesian KG, et al. 2004. Pharmacokinetics and nephro-
toxicity of high dose amikacin administered at extended
intervals to neonatal foals. Am J Vet Res 65:473.

Orsini JA, et al. 1989. Resistance to gentamicin and amikacin
of gram-negative organisms isolated from horses. Am J Vet
Res 50:923.

Orsini JA, et al. 1996. Tissue and serum concentrations of
amikacin after intramuscular and intrauterine administra-
tion to mares in estrus. Can Vet J 37:157.

Papich MG. 2012. Selection of antibiotics for meticillin-
resistant Staphylococcus pseudintermedius: time to revisit
some old drugs? Vet Dermatol 23:352.

Chapter 14. Aminoglycosides and Aminocyclitols 247

Parra-Sanchez A, et al. 2006. Pharmacokinetics and pharma- and treatment of colibacillosis (Andreotis et al., 1980).
codynamics of enrofloxacin and a low dose of amikacin The drug is incorporated into water so that pigs con-
administered via regional intravenous limb perfusion in sume sufficient amounts to obtain 12.5 mg/kg daily for
standing horses. Am J Vet Res 67:1687. 7  days. Efficacy has been reported against naturally
acquired E. coli infections in broilers (Cracknell et al.,
Pinto N, et al. 2011. Pharmacokinetics of amikacin in plasma 1986). Enteritis significantly increases oral absorption
and selected body fluids of healthy horses after a single in chickens, which may be problematic for tissue resi-
intravenous dose. Equine Vet J 43:112. dues (Thomson et al., 1992). Tissue residues would be
expected to be typical of aminoglycosides in general.
Rubin JE, et al. 2011. Antimicrobial susceptibility of Staphylo-
coccus aureus and Staphylococcus pseudintermedius Bibliography
isolated from various animals. Can Vet J 52:153.
Andreotis JS, et al. 1980. An evaluation of apramycin as an
Sparks TA, et al. 1994. Antimicrobial effect of combinations in-feed medication for the treatment of postweaning coli-
of EDTA-Tris and amikacin or neomycin on the microor- bacillosis in pigs. Vet Res Comm 4:131.
ganisms associated with otitis externa in dogs. Vet Res
Commun 18:241. Cracknell VC, et al. 1986. An evaluation of apramycin soluble
powder for the treatment of naturally acquired Escherichia
Taintor J, et al. 2006. Comparison of amikacin concentrations coli infections in broilers. J Vet Pharmacol Ther 9:273.
in normal and inflamed joints of horses following intra-
articular administration. Equine Vet J 38:189. Kadlec K, et al. 2012. Novel and uncommon antimicrobial
resistance genes in livestock-associated methicillin-resist-
Wichtel MG, et al. 1992. Relation between pharmacokinetics ant Staphylococcus aureus. Clin Microbiol Infect 18:745.
of amikacin sulfate and sepsis score in clinically normal
and hospitalized neonatal foals. J Am Vet Med Assoc Livermore DM, et al. 2011. Activity of aminoglycosides,
200:1339. including ACHN-490, against carbapenem-resistant
Enterobacteriaceae isolates. J Antimicrob Chemother 66:48.
Apramycin
Thomson TD, et al. 1992. Effects of intestinal pathology due
Apramycin, like tobramycin, is a nebramycin isolated to coccidial infection on the oral absorption of apramycin
from the fermentation of Streptomyces tenebrans. It has in 4-week-old broiler chickens. Acta Vet Scand Suppl
not been developed for clinical use in humans but has 87:275.
been used in the oral treatment of Gram-negative bacte-
rial enteritis of farm animals. Gentamicin

Apramycin is active against S. aureus, many Gram- Gentamicin is one of the fermentation products of
negative bacteria, and some mycoplasma (Table  14.4). Micromonospora purpurea; because it is not a Streptomyces
Additional studies are required to define its spectrum of product, it is spelled “gentamicin,” not “gentamycin.”
activity. Bacteria with an MIC ≤ 16 μg/ml are regarded
as susceptible. Antimicrobial Activity
Gentamicin is one of the most active aminoglycosides
The unique chemical structure of apramycin resists (Table  14.1). The drug is active against most Gram-
most of the plasmid-mediated degradative enzymes. negative aerobic rods including many Pseudomonas aer-
Resistance is rare among Gram-negative bacteria, so uginosa, against some Gram-positive bacteria, and
that many pathogenic E. coli and Salmonella isolated against mycoplasma. It is usually more active against
from animals are susceptible. The emergence of carbap- streptococci than amikacin. Gentamicin has little activ-
enemases in Enterobacteriaceae is driving a search for ity against mycobacteria or Nocardia and none against
therapeutic alternatives, and there is interest in apramy- anaerobic bacteria or against aerobic bacteria under
cin for its ability to evade of rRNA methylases (Livermore anaerobic conditions. Like all aminoglycosides, it is a
et al., 2011). Livestock-associated methicillin-resistant bactericidal, concentration-dependent killer and pen-
S. aureus containing the apramycin resistance gene etrates phagocytic cells poorly. There is widespread
apmA have been detected (Kadlec et al., 2012). Most of susceptibility among veterinary pathogenic bacteria
these genes are located on multiresistance plasmids, although resistance is sometimes a problem in veterinary
which enable their co-selection and persistence. hospital settings (Peyrou et al., 2003; Sanchez et al., 2002).

Apramycin is approved in some countries as an inject-
able product for the treatment of colibacillosis in calves.
In swine, apramycin is highly effective in prophylaxis

248 Section II. Classes of Antimicrobial Agents

In human hospitals, there have been explosive outbreaks same syringe. Care must be taken when both drugs are
of nosocomial infection caused by gentamicin-resistant administered through the same intravenous line to flush
bacteria of many species. thoroughly between drugs.

t Susceptible bacteria (MIC ≤ 2 mg/ml [dogs and horses] Halothane anesthesia causes significant changes in
or ≤ 4μg/ml [other species]) are most Enterobacteriaceae the pharmacokinetics of gentamicin in horses; total
including Enterobacter spp., E. coli, Klebsiella spp., body clearance and volume of distribution decrease
Proteus spp., Serratia spp., Yersinia spp., Brucella while half-life of elimination increases (Hague et al.,
spp., Campylobacter spp., Haemophilus spp., and 1997; Smith et al., 1988). In horses, concurrent adminis-
Pasteurella spp. Most strains of P. aeruginosa are tration of phenylbutazone with gentamicin decreases
susceptible. Among Gram-positive bacteria, S. aureus the elimination half-life of gentamicin by 23% and
are typically susceptible but susceptibility of strepto- decreases the volume of distribution by 26%; while the
cocci and many other Gram-positive aerobes can be phenylbutazone pharmacokinetics are not affected
variable. Prototheca zopfii are generally susceptible. (Whittem et al., 1996).
Rhodococcus equi is susceptible in vitro, but clinical
efficacy is poor due to poor penetration and activity Toxicity and Adverse Effects
in abscesses. Gentamicin causes the expected aminoglycoside toxic
effect of neuromuscular blockade, that is exacerbated by
t Resistant bacteria (MIC ≥ 8–16 mg/ml) include many anesthetics. It causes minor cardiovascular depressive
Gram-positive aerobes, some Pseudomonas spp., and effects; so it should not be given rapidly IV. Gentamicin
anaerobes. Strains of gentamicin-resistant P. aerugi- is potentially ototoxic, but the major toxic effect is
nosa are commonly susceptible to amikacin or nephrotoxicosis, which limits prolonged use. High
tobramycin. trough concentrations are associated with nephrotoxic-
ity due to gentamicin accumulation in renal tubular epi-
Pharmacokinetic Properties thelial cells. Because of the nephrotoxic potential of
Like amikacin, reported values of distribution for gen- gentamicin, it is best reserved for severe infections.
tamicin range from 0.15 to 0.3 L/kg and plasma elimina- Ideally, serum drug concentrations should be monitored
tion half-lives range from 1 to 2 hours in adult animals. in treated animals. Otherwise, renal function must be
Protein binding is low. The larger volume of distribution carefully monitored.
in neonates means that the dose in should be higher
than in adults, but dosage intervals need to be extended Subclinical renal damage, which occurs with most
(Burton et al., 2012). Gentamicin transfers across the therapeutic regimens, is generally reversible and clini-
placenta and can achieve therapeutic concentrations in cally insignificant. Risk factors for gentamicin-induced
the allantoic fluid in pony mares (Murchie et al., 2006). nephrotoxicity include immaturity or old age, acidosis,
concurrent use of diuretics such as furosemide, daily
Drug Interactions and total dose, fever, dehydration, previous aminoglyco-
Gentamicin is commonly synergistic with beta-lactam side treatment, concurrent treatment with amphotericin
antibiotics against a wide variety of Gram-negative rods, B and non-steroidal anti-inflammatory drugs and, in
including P. aeruginosa. It is commonly synergistic with the dog, pyometra. Fever decreases clearance and the
beta-lactam antibiotics against Gram-positive bacteria volume of distribution, thus increasing plasma gen-
such as Listeria monocytogenes. Gentamicin is synergistic tamicin concentrations.
with trimethoprim-sulfonamide combinations against
E. coli and K. pneumoniae. Antagonism may occur with Currently, high-dose, once-daily gentamicin therapy
chloramphenicol, tetracycline, and erythromycin. is recommended to maximize antimicrobial efficacy and
Combinations of gentamicin and rifampin are antagonis- minimize nephrotoxicity. Monitoring peak and trough
tic against Rhodococcus equi (Giguère et al., 2012). serum concentrations to detect changes in the elimina-
tion half-life is the most proactive way to detect the
Injectable beta-lactam formulations are incompatible onset of nephrotoxicity, but may be difficult to do in a
with gentamicin, so they should not be mixed in the clinical setting. The next best indicator is an increase
in urine GGT and an increase in the urine GGT:urine

Chapter 14. Aminoglycosides and Aminocyclitols 249

Cr ratio. Elevations in serum urea nitrogen and Cr confirm Cats are particularly susceptible to gentamicin
nephrotoxicity, but are not seen for 7 days after signifi- toxicosis, which manifests initially as loss in vestibular
cant renal damage has occurred. Elimination half-lives function, followed by nephrotoxicity. Therapeutic doses
of 24–45 hours have been reported in horses with renal are usually safe in cats treated for reasonable periods (5
toxicity, further prolonging the toxic exposure to the days; Hardy et al., 1985; Short et al., 1986; Waitz et al.,
drug (Sweeney et al., 1988). While peritoneal dialysis is 1971). Monitoring of renal function or therapeutic drug
useful in lowering creatinine and serum urea nitrogen, monitoring is advised in seriously ill cats, for which the
it may not be effective in significantly increasing the drug should be reserved. Nephrotoxicosis in a cat asso-
elimination of the accumulating aminoglycoside. ciated with excessive infusion of gentamicin into an
Nomograms based on age and renal function are used in abscess has been described (Mealey and Boothe, 1994).
calculating gentamicin dosage in human patients but are
not available in veterinary medicine. Recent studies of Antimicrobial-associated diarrhea (AAD) is the most
population pharmacokinetic studies of gentamicin in common adverse effect of antimicrobial therapy in
horses showed that a considerable proportion of the horses. While causality cannot be established, in a
individual variability recognized in gentamicin disposi- review of 5251 horses treated with antimicrobials for
tion could be explained by differences in body weight non-gastrointestinal signs, 32 were diagnosed with
and serum creatinine (Martin Jimenez et al., 1998), and probable AAD, the most frequently used antimicrobials
such data can be used to estimate the dosage for once- in horses with AAD were gentamicin in combination
daily dosing. Renal damage in dogs administered the with penicillin (n = 7; Barr et al., 2012).
recommended dosage of gentamicin is usually mild or
moderate and reversible (Albarellos et al., 2004). Administration and Dosage
Administration and dosages for major use species are
Nephrotoxicity can be decreased by feeding treated shown in Table 14.3. Gentamicin is labeled for intrauter-
animals a high-protein diet/high-calcium diet such as ine use in horses (and cattle in some countries), IM or
alfalfa to large animals and a diet higher than 25% PO use in piglets, SC use in day-old poults and chicks,
protein to small animals, as protein and calcium cati- and IM or SC use in dogs. It is frequently administered
ons compete with aminoglycoside cations for binding IV, SC, IM, and by intra-articular injection, and by intra-
to renal tubular epithelial cells (Behrend et al., 1994; venous or intraosseous perfusion. It is used extra-label
Schumacher et al., 1991). High dietary protein also in many other species as well.
increases glomerular filtration rate and renal blood
flow, thereby reducing aminoglycoside accumulation. Clinical Applications
The sparing effect of the diet may be related to the Clinical uses of gentamicin are shown in Table  14.5.
competitive inhibition by protein at the proximal Gentamicin is bactericidal against aerobic bacteria,
tubule or the nephrotoxic-sparing effect of calcium especially Gram-negative bacteria, and is particularly
(Brashier et al., 1998). useful for its activity against Enterobacteriaceae and

Table 14.5. Applications of gentamicin to clinical infections in animals.

Species Primary Application Comments
Horses Nephrotoxcity limits use
Gram-negative septicemia in foals, pleuropneumonia,
Dogs, cats and surgical prophylaxis for colic surgery. Metritis in Nephrotoxcity and ototoxicity limits use
Cattle, sheep, goats mares. Infectious keratitis Not recommended due to prolonged kidney residues
Pigs
Poultry Gram-negative septicemia. Infectious keratitis. Labelled to treat day-old birds but is also administered in ovo
Otitis externa

Labeled for metritis in cattle in some countries.
Gram-negative septicemia

Neonatal colibacillosis.
Gram-negative septicemia in poults and chicks

250 Section II. Classes of Antimicrobial Agents

Pseudomonas aeruginosa. It is a drug of choice in the (Easter et al., 1997). The risk of nephrotoxicity can be
treatment of severe sepsis caused by Gram-negative reduced by providing a diet high in protein and calcium,
aerobic rods, but the fluoroquinolones have a similar such as alfalfa hay (Schumacher, et al., 1991).
spectrum of activity with better tissue distribution and
safety profiles. In foals, gentamicin is often used in the treatment of
Gram-negative septicemia, but because of its poor pen-
Cattle, Sheep, and Goats. Gentamicin is of limited etration of the blood-brain barrier it is ineffective in the
value in these species because of cost and prolonged treatment of meningitis. The drug should not be used
tissue residues. Gentamicin is not recommended for for more than 5–7 days without monitoring renal toxic-
extra-label use in ruminants in the United States or ity and trough serum concentrations (Raisis et al., 1998).
Canada. Due to renal accumulation, detectable residues
may persist for years following treatment (Chiesa et al., Gentamicin approved for intrauterine use in mares is
2006; Dowling, 2006). It has been used extra-label in the used in the treatment of infectious metritis in mares
treatment of coliform mastitis in dairy cows. One well- caused by susceptible S. zooepidemicus, K. pneumoniae or
conducted field study of cows suffering from coliform P. aeruginosa. Gentamicin should not be used routinely at
mastitis showed no beneficial effects of systemic admin- or before service or insemination to avoid promoting
istration of the drug (Jones and Ward, 1990). The bene- resistance and destroying normal vaginal microflora.
fit of intramammary infusion has been questioned and, Stallions with Klebsiella or Pseudomonas infections of the
experimentally, intramammary gentamicin had no ben- genital tract have been successfully treated with gen-
eficial effect on the course of E. coli mastitis in cows tamicin at 4.4 mg/kg twice daily IM or IV (Hamm, 1978).
(Erskine et al., 1992).
Gentamicin is often a first-line topical therapy for
Swine. Gentamicin is used to treat neonatal colibacil- bacterial ulcerative keratitis, as S. zooepidemicus and
losis in piglets from day 1 to day 3 of age, with either a P. aeruginosa are the most frequent pathogens isolated.
single IM injection or an oral dose of 5 mg. If multiple Susceptibility testing should be done however, as
doses are given or if administered to older piglets, a sig- increasing resistance to gentamicin has been observed
nificantly increased withdrawal time should be followed. for these pathogens and ineffective therapy may be cata-
strophic (Keller and Hendrix, 2005;Sauer et al., 2003).
Horses. Gentamicin is widely used in horses because
of its relatively broad spectrum of activity, the preva- Gentamicin is administered by intra-articular injec-
lence of susceptible bacteria, and the sentimental value tion for the treatment of septic arthritis in horses, as
of horses treated compared to most farm animals. concentrations in synovial fluid achieved by this route
Gentamicin is extensively used in horses for the treat- exceed those achieved by parenteral administration by
ment of pneumonia and pleuropneumonia (Mair, 1991; up to 100 times, thus exceeding the MIC of susceptible
Raidal, 1995; Sweeney et al., 1991). It is often combined pathogens for 24 hours (Lescun et al., 2006;Meijer
with a beta-lactam antibiotic for synergistic activity. et al., 2000). Intraosseous or intravenous regional per-
Metronidazole is often added to the combination for fusion also achieves high local concentrations for the
treatment of pleuropneumonia in horses to extend the treatment of septic arthritis or osteomyelitis (Mattson
spectrum to beta-lactam-resistant anaerobes such as et al., 2004;Werner et al., 2003). High dosage may cause
Bacteroides fragilis. toxic osteonecrosis (Parker et al., 2010). Gentamicin-
impregnated polymethyl methacrylate beads are also
Gentamicin is frequently administered with a beta- successfully used to treat septic arthritis (Booth et al.,
lactam antibiotic to horses undergoing colic surgery 2001; Farnsworth et al., 2001; Haerdi-Landerer et al.,
(Traub-Dargatz et al., 2002). Endotoxemia increases the 2010). Gentamicin-impregnated collagen sponges
elimination half-life of gentamicin in these horses implanted in the tarsocrural joint of horses provides
(Sweeney et al., 1992; van der Harst et al., 2005a,b), but peak concentrations > 20 times the minimum inhibitory
gentamicin pharmacokinetics are not altered by fluid concentrations reported for common pathogens causing
administration (Jones et al., 1998) or peritoneal lavage septic arthritis (Ivester et al., 2006).

Dogs and Cats. The widespread susceptibility of
common bacterial pathogens of dogs and cats makes

Chapter 14. Aminoglycosides and Aminocyclitols 251

gentamicin a popular drug in small animal practice, Brashier MK, et al. 1998. Effect of intravenous calcium
where it is used with excellent efficacy in the treatment administration on gentamicin-induced nephrotoxicosis in
of respiratory tract, skin and soft tissue, ocular (superfi- ponies. Am J Vet Res 59:1055.
cial infections), and gastrointestinal tract infections.
Post-surgical infections in dogs typically involve gen- Brown SA, Garry FB. 1988. Comparison of serum and renal
tamicin-susceptible organisms (Gallagher and Mertens, gentamicin concentrations with fractional urinary excre-
2012). Gentamicin-impregnated polymethyl methacrylate tion tests as indicators of nephrotoxicity. J Vet Pharmacol
beads and regional intravenous gentamicin perfusion can Ther 11:330.
be used for local therapy of musculoskeletal infections
(Vnuk et al., 2012). Local implantation of gentamicin- Burton AJ, et al. 2013. Effect of age on the pharmacokinetics
impregnated collagen sponges in dogs also appears safe of a single daily dose of gentamicin sulfate in healthy foals.
and effective (Delfosse et al., 2011; Renwick et al., 2010). Equine Vet J 45:507.
Gentamicin’s activity against Staphylococcus pseudinterme-
dius and P. aeruginosa has made it especially useful for Chiesa OA, et al. 2006. Use of tissue-fluid correlations to
topical treatment of canine otitis externa (Zamankhan estimate gentamicin residues in kidney tissue of Holstein
Malayeri et al., 2010). But unless predisposing factors are steers. J Vet Pharmacol Ther 29:99.
corrected, P. aeruginosa often becomes resistant in chronic
cases (Hariharan et al., 2006). When applied topically to Delfosse V, et al. 2011. Clinical investigation of local implan-
clinically normal dogs with intact or ruptured tympanic tation of gentamicin-impregnated collagen sponges in
membranes, gentamicin does not cause detectable coch- dogs. Can Vet J 52:627.
lear or vestibular damage (Strain et al., 1995).
Dowling P. 2006. Clinical pharmacology update. Insulin and
Poultry. Gentamicin is administered SC to 1- to 3-day Gentamicin. Can Vet J 47:711.
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and treatment of E. coli, P. aeruginosa, Arizona paracolon Dowling PM, et al. 1996. Pharmacokinetics of gentamicin in
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eggs in hatcheries to prevent infection prior to hatching.
Easter JL, et al. 1997. Effects of postoperative peritoneal
Camelids. Gentamicin is used in camelids for treatment lavage on pharmacokinetics of gentamicin in horses after
of Gram-negative infections. Camelids appear susceptible celiotomy. Am J Vet Res 58:1166.
to gentamicin-induced nephrotoxicty (Hutchison et al.;
1993). A pharmacokinetic study in normal adult llamas Erskine RJ, et al. 1991. Theory, use, and realities of efficacy
demonstrated high peak concentrations and prolonged and food safety of antimicrobial treatment of acute
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the distal metacarpus in standing horses. Vet Surg 33:180. gentamicin pharmacokinetics in colic horses. Vet Res
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Mealey KL, Boothe DM. 1994. Nephrotoxicosis associated
with topical administration of gentamicin in a cat. J Am Vnuk D, et al. 2012. Regional intravenous gentamicin admin-
Vet Med Assoc 204:1919. istration for treatment of postoperative tarso-metatarsal
infection in a dog—a case report. Berl Munch Tierarztl
Meijer MC, et al. 2000. Clinical experiences of treating Wochenschr 125:172.
septic arthritis in the equine by repeated joint lavage: a
series of 39 cases. J Vet Med A Physiol Pathol Clin Med Waitz JA, et al. 1971. Aspects of the chronic toxicity of gen-
47:351. tamicin sulfate in cats. J Infect Dis 124 Suppl:S125.

Murchie TA, et al. 2006. Continuous monitoring of penicillin Werner LA, et al. 2003. Bone gentamicin concentration after
G and gentamicin in allantoic fluid of pregnant pony intra-articular injection or regional intravenous perfusion
mares by in vivo microdialysis. Equine Vet J 38:520. in the horse. Vet Surg 32:559.

Parker RA, et al. 2010. Osteomyelitis and osteonecrosis after Whittem T, et al. 1996. Pharmacokinetic interactions between
intraosseous perfusion with gentamicin. Vet Surg 39:644. repeated dose phenylbutazone and gentamicin in the
horse. J Vet Pharmacol Ther 19:454.
Peyrou M, et al. 2003. [Evolution of bacterial resistance to
certain antibacterial agents in horses in a veterinary hospi- Zamankhan Malayeri H, et al. 2010. Identification and anti-
tal]. Can Vet J 44:978. microbial susceptibility patterns of bacteria causing otitis
externa in dogs. Vet Res Commun 34:435.
Raidal SL. 1995. Equine pleuropneumonia. Br Vet J 151:233.
Raisis AL, et al. 1998. Serum gentamicin concentrations in Spectinomycin

compromised neonatal foals. Equine Vet J 30:324. Spectinomycin (Figure 14.3) is a product of Streptomyces
Renwick AI, et al. 2010. Treatment of lumbosacral discospon- spectabilis. It is an aminocyclitol antibiotic that lacks
most of the toxic effects of the aminoglycoside antibiot-
dylitis by surgical stabilisation and application of a gen- ics but, unfortunately, is limited in application by the
tamicin-impregnated collagen sponge. Vet Comp Orthop ready development of resistance. There are discrepan-
Traumatol 23:266. cies between resistance to the drug in vitro and apparent
Sanchez S, et al. 2002. Characterization of multidrug-resist- efficacy in some cases clinically, which have not been
ant Escherichia coli isolates associated with nosocomial explained. For example, Goren et al. (1988) observed
infections in dogs. J Clin Microbiol 40:3586. high efficacy of orally administered spectinomycin or
Sauer P, et al. 2003. Changes in antibiotic resistance in equine lincomycin-spectinomycin in treating experimentally
bacterial ulcerative keratitis (1991–2000): 65 horses. Vet
Ophthalmol 6:309.
Schumacher J, et al. 1991. Effect of diet on gentamicin-
induced nephrotoxicosis in horses. Am J Vet Res 52:1274.
Short CR, et al. 1986. The nephrotoxic potential of gen-
tamicin in the cat: a pharmacokinetic and histopathologic
investigation. J Vet Pharmacol Ther 9:325.

Chapter 14. Aminoglycosides and Aminocyclitols 253

OH O CH3 Table 14.6. Activity (MIC90) of spectinomycin (μg/ml)
O against selected bacteria and mycoplasma.

H3C HN

HO O Organism MIC90(μg/ml)
OH
Gram-positive aerobes 8
NH O Rhodococcus equi 64
Staphylococcus aureus 64
CH3 Streptococcus pyogenes
32
Figure 14.3. Chemical structure of spectinomycin. Gram-negative aerobes >128
Actinobacillus pleuropneumoniae >256
induced E. coli infections in chickens, despite the Bordetella avium 1
absence of any antimicrobial activity in the serum of B. bronchiseptica >400
these chickens. To explain this discrepancy, they sug- Brucella canis 25
gested that a metabolite or degradation product of the Escherichia coli 32
drug might reach the respiratory tract and interfere with Histophilus somni ≤64
bacterial attachment. This explanation is speculative Klebsiella pneumoniae 32
since it has not been shown that the drug undergoes Ornithobacterium rhinotracheale >128
metabolism in any species. In humans, all of an admin- Pasteurella multocida >256
istered dose is recovered in the urine within 48 hours Proteus spp. ≤64
after injection. Pseudomonas aeruginosa 4
Salmonella spp.
Antimicrobial Activity Taylorella equigenitalis 4
4
Spectinomycin is a usually bacteriostatic, relatively broad- Mycoplasmas 1
spectrum drug that can be bactericidal at concentrations M. bovis 1
4 times MIC. It is not particularly active on a weight basis M. bovigenitalium 4
(Table 14.6). Bacteria are usually regarded as susceptible if M. hyopneumoniae
their MIC is ≤ 20 μg/ml. Susceptibility among aerobic M. hyorhinis
Gram-negative rods is unpredictable because of the pres- M. hyosynoviae
ence of naturally resistant strains. Mycoplasma spp. are
susceptible but P. aeruginosa is resistant. Gram-negative pathogens involved in bovine respira-
tory disease is variable (Welsh et al., 2004). Mycoplasma
bovis isolates can acquire resistance to spectinomycin
(Francoz et al., 2005).

Resistance Drug Interactions

Natural resistance to spectinomycin in many enteric Combination with lincomycin may marginally enhance
bacteria is widespread. Chromosomal one-step muta- spectinomycin’s activity against mycoplasma and
tion to high-level resistance develops readily in vivo and Lawsonia intracellularis.
in vitro, in a manner similar to streptomycin resistance.
Chromosomally resistant strains do not show cross- Toxicity and Adverse Effects
resistance with aminoglycosides. Plasmid-mediated
resistance is uncommon. Vaillancourt et al. (1988) Spectinomycin seems to be relatively non-toxic in ani-
reported a marked drop (from 91%–24%) of in vitro sus- mals; it does not induce ototoxicity or nephrotoxicity but
ceptibility of Actinobacillus pleuropneumoniae isolated may, like the aminoglycosides, cause neuromuscular
over a 5-year period, associated with the widespread blockade. The apparent lack of reported toxic effects may
use of the drug to treat pleuropneumonia in swine. reflect lack of long-term usage. Administration of linco-
They noted, however, discrepancies between in vitro mycin-spectinomycin oral preparations, by parenteral
resistance and apparent field efficacy. Susceptibility of injection to cattle, has produced heavy losses associated
with severe pulmonary edema. Similar problems have

254 Section II. Classes of Antimicrobial Agents

been noted with misuse of spectinomycin, and attributed In dogs, spectinomycin has been administered by IM
to endotoxin contamination (Genetsky et al., 1994). injection for a variety of infections from Gram-negative
bacteria but no reports of efficacy are available. It is
Pharmacokinetic Properties available combined with lincomycin and approved for
the treatment of streptococcal, staphylococcal,
Pharmacokinetic properties are similar to those of the Mycoplasma and Pasteurella infections, in dogs and cats
aminoglycosides. with dosage based on 20 mg/kg IM of the spectinomycin
component administered once or twice daily. The com-
Administration and Dosage bination is effective for the treatment of tonsillitis, con-
junctivitis, laryngitis, and pneumonia in dogs.
Administration and dosages are shown in Table 14.3.
In poultry, spectinomycin is used parenterally in
Clinical Applications young poults as a single injection to control salmonel-
losis, pasteurellosis (fowl cholera), E. coli, and
Spectinomycin has been largely abandoned in human Mycoplasma synoviae. Spectinomycin can be adminis-
medicine because of the rapid development of resistance tered in the water to control mortality associated with
and unpredictable antibiotic susceptibility. The drug is chronic respiratory disease and infectious synovitis in
used in animals in the treatment of mycoplasma infec- chickens. The activity of spectinomycin against myco-
tions, of diseases caused by Enterobacteriaceae (E. coli, plasma is a particularly useful attribute but it is surpris-
diarrhea, septicemia), and of respiratory disease caused ing that the drug administered orally would have any
by Gram-negative bacteria. The development of resist- effect on systemic infections, since it is at best poorly
ance in bacteria limits its long-term use. It is sometimes absorbed from the intestine.
combined with lincomycin to give a broad-spectrum
combination with activity against Gram-positive aero- Bibliography
bic as well as anaerobic bacteria.
Cook B. 1973. Successful treatment of an outbreak of
In cattle, spectinomycin was approved in the United Salmonella dublin infection in calves using spectinomycin.
States and Canada for SC injection (daily for 3–5 days) Vet Rec 93:80.
to treat bovine respiratory disease caused by Mannheimia
hemolytica and Pasteurella multocida, but it is no longer Francoz D, et al. 2005. Determination of Mycoplasma bovis
available. It has been used successfully to treat Salmonella susceptibilities against six antimicrobial agents using the
dublin infection in calves at a dosage of 22 mg/kg SC on E test method. Vet Microbiol 105:57.
the first day and 0.5 g PO twice daily for an additional
4  days (Cook, 1973). Combined the lincomycin, the Genetsky R, et al. 1994. Intravenous spectinomycin-associated
drug was effective in treating Ureaplasma infection in deaths in feedlot cattle. J Vet Diagn Invest 6:266.
rams (Marcus et al., 1994).
Goren E, et al. 1988. Therapeutic efficacy of medicating drink-
In pigs, spectinomycin is available as an oral solution ing water with spectinomycin and lincomycin-spectinomycin
for the treatment of colibacillosis. It is also administered in experimental Escherichia coli infection in poultry. Vet
IM for the treatment of respiratory disease, including Quart 10:191.
A. pleuropneumoniae. Resistance has limited use for this
latter purpose. While not approved for this use, IM Marcus S, et al. 1994. Lincomycin and spectinomycin in the
injection of 10 mg/kg BID for 3 days has been used suc- treatment of breeding rams contaminated with ureaplas-
cessfully to treat pigs severely affected with proliferative mas. Res Vet Sci 57:393.
intestinal adenomatosis. The MIC of spectinomycin
against Lawsonia intracellularis (32 μg/ml) is the lowest McOrist S, et al. 1995. Antimicrobial susceptibility of ileal
among the aminoglycosides but suggests that the organ- symbiont intracellularis isolated from pigs with prolifera-
ism is barely susceptible, at least in vitro (McOrist et al., tive enteropathy. J Clin Microbiol 33:1314.
1995). Spectinomycin is available combined with linco-
mycin for the oral treatment of swine dysentery and the McOrist S, et al. 2000. Therapeutic efficacy of water-soluble
combination is effective therapy for porcine prolifera- lincomycin-spectinomycin powder against porcine prolif-
tive enteropathy (McOrist et al., 2000). eration enteropathy in a European field study. Vet Rec
146(3):61.

Vaillancourt J-P, et al. 1988. Changes in the susceptibility of
Actinobacillus pleuropneumoniae to antimicrobial agents
in Quebec (1981–1986). J Am Vet Med Assoc 193:470.

Welsh RD, et al. 2004. Isolation and antimicrobial suscepti-
bilities of bacterial pathogens from bovine pneumonia:
1994–2002. J Vet Diagn Invest 16:426.

Chapter 14. Aminoglycosides and Aminocyclitols 255

H2C NH2 NH2 NH2 evidence of renal toxicity, there was also evidence of
HO O II dose-dependent differences in pharmacokinetics, sug-
I O gesting that further studies of toxicity and pharma-
cokinetics are required in multiple-dosing studies
CH2 OH before tobramycin can be recommended in cats. After
intravenous administration to horses, tobramycin
NH2 HO OO pharmacokinetics are similar to other aminoglycosides
(Hubenov et al., 2007). Currently, due to the expense of
III OH systemic therapy, tobramycin use in veterinary medicine
is mainly limited to the ophthalmic formulation in the
HO NH2 treatment of bacterial keratitis due to P. aeruginosa.
Emerging resistance to tobramycin has been docu-
Figure 14.4. Chemical structure of tobramycin. mented in equine corneal infections (Sauer et al., 2003)
Tobramycin has also been used in antibiotic-impregnated
Tobramycin polymethyl methacrylate beads for the treatment of
septic arthritis or osteomyelitis in horses (Holcombe
Tobramycin is a naturally occurring deoxykanamycin et al., 1997). Tobramycin-impregnated calcium sulfate
(Figure  14.4) with antimicrobial and pharmacokinetic beads appear safe and effective in the treatment of staph-
properties similar to gentamicin. Tobramycin is structur- ylococcal osteomyelitis in dogs (Ham et al., 2008).
ally related to kanamycin and has 4 times the activity of
gentamicin against Pseudomonas spp., but resistance can Bibliography
emerge in canine isolates (Lin et al., 2012). Tobramycin
is generally not effective against gentamicin-resistant Ham K, et al. 2008. Clinical application of tobramycin-
strains of Enterobacteriaceae. For treatment of serious impregnated calcium sulfate beads in six dogs (2002–
P. aeruginosa infections, tobramycin should be com- 2004). J Am Anim Hosp Assoc 44:320.
bined with an antipseudomonal penicillin. Tobramycin
is less nephrotoxic than gentamicin, although ototoxic Holcombe SJ, et al. 1997. Use of antibiotic-impregnated
properties are similar. In a study of tobramycin pharma- polymethyl methacrylate in horses with open or infected
cokinetics in cats, Jernigan et al. (1988) found persistent fractures or joints: 19 cases (1987–1995). J Am Vet Med
elevations of blood urea nitrogen and serum creatinine, Assoc 211:889.
suggesting possible renal damage 3 weeks after a single
dose (5 mg/kg) of tobramycin. The authors suggested Hubenov H, et al. 2007. Pharmacokinetic studies on tobramy-
that this high dose may have occupied and saturated cin in horses. J Vet Pharmacol Ther 30:353.
binding sites in the kidneys from which the drug was
only slowly released. Blood urea nitrogen concentrations Jernigan AD, et al. 1988. Pharmacokinetics of tobramycin in
rose in fewer cats after a lower dose (3 mg/kg). Besides cats. Am J Vet Res 49:608.

Lin D, et al. 2012. Characterization of antimicrobial resist-
ance of Pseudomonas aeruginosa isolated from canine
infections. J Appl Microbiol 113:16.

Sauer P, et al. 2003. Changes in antibiotic resistance in equine
bacterial ulcerative keratitis (1991–2000): 65  horses. Vet
Ophthalmol 6:309.

Tetracyclines 15

Jérôme R.E. del Castillo

The tetracyclines are the class of antibiotics with the aromatic polyketide synthases. Until recently, the newer
highest use in veterinary medicine. They are first-line tetracyclines were obtained by chemically modifying
drugs in food animals, including aquaculture species, the first-generation molecules (i.e., semisynthesis), but
exotic animals, and honeybees, but their use is much a high-yield enantioselective synthesis route may now
lower in companion animals, horses, and humans. produce several second- and third-generation mole-
They were the first discovered broad-spectrum antibi- cules (e.g., glycylcyclines). Structurally, the carboxam-
otics, acting against Gram-positive and Gram-negative ide group flanked by a β-keto-enol group (carbons
bacteria, mycoplasmas, some mycobacteria, most 1–3), the α-oriented dimethylamine (carbon 4), and
pathogenic alpha-proteobacteria, and several proto- the oxygenated groups on the lower half of the tetracy-
zoan and filarial parasites. The molecular structures of clines (carbons 10–12a) are required to retain antibi-
chlortetracycline and oxytetracycline were elucidated otic activity. The β-keto-enol group of carbons 11, 11a,
shortly after their approval. This achievement spawned and 12 is a chelation site for multivalent cations (e.g.,
a second generation of semisynthetic congeners (e.g., Ca2+), and carbons 5–9 are sites for facultative chemical
doxycycline) with even better pharmacokinetic and substitutions that change the liposolubility of the mol-
pharmacodynamic properties. But the spread of tetra- ecule (Figure  15.1). The latter two properties greatly
cycline resistance and the introduction of new large- influence their pharmacokinetic and pharmacodynamic
spectrum antibiotics limited their medical use between properties.
the 1970s and the 2000s. In the last 20 years, the discovery
of their beneficial non-antibiotic properties, and the The tetracyclines are amphoteric drugs that are
emergence of multiresistant nosocomial pathogens, ionized at all pH values. In solution, they form a mixture
has spurred the development of a new generation of of zwitterions, cations, and anions, respective propor-
tetracyclines that evade most of their resistance mech- tions of which depend on the pH of the medium. At pH
anisms, or are anti-inflammatory drugs devoid of values ranging between 4 and 7, the zwitterionic form
anti-infective properties. predominates; its null net charge favors its passage across
cell membranes. As the tetracyclines are sparingly solu-
Chemistry ble in water, they are formulated as acid or basic salts that
may be administered orally or parenterally. This class of
The tetracyclines are substituted 2-naphtacene carbox- drug molecules is fairly stable at physiological pH values
amides (Figure 15.1). All first-generation congeners with the exception of chlortetracycline, which degrades
are produced by Streptomyces strains that possess in basic mediums at a rate that increases with pH.

Antimicrobial Therapy in Veterinary Medicine, Fifth Edition. Edited by Steeve Giguère, John F. Prescott and Patricia M. Dowling.
© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

257

Figure 15.1. Structures of the tetracyclines scaffold (naphtacene carboxamide showing the carbon numbering) and an anti-
inflammatory-only derivative, and of the most significant first-, second-, and third-generation tetracycline antibiotics.

Chapter 15. Tetracyclines 259

Mechanism of Action followed by the chlorinated tetracyclines, and lastly by
oxytetracycline and tetracycline. It is noteworthy that
The tetracyclines are pleiotropic drugs that classically the decay of chlortetracycline in culture media biases its
are used as protein synthesis inhibitors. Upon binding to estimation of antimicrobial potency, especially against
the 16S RNA (rRNA) and S7 protein of the 30S bacterial slow-growing organisms (e.g., Mycoplasma). Table 15.1
ribosome, they allosterically inhibit the binding of ami- lists the cumulative estimates of MIC for a number of
noacylated transfer RNA (AA-tRNA) to their docking pathogens: they must be considered with caution, as
site (A-site) on the ribosome. This halts the process of their associated MIC distributions must be examined
peptide synthesis. Overall, they exert a bacteriostatic for proper interpretation, since they are conservative
effect on susceptible bacterial pathogens, with time- potency estimates for molecules other than tetracycline,
dependent bactericidal activity that has been proven at and the cumulative MIC estimates are always associated
least for tigecycline and doxycycline. They exert antipar- with exponential measurement error.
asitic activity by inhibiting protein synthesis in endos-
ymbionts or organelles that possess a genome and t Good or moderate activity (MIC ≤ 4 mg/ml): The tetra-
prokaryote-like ribosomal components. For instance, cyclines exhibit good to moderate activity against
they alter the apicoplasts of Plasmodium falciparum, and the following Gram-positive aerobes: Bacillus spp.,
likely of coccidia and Babesia. As a result, their progeny Corynebacterium spp., Erysipelothrix rhusiopathiae,
inherit defective organelles that shorten their lifespan. Listeria monocytogenes, some streptococci and against
In filaria, they kill the endosymbiont Wolbachia pipientis the following Gram-negative bacteria: Actinobacillus
that is essential to the growth and fertility of the nema- spp., Bordetella spp., Borrelia spp., Brucella spp.,
tode, and plays key role in its evasion from the host Campylobacter fetus, Francisella tularensis, Haemo-
immune mechanisms (McHaffie et al., 2012). philus spp., Lawsonia intracellularis, Leptospira spp.,
Mannheimia spp., Pasteurella spp., including P. mul-
The tetracyclines possess an adjunct anti-inflammatory tocida, and Yersinia spp. (Table 15.1). They are also
activity that is valuable in controlling infectious disease. active against Anaplasma spp., Chlamydia and
They inactivate the matrix metalloproteinases by inter- Chlamydophila spp., Coxiella burnetii, Ehrlichia
acting with the structural (not catalytic) Zn2+ and/or spp., Mycoplasma spp., Rickettsia and Neorickettsia,
Ca2+ of these proteins, and they scavenge the reactive and some anaerobes including Actinomyces spp. and
oxygen species. Finally, the tetracyclines have been Fusobacterium spp.
shown to reduce the infectivity of pathogenic prions in
animals and currently are subject to clinical trials against t Variable susceptibility: Because of acquired resistance,
Creutzfeldt-Jakob disease. among Gram-positive bacteria, many isolates of entero-
cocci, staphylococci and; streptococci may be resistant.
Antimicrobial Activity Among Gram-negative bacteria many Enterobacter-
iaceae including Enterobacter spp., E. coli, Klebsiella spp.,
The tetracyclines are classic broad-spectrum antibiotics. Proteus spp. and Salmonella spp. may be resistant.
They exhibit activity against a range of Gram-positive and Anaerobes such as Bacteroides spp. and Clostridium spp.
Gram-negative bacteria, including the Mycoplasmataceae, show variable susceptibility. Some isolates of Mannhe-
Coxiella and Chlamydiales, and alpha-proteobacteria imia haemolytica may also be resistant.
such as Anaplasma spp., Ehrlichia spp., Neorickettsia spp.,
Rickettsia spp., and Wolbachia spp. Their spectrum of t Resistant (MIC ≥ 16 mg/ml): Most Mycobacterium spp.,
activity also includes many protozoan parasites such as some enterobacteria (Proteus mirabilis, Serratia spp.),
Entamoeba histolytica, Giardia lamblia, Leishmania P. aeruginosa, and some Mycoplasma spp. are resistant.
major, Plasmodium falciparum, Trichomonas spp., and
Toxoplasma gondii. Resistance

Tetracycline is the representative molecule in drug To reach the ribosome, tetracyclines must first com-
sensitivity testing because it is more stable in culture plex with Mg2+ to cross the Gram-negative outer cell
media than its congeners. However, the antibacterial wall via a porin. The periplasmic acidity dissociates the
potency of these drugs positively correlates with lipid
solubility: the semisynthetic derivatives are most active,

260 Section II. Classes of Antimicrobial Agents

Table 15.1. In vitro activity (MIC90, μg/ml) of tetracycline against bacteria including Mycoplasma.

Organism MIC90 Organism MIC90

Gram-positive aerobes 16 Staphylococcus aureus > 64
Arcanobacterium pyogenes 4 Streptococcus agalactiae 0.25
Bacillus anthracis ≤ 0.25 S. dysgalactiae
Corynebacterium pseudotuberculosis 4 S. suis > 32
C. renale 0.25 S. uberis 64
Erysipelothrix rhusiopathiae 1 S. equi (ssp. zooepidemicus and equi) 0.5
Listeria monocytogenes 8
Rhodococcus equi Klebsiella pneumoniae > 16
≤ 0.25 Moraxella bovis
Gram-negative aerobes ≥ 16 Manheinemia haemolytica ≥ 16
Actinobacillus spp. ≥ 16 Pasteurella spp. (horse) 1
A. pleuropneumoniae ≥ 16 P. multocida (pig)
Bordetella avium Proteus spp. ≥ 16
B. bronchiseptica (pig) 0.25 Pseudomonas spp. ≤2
Brucella canis 2 Salmonella spp.
Campylobacter fetus ≥ 64 Taylorella equigenitalis 1
C. jejuni ≥ 64 ≥ 16
Escherichia coli 0.5 Clostridium spp ≥ 16
Haemophilus parasuis 2 C. perfringens ≥ 16
Histophilus somni C. difficile
1 Dichelobacter nodosus 0.5
Anaerobes 2
Actinomyces spp. 25 M. hyorhinis 8
Bacteroides fragilis 4 M. hyosynoviae 32
Bacteorides spp. M. ovipneumoniae 16
Fusobacterium necrophorum 0.5* Ureaplasma spp. 0.12
4*
Mycoplasma 16 2
Mycoplasma bovirhinis 0.03 32
M. bovis 0.5 0.5
M. canis 0.06
M. hyopneumoniae 1
M. agalactiae 4

Spirochetes
Borrelia burgdorferi
Leptospira spp.

*Some reports show resistance.

drug-cation complex, and provides motor ions for and ribosome binding; (4) ribosomal 16S RNA muta-
carrier-mediated passage of drug molecules across the tion at the primary binding site of tetracyclines; and (5)
cytoplasmic membrane. stress-induced down-regulation of the porins through
which the drug crosses the outer Gram-negative wall.
Resistance to tetracyclines can be mediated by differ- The first two mechanisms are by far the most common.
ent mechanisms: (1) energy-dependent efflux systems, Currently, almost 50 resistance genes have been reported,
most of which being antiporters that exchange an extra- some of which are mosaic genes.
cellular H+ for a cytoplasmic drug-Mg2+ complex; (2)
ribosomal protection proteins that dissociate the tetra- Acquired resistance to tetracyclines is widespread
cyclines from their binding site near the ribosomal among enteric bacteria and mycobacteria, but they still
AA-tRNA docking site; (3) flavin-dependent enzymatic are useful drugs against many pathogens of veterinary
hydroxylation of carbon-11a, which disrupts the tetra- importance. Fortunately, resistance is extremely rare
cyclines’ β-keto-enol involved in the chelation of cations among obligate intracellular pathogens such as Anaplasma,

Chapter 15. Tetracyclines 261

Chlamydia, and Ehrlichia. However, horizontal trans- horses given 10 mg/kg doxycycline q 12 h for several days,
mission of tetracycline resistance was recently found in the average peak serum concentration was 0.46 μg/ml.
a Chlamydia suis isolate. Tetracycline-resistant bacteria This is in contrast to peak serum concentrations of
may carry more than one tetracycline resistance gene, 3.5 μg/ml in dogs receiving a dose of 5 mg/kg.
which often are on different mobile elements (chapter 3).
The distribution of tetracyclines is highest in richly
Pharmacokinetic Properties perfused organs: kidney > liver ≥ lungs > blood = syno-
via > muscle. Because they are substrates of P-gp, the tet-
The absorption, distribution and elimination of the tet- racyclines cross the blood-brain barrier with difficulty,
racyclines all depend on factors such as their molecular and at a rate that depends on their lipid solubility. The
size, lipid/buffer partition behavior, plasma protein tetracyclines vary in their binding to serum albumin:
binding, the acidity of biological mediums, their expo- doxycycline > minocycline = chlortetracycline > tetracy-
sure to multivalent cations (Ca2+, Mg2+, Zn2+, Cu2+, Fe2+, cline > oxytetracycline. The limited evidence available
Fe3+, Al3+), and the expression level of P-glycoprotein suggests that minocycline has greater capacity than
(P-gp) in the cell membranes they face. other tetracyclines to penetrate cellular barriers, as it
attains higher concentrations in poorly accessible fluids
To be absorbed, the tetracyclines administered as such as tears and prostatic fluid. The tetracyclines are
solid oral dosage or long-acting injectable formulations among a limited number of osteotropic drugs. Their
must undergo the process of drug release. Dissolution in multivalent cation-chelating properties cause their dep-
the gastric fluids is the critical step to the absorption osition in teeth and at sites of new bone formation. This
from solid oral tetracycline forms. Some excipients of feature has toxicological consequences that will be dis-
the injectable products retain the tetracyclines at the cussed further. The drugs cross the placenta to reach the
injection site via different mechanisms that delay their fetus and are secreted in milk, where they reach concen-
absorption; for example, tissue irritation. The type of trations approximating those of serum.
tetracycline salt influences its solubility and release, and
therefore its extent of absorption (i.e., bioavailability). In The tetracyclines are excreted primarily by glomerular
dogs and cats, this parameter varies among oral tetracy- filtration, by biliary secretion at an extent that depends
cline preparations. Water and feed acidifiers improve on their lipid solubility (e.g., approximately 5% of total
the release and absorption of tetracyclines from medi- clearance for doxycycline in dogs), and by intestinal
cated feeds in pigs. excretion via P-gp. Minocycline is also subject to oxido-
reductive reactions, the main metabolites of which are
The bioavailability of these drugs after oral adminis- 9-OH-minocycline and mono-N-demethylated mole-
tration depends on their lipid/buffer partition, and is cules. As glomerular filtration is their mechanism of
hampered by complexation with multivalent cations excretion, impaired renal function can increase their
that precipitate with increasing pH, and by food parti- elimination half-life.
cles (particularly of dairy products). For instance, the
mean oral bioavailabilities of oxytetracycline and chlo- Tetracyclines undergo enterohepatic circulation, with
rtetracycline respectively are 5% and 37% in non-fasting much of the drug excreted in bile being reabsorbed from
calves, and 5% and 28% in fed pigs. Bioavailability is the intestine. This process contributes to the half-life of
further reduced when fed with milk or milk replacer 6–10 hours, which is unusually long for drugs that are
but is much higher in fasted calves and pigs. The oral eliminated mainly by renal excretion.
bioavailability of feed-administered doxycycline is
approximately 22%. Hence, steady-state plasma drug Chlortetracycline has shorter mean residence time
concentrations reached with medicated feeds in food- than other congeners because of its spontaneous degra-
animal species may not cover the whole MIC range of dation in neutral to basic mediums. Neighboring effects
sensitive pathogens, but chlortetracycline and doxycy- of the carbon-7 chlorine on the carbon-6 hydroxyl group
cline are 2–3 doubling dilutions lower (i.e., more potent) result in the production of iso-chlortetracycline, a mole-
than tetracycline. Oral doxycycline is of limited useful- cule devoid of antibiotic activity. Besides, all tetracy-
ness in horses due to the low systemic exposure it achieves, clines are subject to reversible epimerization of carbon-4
presumably as a result of poor oral bioavailability. In at pH values between 2 and 6, especially when exposed
to substances such as phosphate, urea, and multivalent

262 Section II. Classes of Antimicrobial Agents

cations. The 4-epitetracyclines are more pH-stable and also be associated with late pregnancy. In cattle, high doses
water soluble, but their antimicrobial efficacy is much (33mg/kg IV) have led to fatty infiltration of the liver and
lower than the original molecules. severe proximal renal tubule necrosis. Tetracyclines should
be administered to cattle only in recommended doses to
Drug Interactions avoid problems of nephrotoxicosis (Lairmore et al., 1984).
Transient hemoglobinuria with trembling and subnormal
The absorption of tetracyclines is impaired by antacids temperatures lasting 4 hours has been reported with long-
containing Al3+ or other multivalent cations, by iron- acting formulations (Anderson, 1983). Rapid IV adminis-
containing preparations, and by bismuth subsalicylate. tration in cattle has been followed by collapse, probably
Synergism between tetracyclines and tylosin or tiamulin the result of calcium binding and consequent cardiovascu-
against respiratory pathogens including Mycoplasma lar depression (Gyrd-Hansen et al., 1981), although the
and Pasteurella has been described and may occur with propylene glycol vehicle may be responsible. Intravenous
other macrolides and other bacteria. Combination with injections of all forms of tetracyclines should be given
polymyxins may also give synergistic effects by enhanc- slowly to cattle over a period of not less than 5 minutes
ing bacterial uptake of the drugs. Doxycycline is syner- (Gyrd-Hansen et al., 1981).
gistic with rifampin or streptomycin in the treatment of
brucellosis. Doxycycline was synergistic with pyrimeth- Malabsorption because of moderate diarrhea may
amine in the effective treatment of toxoplasmosis in occur in calves after oral administration of therapeutic
experimentally infected mice. doses. In horses, the most feared side effect of tetracyclines
is enterocolitis due to alteration of intestinal microflora
Toxicity and Adverse Effects and superinfection with resistant Salmonella or unidenti-
fied pathogens that may include Clostridium difficile. This
From a toxicologic perspective, the tetracyclines are rela- occurs in only a small percentage of treated horses.
tively safe. They are irritants that may cause vomiting
after oral dosing, and tissue damage at injection site. Oxytetracycline irritates tissues. Marked differences
Similarly to other inhibitors of the bacterial protein syn- have been found in the different formulations of oxytet-
thesis, these antibiotics cause imbalances of the intestinal racyclines in this respect (Nouws et al., 1990). The more
flora. Their ability to bind calcium is associated with irritating the product, the lower the bioavailability
acute cardiac toxicity. They also induce apoptosis in oste- and the greater the associated drug persistence at the
oclasts, which may cause chronic bone toxicity. Their injection site. The long-acting formulations containing
most serious adverse effects are attributed to anhydrotet- glycerol formaldehyde or dimethylacetamide are partic-
racyclines that damage the plasma membranes and bind ularly irritating.
to serum albumin. These tetracycline degradation prod-
ucts that are found in expired or poorly preserved drug Administration to growing puppies or pregnant
products have been associated with renal toxicity, and bitches results in yellow discoloration of primary and, to
likely in hepatic and cardiovascular toxicity. a lesser extent, permanent teeth. But their chronic use in
pig and rodent models induce apoptosis in the osteo-
Although not well documented in veterinary medi- clasts, which hinders the process of bone remodeling.
cine, tetracyclines are associated with dose-related func- This causes an increase in bone mineral density and
tional changes in renal tubules (Riond and Riviere, 1989). conformation.
Tetracycline-induced renal toxicosis may be exacerbated
by dehydration, hemoglobinuria, myoglobinuria, tox- Tetracyclines have antianabolic effects that may pro-
emia, or the presence of other nephrotoxic drugs (Riond duce azotemia. Such effects can be exacerbated by corti-
and Riviere, 1989). Nephrotoxicosis has been reported costeroids. The drugs may also cause metabolic acidosis
especially in foals receiving high doses for the treatment and electrolyte imbalance.
of contracted tendons. In dogs, fatal nephrotoxicosis has
been reported after the IV administration of tetracy- Administration and Dosage
clines at higher than recommended dosages.
Recommended dosages are shown in Table  15.2.
Severe liver damage can follow overdosage of tetracy- Tetracyclines are available both in capsular and tablet forms
clines in animals with preexisting renal failure and may and are usually administered PO to dogs and cats. Milk,
antacids, and ferrous sulfate interfere with absorption.

Chapter 15. Tetracyclines 263

Table 15.2. Usual dosages of tetracyclines in selected domestic animal species.

Species Drug Route Dose (mg/kg) Interval (h) Comments

Dogs and cats Chlortetracycline, oxytetracycline PO 20 8 slow IV
Horses Oxytetracycline IV 10 12 slow IV
Doxycycline, minocycline PO, IV 5–10 12 slow IV
Ruminants Oxytetracycline IV 5 12
Swine Doxycycline PO 10 12 slow IV
Minocycline PO 4 12
IV 2.2 12
Oxytetracycline, tetracycline IM, IV 10 12–24
Long acting IM 20 48
Oxytetracycline, tetracycline IM 10–20 12–24
Long acting IM 20 48
Doxycycline PO 10 12

Because of poor water solubility, oxytetracycline low cost, broad antimicrobial activity, ease of adminis-
dihydrate is subject to “flip-flop” absorption, and cannot tration, and general effectiveness. However, their
reach similar plasma and tissue concentrations than the widespread use has undoubtedly contributed to very
hydrochloride salt. Intramuscular injection of tetra- widespread resistance in Enterobacteriaceae and other
cyclines cannot be recommended for horses or compan- important pathogenic bacteria.
ion animals because of local tissue damage and pain,
and erratic absorption. The recommended dose in cattle The tetracyclines’ capacity to attain effective con-
is 10 mg/kg given IM or preferably IV, because of varia- centrations in most tissues, together with their broad-
bility in absorption. The long-acting oxytetracycline spectrum of activity, makes them particularly useful in
parenteral preparation containing 2-pyrrolidone as the treatment of mixed bacterial infections. The activ-
excipient is approved for IM use in cattle and swine only. ity of the agents against obligate intracellular patho-
Owing to its “flip-flop” absorption kinetics, a single IM gens such as Anaplasma, Chlamydia, Ehrlichia,
dose of 20 mg/kg provides serum concentrations of Rickettsia, and some Mycoplasma makes them the
oxytetracycline above 0.5 μg/ml for 48 hours, but drugs of choice in treatment of infections caused by
appears to offer no advantage over the same dose of the these microorganisms. Although recommended for
conventional drug IM (Nouws, 1986). Subcutaneous the treatment of plague, results in the treatment of
injection in cattle maintains similar serum concentra- experimental infections in animals have sometimes
tions to those following IM administration and appears been disappointing. The lipophilic character of the
to be better tolerated. To prevent adverse effects, it is newer tetracyclines (minocycline, doxycycline) allows
important to differentiate between the conventional and them to attain concentrations in sites such as the pros-
the long-acting formulation in dosage decisions. tate, which are poorly accessible to older members of
the group. One disadvantage of tetracyclines over a
Clinical Applications number of other antimicrobial drugs is their bacterio-
static action, so that treatment may need to be for
The primary indications for tetracyclines are in the longer than with bactericidal drugs.
treatment of bacterial pathogens involved in the bovine
and porcine respiratory disease complexes, borreliosis, Tetracyclines are commonly used in the treatment of
brucellosis, chlamydiosis, ehrlichiosis, Lawsonia prolif- brucellosis, usually in combination with rifampin or
erative enteropathy, leptospirosis, listeriosis, porcine streptomycin. Doxycycline and minocycline are more
mycoplasmosis, rickettsiosis, and tularemia. The older effective than older tetracyclines because of better
tetracyclines have been used for many years in manag- penetration into cells. Treatment with doxycycline
ing infectious diseases in food animals because of their should last 6 weeks and with streptomycin 7–14 days.
Tetracyclines (particularly minocycline and doxycycline)

264 Section II. Classes of Antimicrobial Agents

are also used in the treatment of infections caused by acting tetracyclines are effective in preventing Babesia
other intracellular bacteria, including Coxiella and bovis and B. bigemina (redwater) in cattle. Tetracyclines
Ehrlichia. are used in the treatment of, and prevention against,
heartwater disease caused by Ehrlichia ruminantium
Cattle, Sheep, and Goats (Mebus and Logan, 1988). The drugs are also used in the
Many of the microorganisms that cause bovine pneumo- prophylaxis of East Coast fever caused by Theileria
nia are susceptible to tetracyclines at concentrations that parva (Chumo et al., 1989) and tickborne fever caused
can be achieved in lung tissue. The drugs are generally by Anaplasma phagocytophilum (Cranwell, 1990).
useful in the treatment of bovine pneumonias and also in
their prophylaxis, especially in feedlots. Nevertheless, For infectious keratoconjunctivitis in cattle, 2 doses of
increasing resistance in Mannheimia haemolytica and the long-acting preparation given 3 days apart can be
variable susceptibility of Mycoplasma bovis limits their recommended (George et al., 1988). Long-acting tetracy-
effectiveness. The long-acting parenteral formulation, clines produced moderate cure rates in cattle with der-
which must be administered by IM injection (or in some matophilosis. Long-acting tetracyclines (at 3- to 4-day
formulations, SC), 20 mg/kg at 48-hour intervals on 2–4 intervals for 5 treatments) combined with streptomycin
occasions, may be adequate in treating lower respiratory (IM daily for 7 days) successfully treated 14 of 18 cows
disease in cattle, sheep, and goats. with B. abortus infection (Nicoletti et al., 1985). Adminis-
tration once daily as a topical spray (25 mg/ml) was effec-
If tetracyclines are administered orally to feedlot cattle tive in controlling bovine papillomatous digital dermatitis,
in the prophylaxis of pneumonia, they should be given in the efficacy increasing with an increasing number of days
feed and not water. Administration in water may increase of applications (Shearer and Elliott, 1998).
mortality (Martin et al., 1982), possibly because of the
difficulty of ensuring that even amounts are ingested. Tetracyclines achieve milk concentrations approxi-
While prophylactic administration of drug in the ration mating those of blood, but because of poor bioavailability
appears often to reduce pneumonia and to improve after IM injection, they are best given IV. They are
growth and feed conversion efficiency, the cost-to-benefit second-choice parenteral antibiotics for serious infec-
ratio may not justify this approach. In addition, such a tions of the udder caused by Gram-positive bacteria and
practice tends to promote resistance among Mannheimia possibly by coliforms, although susceptibility among the
organisms. In prophylaxis of feedlot pneumonia, paren- latter organisms is uncommon. Repeated intramammary
teral administration gives better effects than oral admin- administration of tetracycline in combination with tylosin
istration because of higher bioavailability. An approach cured experimentally induced Mycoplasma californicum
shown to be useful is to inject tetracyclines when animals mastitis in cows (Ball and Campbell, 1989).
enter feedlots or to inject a single dose of long-acting tet-
racycline to all animals as soon as some in the lots appear In enzootic abortion in sheep caused by Chlamydophila
to be developing pneumonia. abortus, experimental and field evidence suggests that
2 treatments of 20 mg/kg of the long-acting preparation
Clostridial infections and listeriosis can be treated by at 2-week intervals, starting 6–8 weeks before lambing,
tetracyclines. A recommended dosage in neural listeri- will reduce the prevalence of abortions. The drug may
osis is 10 mg/kg/day IV, but clinical trials are needed to be most useful at the start of outbreaks (Greig and
determine whether the same dose given twice daily or Linklater, 1985). Tetracycline is the drug of choice in the
the use of ampicillin or penicillin G might not be more prevention and treatment of Q fever (Coxiella burnetii).
effective. In listeriosis, IV administration of the conven- Lambs can be protected from the rickettsial agent of
tional preparation (parenteral aqueous solution) is pre- tickborne fever and associated infections by a single
ferred. In human medicine, minocycline is a recognized injection of long-acting tetracycline formulation (Brodie
alternative to ampicillin. et al., 1986). Duration of the effect is between 2 and
3 weeks (Brodie et al., 1988). A single injection of long-
Oxytetracycline is the drug of choice in acute acting tetracycline with topical tetracycline is an effective
Anaplasma marginale infections. However, short-term treatment of ovine keratoconjunctivitis caused by
therapy with oxytetracyclines fails to clear the A. marginale Mycoplasma conjunctivae (Hosie, 1988; Hosie and Greig,
infections in carrier cattle (Coetzee et al., 2005). Long- 1995). Long-acting oxytetracycline was highly successful

Chapter 15. Tetracyclines 265

in preventing M. haemolytica pneumonia in sheep many other antimicrobial agents, cause enterocolitis, the
(Appleyard and Gilmour, 1990), and has been used suc- vast majority of treated horses do not exhibit side effects.
cessfully in the treatment of ovine footrot (Grogono- Nevertheless, the main factor limiting the use of tetracy-
Thomas et al., 1994), and dermatophilosis (Jordan and clines in horses is their limited spectrum against com-
Venning, 1995). mon equine pathogens as well as the irritant nature of
injectable products.
Long-acting tetracyclines combined with streptomycin
were shown to successfully treat about 80% or more of Oxytetracycline is active in vitro against most equine
rams with Brucella ovis infection (Marin et al., 1989; non-enteric Gram-negatives such as Actinobacillus
Dargatz et al., 1990). Daily intraperitoneal injections of spp. and Pasteurella spp., and approximately 70% of
1000mg oxytetracycline hydrochloride eliminated Brucella Staphylococcus spp. However, at clinically achievable
melitensis infection from sheep (Radwan et al., 1989). concentrations, oxytetracycline is active against only
50–60% of Enterobacteriaceae and β-hemolytic strepto-
Swine cocci. Doxycycline is generally safe when administered
Tetracyclines are commonly used in swine to prevent orally to horses, but it has poor bioavailability.
and treat atrophic rhinitis and bacteria associated with Oxytetracycline or doxycycline is the treatment of
the porcine respiratory complex (A. pleuropneumoniae, choice for infections caused by A. phagocytophilum,
M. hyopneumoniae, P. multocida). They also are effective B. burgdorferi, and N. risticii in horses. These microor-
against L. intracellularis. Field outbreaks of Pasteurella ganisms typically have a very low MIC (≤ 0.25 μg/ml).
pneumonia have been controlled by feed medication Oxytetracycline is also highly effective in the treatment
(200–400 g/ton). Feed medication with chlortetracy- of A. phagocytophilum and N. risticii infections in
cline, 100 g/ton, has been used to control adenomatosis horses (Madigan and Grible, 1987; Palmer et al, 1992).
and at a much higher level, 800 g/ton, to eradicate Administration of oxytetracycline to ponies experimen-
Leptospira from the kidneys of swine (Stahlheim, 1967). tally infected with B. burgdorferi by tick exposure
Tetracyclines may be effective against erysipelas and resulted in elimination of persistent infection. In con-
Haemophilus infections, but these pathogens are better trast, doxycycline or ceftiofur were inconsistent in elim-
controlled with the beta-lactams. Enterotoxigenic E. coli inating persistent infection in this experimental model
and S. suis are usually resistant. Tetracyclines in feed or (Chang et al., 2005). Oxytetracycline or doxycycline has
water have been used successfully to control streptococ- been used successfully in the treatment of infections by
cal lymphadenitis and M. hyopneumoniae infection. Lawsonia infection in foals (Sampier et al., 2006).

Orally administered oxytetracycline in pigs has an Dogs and Cats
average bioavailability of 5% across studies, tetracycline Tetracyclines are drugs of choice for A. phagocytophi-
is 18% bioavailable, chlortetracycline is between 18% lum, Ehrlichia canis, and Rickettsia rickettsii infections.
and 28% bioavailable, and doxycycline is 22% bioavail- Doxycycline administered orally to dogs infected with
able on average. The long-acting oxytetracycline formu- R. rickettsii is effective in preventing the disease or treat-
lations were more effective than the conventional ing acute illness but may not remove the carrier state. In
formulations in preventing experimental A. pleuropneu- experimental Brucella canis infection, the most effective
moniae infections when administered 48 hours before of several treatments, was minocycline (22 mg/kg every
challenge, but no more effective in treatment. An aver- 12 hours for 14 days) co-administered with streptomy-
age dose of 11 mg doxycycline/kg bodyweight q 24 h in cin (11 mg/kg every 12 hours for 7 days), but effective-
feed for 8 days was effective in controlling pneumonia ness must be monitored in the laboratory (Flores-Castro
due to P. multocida and M. hyopneumoniae in pigs and Carmichael, 1978). Field efficacy of tetracycline and
(Bousquet et al., 1998). streptomycin was 74% in one study (Nicoletti and Chase,
1987). Tetracycline hydrochloride, 10 mg/kg PO every
Horses 8 hours, was successful for the treatment of P. aeruginosa
The clinical use of oxytetracycline in horses has long urinary tract infections in dogs because of the high urine
been controversial because of early anecdotal reports of concentrations of the drug attained (Ling et al., 1980).
severe enterocolitis. While oxytetracycline may, like

266 Section II. Classes of Antimicrobial Agents

Other indications in dogs include treatment of Lyme reducing inflammatory cytokine and nitric oxide pro-
borreliosis and leptospirosis. Minocycline delivered in a duction. Minocycline is neuroprotective in many
subgingival local delivery system improved the clinical experimental models of neurodegenerative diseases,
and microbiologic response in dogs with periodontitis central nervous system injury, and viral encephalitis
following root scaling and planing (Hayashi et al., 1998). owing to its antiapoptotic and reactive oxygen species-
Doxycycline administered orally for 3 weeks achieved scavenging properties.
complete remission of about half of canine patients with
superficial pyoderma and partial remission in another Its matrix-metalloproteinase inhibitory effects have
40%, but complete remission in only 14% of patients been shown to be beneficial for various conditions such
with deep pyoderma and partial remission in another as rheumatoid arthritis, gingivitis, acute lung injury,
51% (Bettenay et al., 1998). myocardial disease, and cancer. This might be the main
mechanism of action in the treatment of contracted ten-
Cats suffering from Chlamydophila felis infection of the dons in foals. In one study, intravenous administration
upper respiratory tract and conjunctiva should be treated of oxytetracycline, at a dose of 44 mg/kg, resulted in a
with tetracyclines for 14 days to eliminate the organism decrease in the angle of the metacarpophalangeal joint
and to remove the latent carrier state. Tetracyclines are for approximately 96 hours. These high doses of oxytet-
drugs of choice for the treatment of Mycoplasma hae- racycline to foals with preexisting renal damage or
mofelis. Prolonged oral treatment with doxycycline does hypovolemia, or to foals unable to nurse sufficiently,
not eliminate the carrier state in Bartonella henselae or may result in acute renal failure.
B. clarridgeae infection (Kordick et al., 1997). Treatment
by tetracyclines of a cat with Yersinia pestis infection was Glycylcyclines
only temporarily effective (Culver, 1987).

Poultry The glycylcyclines are the first approved members of the
Tetracyclines are effective in the treatment of chlamydo- third-generation tetracyclines. They retain the mecha-
philosis if administered for prolonged periods. nism of action of the tetracyclines and circumvent their
Tetracycline or chlortetracycline can be administered in main resistance mechanisms (i.e., efflux pump and ribo-
1% medicated feed (45 days), and doxycycline has been somal protection system), but they are substrates of the
administered at 100 mg/kg IM at 5-day intervals on 6 or bacterial hydrolases.
7 occasions (Gylsdorff, 1987) or orally twice daily for
20 days. Tetracyclines are also used in the treatment of Tigecycline is a minocycline holding a tert-butyl-
chronic respiratory disease (Mycoplasma gallisepticum) glycylamino group on carbon-9 (Garrison et al., 2005).
and infectious synovitis (Mycoplasma synoviae), as well Tigecycline is available only as an injectable formulation,
as of fowl cholera (P. multocida). Prolonged administra- which restricts its use to hospital settings. It is unsuitable
tion of oxytetracycline (250 ppm) in feed is required to for oral administration due to its higher formula weight
control M. gallisepticum infection in birds. One report (584Da), and lipid/buffer partition coefficient (log-P>10).
noted the surprising efficacy of tetracycline sorbate in
the oral treatment of naturally occurring Aspergillus Tigecycline binds 5 times more strongly to ribosomes
fumigatus infection (Roy et al., 1991). than minocycline or tetracycline, which may lead to
decreased sensitivity toward ribosomal protection
Uses Unrelated to Their Antibacterial Activity resistance mechanisms. It is active against a broad range
of Gram-positive, Gram-negative and anaerobic micro-
Tetracyclines have a number of non-antibiotic effects organisms including multidrug-resistant strains of
that are better documented for the second- and third- Staphylococcus spp. and Enterococcus spp., but is not
generation molecules. They include anti-inflammatory active against Pseudomonas spp. (Garrison et al., 2005).
properties, immunosuppression, inhibition of lipase Several laboratory animal studies describing the efficacy
and collagenase, antinociceptive, antiosteoporotic, and of tigecycline have been published. In people, nausea
wound-healing effects. Experimentally, tetracyclines and vomiting are the most important side effects. There
have protected mice from endotoxin-induced shock by are currently no published studies evaluating tigecycline
in domestic animal species.

Chapter 15. Tetracyclines 267

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Chloramphenicol, Thiamphenicol, 16
and Florfenicol

Patricia M. Dowling

Chloramphenicol is a stable, lipid-soluble, neutral Antimicrobial Activity
compound. It is a derivative of dichloracetic acid and
contains a nitrobenzene moiety. This p-nitro group Chloramphenicol is active against a wide range of
is  associated with idiosyncratic (non-dose-dependent) Gram-positive and many Gram-negative bacteria
aplastic anemia in humans (Figure 16.1). Thiamphenicol (Table  16.1), against which it is usually bacteriostatic.
has a similar antibacterial spectrum to chloramphenicol Anaerobic bacteria are inhibited at usual therapeu-
but differs from the parent compound in that the p-nitro tic  concentrations (5–15 μg/ml). Chloramphenicol
group attached to the benzene ring is replaced by a suppresses rickettsial and chlamydial growth. While
sulfomethyl group. Florfenicol is a structural analogue mycoplasma often show susceptibility in vitro, chloram-
of thiamphenicol that also lacks the p-nitro group, phenicol therapy of mycoplasma pulmonary infections
and  it  is more active than thiamphenicol. Neither is often ineffective.
thiamphenicol nor florfenicol are associated with dose-
independent aplastic anemia in humans or any other t Susceptible organisms (MIC ≤ 8 mg/ml) include among
species, but both are associated with dose-dependent Gram-positive aerobic bacteria, including Actino-
bone marrow suppression. myces spp., Trueperella pyogenes, Bacillus anthracis,
Corynebacterium spp., Erysipelothrix rhusiopathiae,
Chloramphenicol Listeria monocytogenes, many Enterococcus spp.,
Staphylococcus spp., and Streptococcus spp.
Mechanism of Action Methicillin-resistant Staphylococcus aureus (MRSA)
and Staphylococcus pseudintermedius (MRSP) have
Chloramphenicol is a potent inhibitor of microbial emerged as a significant pathogens in companion
protein synthesis. It binds irreversibly to a receptor animals. Two major clonal MRSP lineages have
site  on the 50S subunit of the bacterial ribosome, disseminated in Europe and North America. Isolates
inhibiting peptidyl transferase and preventing originating from North America are often susceptible
the  amino acid transfer to growing peptide chains to chloramphenicol, whereas isolates from Europe
and  subsequently inhibiting protein formation. are  often resistant to chloramphenicol (Perreten
Chloramphenicol also inhibits mitochondrial protein et  al.,  2010). Staphylococcus schleiferi isolated from
synthesis in mammalian bone marrow cells in a pyoderma dogs is typically susceptible (Vanni et al.,
dose-dependent manner. 2009). Typically susceptible Gram-negative aerobic
bacteria include Actinobacillus spp., Bordetella

Antimicrobial Therapy in Veterinary Medicine, Fifth Edition. Edited by Steeve Giguère, John F. Prescott and Patricia M. Dowling.
© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

269

270 Section II. Classes of Antimicrobial Agents

O systems, inactivation by phosphotransferases, and
H NH C CHCI2 mutations of the target site or permeability barriers
(Schwarz et al., 2004). The CAT genes are commonly
O2N C C CH2OH found on plasmids in Enterobacteriaceae and
Pasteurellaceae, and most of these plasmids carry one
OH H O or  more additional resistance genes. The efflux of
Chloramphenicol C CHCl2 chloramphenicol from bacteria can be mediated by
either specific transporters or multidrug transporters.
H NH Specific transporters tend to have a substrate spectrum
limited to a small number of structurally related
H3C SO2 C C CH2F compounds while the multidrug transporters often
have a wide range of unrelated substances as substrates.
OH H Specific transporters commonly mediate higher levels
of  resistance compared to multidrug transporters.
Florfenicol O Many of the genes coding for the CAT genes or specific
transporters are located on mobile genetic elements,
H NH C CHCI2 such as plasmids, transposons or gene cassettes. When
plasmids mediating resistance to chloramphenicol are
H3C SO2 C C CH2OH transferred from one bacterium to another, they are not
always able to replicate in the new host. Recombination
OH H between the new plasmid and the plasmids already
Thiamphenicol resident in the new host effectively circumvents replica-
tion problems. Such recombination may lead to the
Figure 16.1. Chemical structure of chloramphenicol, formation of novel resistance plasmids that carry the
florfenicol, and thiamphenicol. resistance genes of both parental plasmids and are well
adapted to replication in the new host.
bronchiseptica, Brucella canis, Enterobacteriaceae
(including many E. coli), Klebsiella spp., Proteus spp., Pharmacokinetic Properties
and Salmonella spp., Haemophilus spp., Histophilus
somni, Leptospira spp., Moraxella bovis, Mannheimia In monogastric animals and pre-ruminant calves,
hemolytica, and Pasteurella spp. Anaerobes (Bacteroides chloramphenicol is typically well absorbed from
spp., Clostridium spp., Prevotella spp., Porphyromonas the  gastrointestinal tract. The oral bioavailability of
spp.) are commonly susceptible, including penicillin- chloramphenicol in foals is 83%, but only 40% after a
resistant Bacteroides fragilis. single administration in mares; declining to 20% after
t Intermediately susceptible organisms (MIC = 16 mg/ml) 5 doses (Brumbaugh et al., 1983; Gronwall et al., 1986).
include Rhodococcus equi. Chloramphenicol palmitate is poorly absorbed in cats.
t Resistant organisms (MIC ≥ 32 mg/ml) include In ruminants, orally administered chloramphenicol
Mycobacterium spp. and Nocardia spp. Resistance is  inactivated in the rumen. The apparent volume of
often emerges in Gram-negative enteric bacteria distribution of chloramphenicol is large (> 1 L/kg) in all
such as E. coli. species. This can be attributed to widespread distribu-
tion, as partitioning of the drug is independent of pH
The most frequently encountered mechanism of and there is no evidence of selective tissue binding.
bacterial resistance to chloramphenicol is enzymatic Because of its lipid solubility and moderately low
inactivation by acetylation of the drug by chloram- protein  binding (30–46%), chloramphenicol attains
phenicol acetyltransferases (CATs). Acetylation of the effective concentrations in most tissues and body fluids,
hydroxyl groups on chloramphenicol prevents drug including cerebrospinal fluid (CSF) and the central
binding to the 50S ribosomal subunit. There are also nervous system. Chloramphenicol may achieve CSF
reports of other mechanisms of resistance, such as efflux concentrations up to 50% of plasma concentrations

Chapter 16. Chloramphenicol, Thiamphenicol, and Florfenicol 271

Table 16.1. Activity (MIC90) of chloramphenicol (μg/ml) against selected bacteria and mycoplasma.

Organism MIC90 Organism MIC90

Gram-positive aerobes 1 8
A. pyogenes 2 L. monocytogenes 8
B. anthracis 4 S. aureus 4
C. renale > 32 S. dysgalactiae 2
Enterococcus spp. 2 S. uberis
E.rhusiopathiae > 32
4 Klebsiella spp. > 32
Gram-negative aerobes 8 Pasteurella spp.
Actinobacillus spp. 4 M. haemolytica 2
B. bronchiseptica > 32 P. multocida 2
B. canis > 32 Proteus spp. > 32
Enterobacter spp. 1 P. aeruginosa > 32
E. coli
H. somni 8 D. nodosus 0.25
8 Fusobacterium spp. 1
Anaerobes 4 F. necrophorum 2
Bacteroides spp. 4 S. hyodysenteriae 4
B. fragilis
C. difficile 8 M. hyopneumoniae 4
C. perfringens 64 M. ovipneumoniae 16
8
Mycoplasma
M. bovis
M. bovirhinis
M. canis

when the meninges are normal and more when In newborn animals the elimination half-life of chlo-
inflammation is present. Topical ophthalmic formula- ramphenicol is considerably longer than in adult animals
tions achieve therapeutic concentrations in the aqueous of the same species. This is due mainly to immature glu-
humor. Chloramphenicol readily diffuses into milk, and curonide conjugation mechanisms. Glucuronide conju-
pleural and ascitic fluids. It readily crosses the placenta, gation develops most rapidly in foals, so that the half-life
achieving concentrations 75% of those in maternal in the 1-week-old foal approaches that of the adult horse.
plasma. This may be of clinical significance, as the fetal
liver is deficient in glucuronyl transferase activity. Drug Interactions
Penetration of the blood-prostate barrier is relatively
poor unless inflammation is present. Chloramphenicol should not be used concurrently with
bactericidal antimicrobials in treating infections where
The elimination half-life of chloramphenicol varies host defenses are poor. Concurrent chloramphenicol
widely between species. Elimination is primarily by and penicillin G have been shown to be antagonistic in
hepatic metabolism by conjugation with glucuronic acid. treating bacterial meningitis and endocarditis in humans.
Its elimination is short in horses (1 hour; Sisodia et al., Chloramphenicol acts on the same ribosomal site as
1975) and long in cats (5–6 hours) because of feline macrolides antibiotics. Chloramphenicol is antagonistic
deficiencies in glucuronide conjugation (Watson, 1991). to the fluoroquinolones, as inhibition of protein synthesis
A fraction of the dose is excreted unchanged by glomeru- by chloramphenicol interferes with the  production of
lar filtration in the urine of dogs (10%) and cats (20%), autolysins necessary for cell lysis after the fluoroquinolone
while a negligible amount is eliminated by renal excre- interferes with bacterial DNA supercoiling.
tion in herbivores. The metabolites, which are inactive,
are excreted in the urine and to a much lesser extent in the Because chloramphenicol inhibits microsomal
bile. The glucuronide conjugate excreted in bile can  be enzyme activity, hepatic metabolism (oxidative reactions
hydrolyzed by intestinal flora to liberate the parent drug. and glucuronide conjugation) of drugs given concur-
rently is slowed, resulting in prolonged pharmacologic

272 Section II. Classes of Antimicrobial Agents

effect. Thus chloramphenicol markedly prolongs the to other antimicrobials. Therapeutic efficacy is
effect of barbiturates, and fatal effects have been maximized by maintaining an average steady-state
observed in epileptic dogs treated concurrently with plasma concentration of 5–10 μg/ml.
phenobarbital (Adams and Dixit, 1970).
Chloramphenicol is available for either oral (free
Toxicity and Adverse Effects base  or palmitate ester) or parenteral (sodium succi-
nate) administration. For local treatment of eye or
The main toxic effects of chloramphenicol in humans ear infections caused by susceptible organisms, topical
are bone marrow depression, which can be either an preparations are available.
idiosyncratic, non-dose-dependent aplastic anemia or
a dose-dependent anemia from suppression of protein Because the drug is well absorbed from the
synthesis. Aplastic anemia appears to be a genetically gastrointestinal tract in small animals, it can be given
determined idiosyncrasy of individual humans. The orally as either the base or the palmitate ester. The ester
incidence of fatal aplastic anemia has been estimated is hydrolyzed prior to absorption of the free base.
as 1 in every 25,000–60,000 humans who use the The  intake of food does not influence bioavailability.
drug. A few cases of aplastic anemia in humans have Subcutaneous injection of chloramphenicol sodium
occurred following contact exposure (ophthalmic use, succinate is an alternative to oral administration. While
medicated sprays, handling), so that veterinarians both routes may provide equivalent concentrations, the
and  owners should wear protective gloves and face oral route is preferable as injection of the parenteral
masks  when handling chloramphenicol products preparation is painful. The total length of treatment
(Wallerstein et al., 1969). should not exceed 10 days, especially in cats. Do not
administer chloramphenicol to patients with evidence
A “gray baby” syndrome occurs in newborn infants of or suspected bone marrow suppression.
because their deficiency in glucuronic acid conjugation
causes a dose-dependent anemia. In animals, chloram- The short half-life of chloramphenicol in horses
phenicol toxicity is related to both the dose and dura- (1  hour), together with its generally bacteriostatic
tion  of treatment, and cats are more likely than dogs action, makes IV administration impractical. Oral
to develop toxicity. In cats, clinical signs of toxicity may tablets of the free base drug can be administered PO or
be seen when the usual maintenance dosage of 25 mg/kg the sodium succinate formulation can be given by IM
of  base or palmitate ester is given twice daily for injection. After absorption from injection sites, the
21 days (Watson, 1991). Chloramphenicol causes changes inactive succinate ester is rapidly hydrolyzed to
in the  peripheral blood and bone marrow due to the active drug.
reversible, dose-related disturbances in red cell matura-
tion. Administration for less than 10 days using the Because of the risks of idiosyncratic aplastic anemia
maintenance dose is not likely to cause toxicity in either in humans, chloramphenicol is banned for use in food
dogs or cats, unless the animals have depressed hepatic animals in most countries. The drug should not be used
microsomal enzyme activity or severely impaired renal in the early neonatal period unless plasma concentra-
function. Use in dogs for MRSA and MRSP infections tions are monitored, and should be used with caution
is  associated with frequent adverse gastrointestinal in  pregnant animals because of the potential adverse
effects (vomiting, diarrhea, weight loss, nausea, anorexia effects on the fetus.
and decreased appetite), as well as lethargy, shaking,
increased liver enzymes, and anemia (Bryan et al., 2012). Clinical Applications

Administration and Dosage The potential for idiosyncratic fatal aplastic anemia in
humans has led to prohibition of chloramphenicol use
Recommended drug dosages of chloramphenicol are in food animals in many parts of the world. Florfenicol
given in Table 16.2. is the appropriate analogue to use in food animals. With
the development of fluoroquinolone antimicrobials for
Chloramphenicol is a broad-spectrum, time-dependent companion animals, there were few primary indications
bacteriostatic drug that can attain effective concentra- for the use of chloramphenicol, but it was still considered
tions at sites of infection that are relatively inaccessible for some anaerobic infections, serious ocular infections,
prostatitis, otitis media/interna and salmonellosis in

Chapter 16. Chloramphenicol, Thiamphenicol, and Florfenicol 273

Table 16.2. Usual systemic dosing rate (mg/kg) of chloramphenicol in animals.*

Species Dosage Form Route Dose (mg/kg) Interval (h) Comments
Limit to 10 days of therapy
Dogs, cats Base, palmitate Oral 50 12
Horses Sodium succinate IV, IM, SC 25–50 8–12
Sodium succinate IM 30–50 6
Base, palmitate PO 25–50 6–8

*Owners should be warned of the risks of their exposure to chloramphenicol.

horses, dogs and cats. Use in dogs and cats has been Vanni M, et al. 2009. Antimicrobial susceptibility of
increasing in frequency due to the increase in MRSA Staphylococcus intermedius and Staphylococcus schleiferi
and MRSP infections, but chloramphenicol is associated isolated from dogs. Res Vet Sci 87:192.
with more adverse effects (mainly gastrointestinal) than
other treatment options such as doxycycline, clindamy- Wallerstein RO, et al. 1969. Statewide study of chlorampheni-
cin and amikacin (Bryan et al., 2012). Human toxicity col therapy and fatal aplastic anemia. JAMA 208:2045.
from handling chloramphenicol should be discussed
with the owner and appropriate precautions taken Watson AD. 1991. Chloramphenicol 2. Clinical pharmacology
when prescribing chloramphenicol for use in dogs and in dogs and cats. Aust Vet J 68:2.
cats. In addition, the zoonotic potential of animal-origin
staphylococci should be discussed with owners Thiamphenicol
(Guardabassi et al., 2004).
Thiamphenicol is a derivative of chloramphenicol, in
Bibliography which the p-nitro group been replaced by a sulfomethxyl
group. Thiamphenicol is generally 1–2 times less active
Adams HR, Dixit BN. 1970. Prolongation of pentobarbital than chloramphenicol, although it has equal activity
anesthesia by chloramphenicol in dogs and cats. J Am Vet against Haemophilus, B. fragilis, and streptococci. Cross-
Med Assoc 156:902. resistance with chloramphenicol is complete in bacteria that
possess CATs. Absorption and distribution are similar to
Brumbaugh GW, et al. 1983. Pharmacokinetics of chloram- chloramphenicol, and it is also equally well distributed
phenicol in the neonatal horse. J Vet Pharmacol Ther into tissues. Oral bioavailability in pre-ruminant lambs
6:219. and calves is 60% (Mengozzi et al., 2002). Thiamphenicol
is not eliminated by hepatic gluronide conjugation but
Bryan J, et al. 2012. Treatment outcome of dogs with excreted unchanged in the urine. Unlike chloramphenicol,
methicillin-resistant and methicillin-susceptible Staphylo- its elimination is unaffected by liver disease and by the
coccus pseudintermedius pyoderma. Vet Dermatol 23:361. use of other drugs metabolized in the liver. The pharma-
cokinetic parameters of thiamphenicol follow allometric
Gronwall R, et al. 1986. Body fluid concentrations and phar- scaling, in that values for elimination half-life and
macokinetics of chloramphenicol given to mares intrave- volume of distribution increase with body size from mice
nously or by repeated gavage. Am J Vet Res 47:2591. through rats, rabbits, dogs, pigs, sheep and calves (Castells
et al., 2001). Therapeutic concentrations are achieved
Guardabassi L, et al. 2004. Transmission of multiple in milk of lactating cows (Abdennebi et al., 1994).
antimicrobial-resistant Staphylococcus intermedius
between dogs affected by deep pyoderma and their owners. One reason for major interest in thiamphenicol is
Vet Microbiol 98:23. that, because it lacks the p-nitro group, it does not
induce irreversible bone marrow aplasia in humans,
Perreten V, et al. 2010. Clonal spread of methicillin-resistant although it may cause dose-dependent bone marrow
Staphylococcus pseudintermedius in Europe and North suppression more frequently than chloramphenicol.
America: an international multicentre study. J Antimicrob
Chemother 65:1145. Thiamphenicol is used extensively in Europe and
Japan but is not available in North America. Apart from
Schwarz S, et al. 2004. Molecular basis of bacterial resistance
to chloramphenicol and florfenicol. FEMS Microbiol Rev
28:519.

Sisodia CS, et al. 1975. A pharmacological study of chloram-
phenicol in horses. Can J Comp Med 39:216.

274 Section II. Classes of Antimicrobial Agents

its bacteriostatic character and lower activity than Table 16.3. Activity (MIC90) of florfenicol (μg/ml) against
chloramphenicol, thiamphenicol appears underutilized selected bacteria and mycoplasma.
in the treatment of many infections caused by suscepti-
ble organisms. While detailed dosage information is Organism MIC90
not  available because of the lack of pharmacokinetic
and  clinical studies, suitable dosage in animals would Porcine 0.5
appear to be similar to that of chloramphenicol. Dosages A. pleuropneumonia 0.5
for cattle and pigs are 10–30 mg/kg IM every 24 hours, P. multocida 8
30 mg/kg PO every 12 hours for pre-ruminant lambs B. bronchispetica 2
and every 24 hours for pre-ruminant calves, or S. suis
50–200 ppm in feed for pigs and 100–500 ppm in feed 2
for chickens. Bovine 0.5
M. haemolytica 2
Bibliography P. multocida 1.56
H. somni 32
Abdennebi EH, et al. 1994. Thiamphenicol pharmacokinetics A. pyogenes 4
in beef and dairy cattle. J Vet Pharmacol Ther 17:365. Salmonella dublin 32.0
M. bovis
Castells G, et al. 2001. Allometric analysis of thiamphenicol L. monocytogenes 0.25
disposition among seven mammalian species. J Vet 1.6
Pharmacol Ther 24:193. Fish 0.5
Edwardsiella ictaluri 0.6
Mengozzi G, et al. 2002. A comparative kinetic study of thia- Aeromonas salmonicid 32.0
mphenicol in pre-ruminant lambs and calves. Res Vet Sci Vibrio anguillarum
73:291. Photobacterium damsela
Chryseobacterium spp.

Florfenicol The mutant prevention concentration for Mannheimia
haemolytica is ≥ 32 μg/ml (Blondeau et al., 2012).
Florfenicol is a fluorinated derivative of thiamphenicol, Fusobacterium necrophorum, Bacteroides melaninogeni-
in which the hydroxyl group at C-3 has been replaced cus and Moraxella bovis are highly susceptible. The
with fluorine. Florfenicol is a potent inhibitor of MIC90 for Enterobacteriaceae, which are less susceptible,
microbial protein synthesis with the same mechanisms is higher; for example, for Salmonella dublin it is 32μg/ml.
of action as chloramphenicol. Like thiamphenicol, Florfenicol is active against a number of important
florfenicol does not cause idiosyncratic aplastic anemia bacterial pathogens of fish including Aeromonas
in humans but can cause dose-dependent bone marrow salmonicida, Vibrio salmonicida, Vibrio anguillarum and
suppression in animals. Yersinia ruckeri in salmon and trout and Edwardsiella
ictaluri in catfish.
Antimicrobial Activity
Because of the substitution of a hydroxyl group with
Florfenicol is slightly more active than chloramphenicol a  fluorine molecule, florfenicol is less susceptible to
in its range of antimicrobial activity (Table  16.3). resistance from bacteria expressing CAT enzymes. But
Florfenicol remains highly active against the pathogens new mechanisms of bacterial resistance to chloram-
involved in bovine respiratory disease (Portis et al., phenicol and florfenicol are being identified (Liu et al.,
2012). It is bactericidal against Histophilus somni and 2012; Tao et al., 2012). Florfenicol resistance in Gram-
Pasteurella spp. at concentrations only one dilution negative bacteria is related to plasmid transfer of the
above those that are bacteriostatic. The MIC90 for floR gene. This gene codes for a membrane-associated
Actinobacillus pleuropneumoniae, Histophilus somni, exporter protein that promotes efflux of chlorampheni-
Mannheimia haemolytica, Trueperella pyogenes, col and florfenicol (Schwarz et al., 2004). In cases of
Pasteurella multocida and Streptococcus suis is ≤ 2 μg/ml. neonatal calf diarrhea from E. coli, if floR is present,
the MIC range is 16 to ≥ 256 μg/ml (White et al., 2000).

Chapter 16. Chloramphenicol, Thiamphenicol, and Florfenicol 275

The  floR gene was identified in Pasteurella multocida alpacas, and dogs (Alcorn et al., 2004; Ali et al., 2003;
isolated from a calf in 2005 (Kehrenberg and Schwarz, Atef et al., 2001; Holmes et al., 2012; Kim et al., 2011;
2005) and has now been identified a bovine isolate of Koc et al., 2009; Lane et al., 2004; Lane et al., 2008; Palma
Mannheimia haemolytica (Katsuda et al., 2012). After a et al., 2011; Shen et al., 2004).
single dose of florfenicol, feedlot cattle show a shift in
fecal flora to multiresistant E. coli, likely due to selection Drug Interactions
for plasmids containing the floR gene linked with other
resistance genes. The antimicrobial resistance associated There are no published data on adverse drug interac-
with florfenicol treatment declined over 4 weeks post- tions with florfenicol. Mechanistically, interactions
treatment but a higher proportion of fecal E. coli were should be similar to those seen with chloramphenicol.
resistant than when the cattle entered the feedlot (Berge
et al., 2005). Toxicity and Adverse Effects

Pharmacokinetic Properties Transient diarrhea or inappetance may occur in cattle
treated with florfenicol, but resolves within a few days of
The oral bioavailability of florfenicol in horses is 83% discontinuing treatment. In swine, peri-anal inflamma-
(McKellar and Varma, 1996). It is 89% in 2- to 5-week- tion and/or rectal eversion may occur in treated
old calves, but decreases when administered with milk animals,  but should resolve completely within 1 week.
replacers (Varma et al., 1986). After intramuscular injec- The injectable florfenicol formulations for cattle and
tion, bioavailability is 81% in horses and 38% in lactat- swine are only labeled for a maximum of 2 doses, so
ing dairy cattle but 54% after intramammary infusion bone marrow suppression has not been reported
(Soback et al., 1995). Ten hours after IM administration with  clinical use in these species. Potentially fatal
to dairy cows, milk concentrations peak at 1.6 μg/ml bone marrow suppression, from suppression of protein
and  it takes at least 5 days for milk concentrations to synthesis in erythroid cells, has been documented with
deplete to undetectable concentrations. Milk depletion over dose or prolonged florfenicol administration
is significantly prolonged with subcutaneous administra- (Holmes, et al., 2012; Tuttle et al., 2006).
tion, so administration by this route should be avoided
in dairy cows. While values of volume of distribution for Administration and Dosage
florfenicol are slightly lower than for chloramphenicol,
florfenicol is well distributed into many tissues includ- Florfenicol is approved in numerous countries in
ing lungs, muscle, bile, kidney and urine. With IV beef  cattle for the treatment of respiratory disease,
administration, cerebrospinal fluid concentrations are pododermatitis and keratoconjunctivitis caused by
46% of plasma concentrations, achieving potentially highly susceptible bacteria (MIC ≤ 2 μg/ml) at 20 mg/kg
therapeutic concentrations for H. somni, but not IM twice at a 48-hour interval or 40 mg/kg SC once.
Gram-negative enteric bacteria (de Craene et al., 1997). Each injection site should not exceed 10 ml. The label
With IM administration to beef calves, the serum dosage does not result in concentrations that would
concentration of florfenicol remains above 1 μg/ml for be effective against Gram-negative enteric pathogens. In
22 hours (Lobell et al., 1994). The commercially available some countries, florfenicol is approved for the treatment
formulation of florfenicol is long-acting, so that “flip- of swine respiratory disease from Actinobacillus pleuro-
flop” kinetics occurs, where elimination is prolonged pneumoniae and Pasteurella multocida at 15 mg/kg IM
due to slow absorption from the IM or SC injection site. twice at a 48-hour interval. In swine it should be injected
In cattle, 64% of a dose is excreted as parent drug in the into the neck at no more than 5 ml per site.
urine. Florfenicol amine is the slowest metabolite to
deplete from the liver and is used as the marker residue In the United States, florfenicol is approved as a
for withdrawal times. premix for swine for the control of swine respiratory
disease associated with Actinobacillus pleuropneumo-
While not approved, florfenicol is used extra-label in niae, Pasteurella multocida, Streptococcus suis, and
a number of species. Pharmacokinetics have been Bordetella bronchiseptica. In Canada, florfenicol is
described in sheep, goats, North American elk, rabbits, approved as a 2.3% concentrate solution for oral
administration to swine for the treatment of swine
respiratory disease associated with Actinobacillus

276 Section II. Classes of Antimicrobial Agents

pleuropneumoniae and Pasteurella multocida and to The use of florfenicol in horses is not recommended.
broiler chickens for the treatment and control of air Despite a high oral bioavailability and good tissue
sacculitis associated with E. coli susceptible to florfenicol. distribution, florfenicol administration to horses altered
As well in Canada, florfenicol is approved for the fecal consistency with single doses administered IV, PO,
treatment of furunculosis caused by susceptible strains or IM (McKellar and Varma, 1996). In a chronic dosing
of Aeromonas salmonicida in salmon. In the United study using the cattle formulation at 20 mg/kg IM every
States, it is approved for control of catfish mortality due 48 hours, all horses remained clinically normal but had
to enteric septicemia associated with Edwardsiella significant alterations in fecal flora (Dowling, 2001).
ictaluri. In Japan, florfenicol is labeled for the treatment
of pseudotuberculosis and streptococcosis in Perciformes Bibliography
(yellowtail, amberjack, red sea bream, tilapia, etc.) and
for the treatment of edwardsiellosis disease in eel. The Alcorn J, et al. 2004. Pharmacokinetics of florfenicol in North
fish formulation is mixed in unmedicated feed prior American elk (Cervus elaphus). J Vet Pharmacol Ther
to pelleting or used to surface coat pelleted feed and fed 27:289.
to  deliver 10 mg/kg per day for 10 consecutive days
(Gaikowski et al., 2003). Ali BH, et al. 2003. Comparative plasma pharmacokinetics
and tolerance of florfenicol following intramuscular and
Clinical Applications intravenous administration to camels, sheep and goats.
Vet Res Commun 27:475.
Currently, florfenicol is used for metaphylaxis and
for  treatment of bovine respiratory disease caused by Atef M, et al. 2001. Disposition kinetics of florfenicol in goats
highly susceptible bacteria such as Mannheimia, by using two analytical methods. J Vet Med A Physiol
Pasteurella and Histophilus (Hoar et al., 1998). The same Pathol Clin Med 48:129.
dosage regimen will treat pododermatitis caused by
Fusobacterium necrophorum and Bacteroides melanino- Berge AC, et al. 2005. Assessing the effect of a single dose
genicus and infectious bovine keratoconjunctivitis florfenicol treatment in feedlot cattle on the antimicrobial
caused by Morexella bovis, but penicillin or oxytetracy- resistance patterns in faecal Escherichia coli. Vet Res 36:723.
cline are less expensive and narrower in antimicrobial
spectrum and should be used as first line treatments for Blondeau JM, et al. 2012. Comparative minimum inhibitory
these infections. When administered to lactating dairy and mutant prevention drug concentrations of enro-
cows, florfenicol readily crosses into milk, and residues floxacin, ceftiofur, florfenicol, tilmicosin and tulathromycin
are more prolonged after SC than IM administration. against bovine clinical isolates of Mannheimia haemolytica.
While it has high systemic bioavailability, intramam- Vet Microbiol 160:85.
mary administration of florfenicol for the treatment of
bovine mastitis caused by a variety of pathogens had no Ciprian A, et al. 2012. Florfenicol feed supplemented
advantage over cloxacillin (Wilson et al., 1996). decrease  the clinical effects of Mycoplasma hyopneumoniae
experimental infection in swine in Mexico. Res Vet Sci 92:191.
Florfenicol in feed or by injection reduces illness due to
Actinobacillus pleuropneumoniae and M. hyopneumoniae De Craene BA, et al. 1997. Pharmacokinetics of florfenicol
in pigs.(Ciprian et al., 2012; Del Pozo Sacristan in  cerebrospinal fluid and plasma of calves. Antimicrob
et al., 2012; Palacios-Arriaga et al., 2000). Oral florfenicol Agents Chemother 41:1991.
is effective in broiler chickens for the treatment and
control of air sacculitis associated with E. coli susceptible Del Pozo Sacristan R, et al. 2012. Efficacy of florfenicol
to florfenicol. injection in the treatment of Mycoplasma hyopneumoniae
induced respiratory disease in pigs. Vet J 194:420.
Florfenicol is used in the treatment of susceptible
bacterial diseases of fish, including furunculosis in Dowling PM. 2001. 19th Annual American College of
salmon and vibriosis in salmon and cod, pseudotuber- Veterianry Internal Medicine Forum, Denver, CO, p. 198.
culosis in Japanese yellowtail, enteric septicemia in
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efficacy and safety of florfenicol and tilmicosin for the
treatment of undifferentiated bovine respiratory disease
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Holmes K, et al. 2012. Florfenicol pharmacokinetics in
healthy adult alpacas after subcutaneous and intramuscu-
lar injection. J Vet Pharmacol Ther 35:382.

Katsuda K, et al. 2012. Plasmid-mediated florfenicol resist-
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Sulfonamides, Diaminopyrimidines, 17
and Their Combinations

John F. Prescott

The value of the sulfonamides as single antimicrobial The sodium salts of sulfonamides are readily soluble
agents has been greatly diminished both by widespread in water, and parenteral preparations are available for IV
acquired resistance and by their relatively low potency injection. These solutions are highly alkaline in reaction,
compared to more modern antimicrobial drugs. with the notable exception of sodium sulfacetamide,
However, when combined with antibacterial diaminopy- which is nearly neutral and is available as an ophthalmic
rimidines such as trimethoprim, resistance occurs less prepa ration.
frequently and thus their usefulness has been enhanced.
Certain sulfonamide molecules are designed for low
Sulfonamides solubility (e.g., phthalylsulfathiazole), so they are slowly
absorbed and are intended for use in treatment of enteric
Chemistry infections.

The sulfonamides are derivatives of sulfanilamide, which Mechanism of Action
contains the structural prerequisites for antibacterial
activity. The sulfonamides differ in the radical (R) attached Sulfonamides interfere with the biosynthesis of folic
to the amido (–SO2NHR) group or occasionally in the acid in bacterial cells by competitively preventing para-
substituent on the amino (–NH2) group (Figure 17.1). aminobenzoic acid (PABA) from incorporation into the
folic (pteroylglutamic) acid molecule. Specifically, sul-
The various derivatives differ in physicochemical and fonamides compete with PABA for the enzyme dihy-
pharmacokinetic properties and in degree of antimicro- dropteroate synthetase. Their selective bacteriostatic
bial activity. As a group, sulfonamides are quite insolu- action depends on the difference between bacterial and
ble; they are more soluble at an alkaline pH than at an mammalian cells in the source of folic acid. Susceptible
acid pH. In a mixture of sulfonamides, each component microorganisms must synthesize folic acid, whereas
drug exhibits its own solubility. An example is the trisul- mammalian cells use preformed folic acid. The bacterio-
fapyrimidine preparation, in which the antibacterial static action can be reversed by an excess of PABA, so
activity of the combined sulfonamides is additive, but that any tissue exudates and necrotic tissue should be
the agents behave independently with respect to solubil- removed if animals are to be treated with sulfonamides.
ity. This mixture was developed to offset the precipita-
tion of sulfonamide crystals in acidic fluid in the distal Antimicrobial Activity
renal tubules and ureters.
Sulfonamides are broad-spectrum antimicrobial agents,
inhibiting bacteria, toxoplasma, and other protozoal agents

Antimicrobial Therapy in Veterinary Medicine, Fifth Edition. Edited by Steeve Giguère, John F. Prescott and Patricia M. Dowling.
© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

279

280 Section II. Classes of Antimicrobial Agents SO2 NH
SO2NH2

NN

CH3 CH
NH2
NH2

Sulfanilamide Sulphamethazine

SO2 NH SO2 NH
N
N

NS

NH2 NH CO
Sulfadiazine

COOH

SO2 NH Phthalylsulfathiazole

N CH3
O

NH2
Sulfamethoxazole

Figure 17.1. Structural formulas of some sulfonamides.

such as coccidia, but their antibacterial activity is signifi- infections are not agreed because of difficulties in
cantly limited by the extensive resistance that has developed both determining MIC and variability in serum con-
over 70 years. Different sulfonamides may show quantita- centrations with different drugs and different doses.
tive but not necessarily qualitative differences in activity. An MIC of 8–32 μg/ml is a reasonable definition of
susceptibility for short-acting systemic sulfonamides;
The MIC of sulfonamides is markedly affected by an MIC of ≥ 64–128 μg/ml can be interpreted as
the composition of the medium and the bacterial evidence of resistance.
inoculum concentration. Because of this, in vitro tests
may sometimes falsely report a bacterium to be resist- Sulfonamide susceptibility testing in veterinary
ant. This will not be the case if proper quality control laboratories is usually done with high-potency triple-
with a thymidine-sensitive strain of Enterococcus sulfonamide disks, designed to determine susceptibility
faecalis is used. In agar diffusion tests, Mueller-Hinton to the high concentrations in the urinary tract (≥ 100 μg/
agar containing lysed horse blood is the ideal medium ml); extrapolation of susceptibility to systemic infec-
because it contains thymidine phosphorylase that tions is thus not appropriate. The CLSI criteria describe
decreases the quantity of thymidine in the medium. susceptibility in bacteria for urinary tract infections as
The criteria of susceptibility for bacteria in systemic those having an MIC of ≥ 256 μg/ml.

Chapter 17. Sulfonamides, Diaminopyrimidines, and Their Combinations 281

Table 17.1. Activity of sulfonamides, trimethoprim, and trimethoprim-sulfamethoxazole (μg/ml) against selected bacteria.

Organism Sulfonamidea Trimethoprim Trimethoprim-Sulfamethoxazole
MIC90 MIC90 MIC90b
Gram-positive aerobes
Arcanobacterium pyogenes 32 8 0.13
Corynebacterium pseudotuberculosis ≤ 0.5
C. renale > 64 0.13
Erysipelothrix rhusiopathiae 8 0.06 0.06
Listeria monocytogenes 8 128 0.03
Nocardia asteroides 64 8
Rhodococcus equi 128 2 32
Staphylococcus aureus > 128 0.5 0.25
Streptococcus agalactiae 4 0.06
S. dysgalactiae 32 4 0.06
S. uberis 32 2 0.5
Beta-hemolytic streptococci > 256 2
> 128 64
Gram-positive anaerobes > 128 ≤ 0.06
Clostridium perfringens 2 8
16
Gram-negative aerobes 4 ≤ 0.06
Actinobacillus spp. 64 0.06
A. pleuropneumoniaec ≥ 128 ≥ 512
Bordetella bronchisepticac > 256 1 ≥ 512
Brucella abortus ≤ 0.5
B. canis 16 4
Campylobacter jejuni 2 > 64 ≤ 0.5
Escherichia colic ≥ 256 < 0.15
Histophilus somni ≥ 128 4
Klebsiella pneumoniaec ≥ 128 8 ≤ 0.5
Moraxella bovis ≥ 128 512 128
Pasteurella multocida > 64 4
Proteus spp. > 128 0.5
Pseudomonas aeruginosa > 256 1
Salmonella spp.c > 515 8
Taylorella equigenitalis 128
Yersinia enterocolitica > 128
> 128

aMainly sulfadimethoxine.
bSingle figures refer to trimethoprim concentration; trimthoprim-sulfonamide ratio is 1:19.
cMany of these isolates are now reported as resistant to the combination; this table is partly designed to illustrate the synergism that can occur
between sulfonamides and trimethoprim. Because of increasing resistance, susceptibility testing under properly controlled conditions is often required.

t Good susceptibility: Bacillus spp., Brucella spp., (including Enterobacter spp., E. coli, Klebsiella spp.,
E.  rhusiopathiae, L. monocytogenes, Nocardia spp., Proteus spp.), Actinobacillus spp., Haemophilus and
pyogenic Streptococcus spp., Chlamydia and Histophilus spp., Pasteurella spp., Pseudomonas
Chlamydophila spp., coccidia, Pneumocystis carinii, spp.  Anaerobes such as Bacteroides spp. and
and Cryptosporidium spp. Fusobacterium spp. are often susceptible in vitro if
the medium is depleted of thymidine; this is, how-
t Moderate susceptibility, but often variable because of ever, often not the case in vivo. Clostridium spp.
acquired resistance (Table  17.1) includes among (other than C. perfringens) and anaerobic cocci are
Gram-positive aerobes: staphylococci, some entero- often resistant.
cocci. Gram-negative aerobes: Enterobacteriaceae

282 Section II. Classes of Antimicrobial Agents

t Resistant: Mycobacterium spp., Mycoplasma spp., Sulfonamides are eliminated by a combination of
most obligate intracellular pathogens (such as C. burnetii renal excretion and biotransformation. This combina-
and Rickettsia spp.), P. aeruginosa, and spirochetes are tion contributes to species variations in the half-lives of
resistant. individual drugs. Sulfadimethoxine, for example, has
half-lives of 12.5 hours in cattle, 8.6 hours in goats, 11.3
Resistance hours in horses, 15.5 hours in swine, 13.2 hours in dogs,
and 10.2 hours in cats. These relatively long half-lives
Chromosomal mutation to resistance develops slowly have been attributed to extensive binding to plasma
and gradually and results from impairment of drug pen- albumin and pH-dependent passive reabsorption of the
etration, production of an insensitive dihydropteroate drug from acidic distal renal tubule fluid.
enzyme, or hyperproduction of PABA. Plasmid- and
integron-mediated resistance, often encoded by sul1, Sulfonamides undergo metabolic alterations to a vari-
sul2 or sul3 genes sometimes linked to other resistance able extent in the tissues, especially the liver. Acetylation
genes including trimethoprim (dfr) resistance genes or (which is the principal metabolic pathway for most
streptomycin (strA, strB), is far more common and in sulfonamides), glucuronide conjugation, and aromatic
enteric bacteria is the result of impaired drug penetration hydroxylation take place in humans and in all domestic
or the production of additional, sulfonamide-resistant, animals except dogs. It appears that dogs cannot acetylate
dihydropteroate synthetase enzymes (Maynard et al., aromatic amines. Acetylation takes place in the reticulo-
2003; Sheikh et al., 2012). Resistance to sulfonamides is endothelial rather than the parenchymal cells of the liver
extensively documented as widespread in bacteria and other tissues such as the lungs. This metabolic
isolated from animals, particularly farmed animals, reaction has clinical significance, since the acetyl deriva-
reflecting extensive use of the drug over many years. tive of most sulfonamides (except sulfapyrimidines) has
A restriction of the sul3 resistance gene to largely por- lower aqueous solubility than the parent compound.
cine E. coli has been noted (Kozak et al., 2009; Wu et al., Acetylation therefore increases the risk of damage to the
2010). There is complete cross-resistance between the renal tubules due to precipitation. Aromatic hydroxyla-
sulfonamides. tion, which may be the principal metabolic pathway for
sulfonamides in ruminants, and glucuronide conjuga-
Pharmacokinetic Properties tion are microsomal-mediated metabolic reactions. The
glucuronide conjugates are highly water-soluble and are
The sulfonamides constitute a series of weak organic rapidly excreted.
acids with pKa values ranging from 10.4 for sulfanila-
mides to 5.0 for sulfisoxazole. They exist predominantly Renal excretion mechanisms include glomerular
in the non-ionized form in biologic fluids of pH lower filtration of free (unbound) drug in the plasma, active
than their pKa. It is the non-ionized moiety that diffuses carrier-mediated proximal tubular excretion of ionized
through cell membranes and penetrates cellular barriers. unchanged drug and metabolites, and passive reabsorp-
tion of non-ionized drug from distal tubular fluid. The
Most sulfonamides are rapidly absorbed from the gas- extent of reabsorption is determined by the pKa of the
trointestinal tract and distribute widely to all tissues and sulfonamide and the pH of the fluid in the distal tubules.
body fluids, including synovial and cerebrospinal fluids. Urinary alkalinization increases both the fraction of the
The sulfonamides are bound to plasma proteins to an dose that is eliminated by renal excretion (unchanged in
extent varying from 15% to 90%. In addition, there is urine) and the solubility of sulfonamides in the urine.
variation among species in binding of individual sul-
fonamides. Extensive (> 80%) protein binding increases Drug Interactions
half-life. In any one species, the extent of protein bind-
ing, apparent volume of distribution, and half-life vary The important synergistic interaction of sulfonamides
widely among individual sulfonamides. This informa- with antibacterial diaminopyrimidines such as trimeth-
tion, together with designating 100 μg/ml as the desired oprim and baquiloprim is discussed below under
steady-state plasma sulfonamide concentration, facili- diaminopyrimidines.
tates calculation of dosages.
The agents appear not to antagonize the bacteri-
cidal effect of penicillins, but the procaine of procaine

Chapter 17. Sulfonamides, Diaminopyrimidines, and Their Combinations 283

penicillin is an analog of PABA that will antagonize used in the treatment of tear staining syndrome in dogs
sulfonamides. Combination with pyrimethamine is the (YounSok et al., 2008).
treatment of choice for toxoplasmosis and some other
protozoal infections. Renal tubular damage can be minimized by ensuring
that the patient is well hydrated throughout the course
Toxicity and Adverse Effects of treatment, by administering the most soluble sulfona-
mides, and by alkalinizing the urine. Prolonged dosage
The sulfonamides can produce a wide variety of usually with sulfa-ethoxypiridine in dogs has produced cata-
reversible side effects, some of which may have an allergic racts. Sulfaquinoxaline has caused hypothrombinemia,
basis and others are the result of direct toxicity. The more hemorrhage, and death in puppies given the drug orally
common adverse effects are urinary tract disturbances for control of coccidiosis; hemorrhagic diathesis was
(crystalluria, hematuria, or even obstruction), hematopoi- reported in other species because of the antagonistic
etic disorders (thrombocytopenia, anemia, leukopenia), effect of this drug on vitamin K.
and dermatologic reactions. Significant reactions, how-
ever, are generally uncommon in animals treated with Rare additional adverse effects reported include:
conventional doses of common sulfonamides (other than hepatic necrosis leading to death or euthanasia, devel-
sulfaquinoxaline) for less than 2 weeks. oping in some cases within days of treatment (Twedt
et  al., 1997) and hypothyroidism associated with pro-
In a small proportion (approximately 0.25%) of longed treatment (Torres et al., 1996). An unusual goi-
humans or animals, sulfonamide therapy can produce trogenic effect in swine, which increased the number of
idiosyncratic drug reactions, which are unpredictable stillborn or weak piglets born to sows fed sulfadimeth-
and rare events occurring 10 days to 3 weeks after first oxine and ormetoprim in late gestation, was described
exposure. The syndrome in dogs includes fever, by Blackwell et al. (1989). Goitrous hypothyroidism has
arthropathy, blood dyscracias, epistaxis, hepatopathy, also been described in a young dog treated with tri-
skin eruptions of various types, uveitis, and keratocon- methoprim-sulfamethoxazole (Seelig et al., 2008).
junctivitis sicca (Trepanier, 2004). These reactions are Congenital defects have been described in foals born to
sometimes described as hypersensitivity reactions mares treated for equine protozoal myeloencephalitis
(drug fever, urticaria) since they seem to involve during pregnancy (Toribio et al., 1998).
immune reactions such as a T-cell-mediated response
to proteins haptenated by sulfonamide metabolites Administration and Dosage
(Trepanier, 2004) but may involve a limited capacity to
detoxify metabolites of sulfonamides. Idiosyncratic In treating systemic diseases with sulfonamides, it is
reactions recur if individuals are retreated with sulfona- desirable to initiate therapy with a priming dose and to
mides. In dogs, serious but reversible sulfadiazine- administer maintenance doses, each one-half the prim-
induced reactions have been described in a number of ing dose, at intervals approximately equal to the half-life
reports on Doberman Pinschers, in which sulfonamides of the drug (Table 17.2). When the drug is administered
should probably be avoided. orally, the dose level must compensate for incomplete
systemic availability from the oral preparation, that is, %
Some adverse effects are associated with particular bioavailability of oral preparations.
sulfonamides. Sulfadiazine and sulfasalazine given for
long periods to dogs as a “geriatric stimulant” have Although a large number of sulfonamide preparations
caused keratoconjunctivitis sicca (KCS), which was not are available for use in veterinary medicine, many of
always fully reversible when the drug was discontinued. these are different dosage forms of sulfamethazine. This
However, in one study KCS determined by decreased sulfonamide is most widely used in food-producing ani-
tear production occurred in 15% of 33 dogs treated with mals and can attain effective plasma concentrations
trimethoprim-sulfadiazine combination, within the first when administered either orally or parenterally. Because
week of treatment (Berger et al., 1995). This effect of their alkalinity, most parenteral preparations should
occurred in dogs weighing less than 12 kg, suggesting be administered only by IV injection. Rapid IV injection
that dosage must be particularly carefully calculated for of high doses of sulfonamide preparations should be
small dogs. Trimethoprim-sulfamethoxazole has been avoided. Sulfamethazine therapy should be initiated
with  an IV priming dose of 100 mg/kg, and effective

284 Section II. Classes of Antimicrobial Agents

Table 17.2. Examples of usual dosages of sulfonamides in animals.

Drug Route Dose (mg/kg) Dosing interval (h) Comment

Short-acting sulfadiazine, sulfamethazine, IV, PO 50–60 12 Double first dose
trisulfapyramidine (triple sulfas)
PO 50 12 Double first dose
Sulfamethoxazole PO, IV, IM, SC 27.5 24 Double first dose
Intermediate-acting sulfadimethoxine PO 137.5 96
PO, IV 50 12 Double first dose
(sustained release, cattle) PO 50 Urinary tract infections
sulfadiazine PO 100 8
Sulfisoxazole PO 25 12 See text
Gut-active phthalylsulfathiazole Topical 12
Special-use salicylazosulfapyridine
silver sulfadiazine

Table 17.3. Usual dosages of potentiated sulfonamide combinations in animals.

Drug (Species) Route Dose (mg/kg) Dosing Interval (h) Comment

Trimethoprim-sulfonamide PO, IV, IM (15–)30 12(–24) Not IM in horses
Ormetoprim-sulfadimethoxine PO 27.5 24 Double first dose
Baquiloprim-sulfadimethoxine
PO 30 48
Dogs PO 30 24
Cats IM 10 24
Cattle, swine

concentrations can then be maintained by administering should be based on quantitative susceptibility of the
maintenance doses of 50 mg/kg PO at 12-hour intervals. pathogenic microorganisms and the site of infection.
At least one prolonged-release oral preparation of sul-
famethazine is available for use in calves and could be Sulfisoxazole has higher aqueous solubility than most
administered to sheep and goats. This is a convenient other members of the class. Its solubility in urine increases
form of maintenance therapy in that a single dose pro- markedly with increase in urinary pH. It has a half-life in
vides an effective level for 36–48 hours. Different oral dogs of 4.5 hours, and because it is eliminated largely by
forms have different systemic availability (Table 17.3). renal excretion, sulfisoxazole is present in high concen-
trations unchanged in the urine. This makes sulfisoxazole
Sulfadimethoxine preparations are more widely used an effective agent in the treatment of urinary tract infec-
in companion animals. The parenteral preparation tions caused by susceptible organisms. The usual oral
(40%), containing sulfadimethoxine sodium in solution, dosage is 50 mg/kg administered at 8-hour intervals.
is suitable for IV administration to horses. Having initi-
ated therapy with a priming dose of 50 mg/kg, effective Unlike the sodium salts of other sulfonamides, sodium
concentrations can be maintained with maintenance sulfacetamide is nearly neutral. It is the only sulfonamide
dosage of 25 mg/kg administered IV at 12-hour intervals. available for topical ophthalmic use. When a 30% solu-
In dogs and cats, sulfadimethoxine can be administered tion is applied to the conjunctivae, it penetrates well and
either as the parenteral solution IV or as the oral suspen- attains high concentrations in ocular fluids and tissues.
sion. Therapy should be initiated with a priming dose
(55 mg/kg, IV), and therapeutic concentrations can be Clinical Applications
maintained either by administering maintenance doses
IV (27.5 mg/kg) or PO (55 mg/kg) at preferably 12-hour Widespread resistance greatly limits the effectiveness of
or 24-hour intervals. Selection of the dosing interval sulfonamides in treating bacterial diseases of animals, so
that indications for primary use are few. Trimethoprim-
or  other antibacterial diaminopyrimidine-sulfonamide

Chapter 17. Sulfonamides, Diaminopyrimidines, and Their Combinations 285

combinations have largely replaced sulfonamides as thera- (20 mg/kg PO SID or BID, for up to 12 weeks or longer)
peutic agents used in companion animals, although resist- combined with pyrimethamine (1.0 mg/kg PO SID, for
ance also increasingly limits their use. Purulent material up to 120 days or longer; Dubey et al., 2001). Dapsone
must always be removed, since free purines neutralizes the alone (3 mg/kg PO SID) has been used successfully in
effect of sulfonamides. Primary uses include treatment the treatment of Pneumocystis carinii pneumonia in a
of toxoplasmosis (when combined with pyrimethamine), foal (Clark-Price et al., 2004).
of chlamydiosis, of Pneumocystis carinii, and possibly of
nocardiosis (combined with minocycline), and the use of Dogs and Cats
sulfasalazine in the treatment of chronic colitis. Use of sulfisoxazole to treat urinary tract infections in
dogs has been largely replaced by antibiotics that are more
Cattle, Sheep, and Goats effective because of their broader spectrum of activity or
Widespread resistance limits the use of sulfonamides in bactericidal action. Sulfonamides are one of the drugs of
these animals, and it is best to give these agents in com- choice in the treatment of Nocardia infections; effective-
bination with trimethoprim. Orally administered, long- ness may be increased by concurrent administration with
acting, sustained-release dosage forms result in effective minocycline. Silver sulfadiazine cream has been used as a
plasma concentrations for 3–5 days. Such a preparation treatment in chronic otitis externa caused by multire-
has been effective in clinical trials assessing prevention sistant P. aeruginosa, as the drug acts as a broad-spectrum
and treatment of feedlot pneumonia, an unexpected antimicrobial antiseptic. This preparation has been effec-
result in view of the resistance reported in bovine tive in controlling bacteria that infect burn wounds in
Mannheimia and Pasteurella. Sulfonamides are used suc- human patients; activity is almost certainly the result only
cessfully to treat bovine interdigital necrobacillosis and of the silver component.
coccidiosis. Sulfadimethoxine is the only sulfonamide
approved for use in dairy cows over 20 months of age in Sulfasalazine (salicylsulfapyridine) has been recom-
the United States; extra-label use in dairy cows is prohib- mended as a drug of choice in the treatment of chronic
ited. Sustained-release oral sulfamethazine and orally colitis in dogs. It is hydrolyzed by colonic bacteria to
administered pyrimethamine, 0.5 mg/kg once daily, yield sulfapyridine and 5-aminosalicylate; it is likely that
might be drugs of choice in preventing outbreaks of the anti-inflammatory effect of the latter is responsible
Toxoplasma abortion in sheep. Sulfonamides have been for the therapeutic effect. Comparably high concentra-
used with chlortetracyclines in feedlot lambs to improve tions of salicylate cannot be achieved in the colon by
performance and prevent clostridial enterotoxemias. oral administration. The dosage of sulfasalazine for the
dog is 25 mg/kg PO 3 times daily. The same dose in cats
Swine may induce salicylate poisoning. Some have suggested
Sulfonamides have been used to promote growth and to that a low dose of corticosteroid be administered simul-
control group E streptococcal infections and atrophic taneously to reduce the overall duration of therapy,
rhinitis in swine. The sulfonamides are often combined which is 3–4 weeks when the drug is administered alone.
with chlortetracycline. In the United States, there have This dual dosage may decrease the frequency of kerato-
been moves to ban the use of sulfonamides for use in conjunctivitis sicca. In most cases of sulfasalazine treat-
swine because of persistent problems of residues in ment, cure is achieved within 4 weeks, and treatment
carcasses in excess of legally permitted concentrations should not be continued beyond this time without histo-
and evidence from chronic toxicity studies in mice that logic confirmation of colonic inflammation.
sulfamethazine was linked to the production of thyroid
adenomas in rodents. Dapsone (diaminodiphenylsulphone) has been used
in the treatment of dermatitis herpetiformis in dogs and
in the treatment of leprosy in humans.

Horses Poultry
Sulfonamides are used in horses in combination with Sulfonamides have been used in the prevention and
antibacterial diaminopyrimidines. For the treatment treatment of coccidiosis, infectious coryza, pullorum
of equine protozoal myeloencephalitis, sulfadiazine disease, and fowl typhoid.

286 Section II. Classes of Antimicrobial Agents

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Trepanier LA. 2004. Idosyncratic toxicity associated with poten- Figure 17.2. Structural formulas of some diamino-
tiated sulfonamides in the dog. J Vet Pharm Therap 27:129. pyrimidines.

Toribio RE, et al. 1998. Congenital defects in newborn foals for bacterial dihydrofloate reductases (aditoprim,
of mares treated for equine protozoal myeloencephalitis baquiloprim, ormetoprim, trimethoprim), others for
during pregnancy. J Am Vet Med Assoc 212:697. protozoal enzymes (pyrimethamine), and others for
mammalian enzymes (methyltrexate). The earliest anti-
Torres SMF, et al. 1996. Hypothyroidism in a dog associated bacterial diaminopyrimidine introduced for clinical use
with trimethoprim-sulphadiazine therapy. Vet Dermatol 7:105 was trimethoprim (Figure 17.2), a synthetic drug that is
widely used in combination with sulfonamides. It is a
Twedt DC, et al. 1997. Association of hepatic necrosis with weak base with a pKa of about 7.6 and is poorly soluble
trimethoprim-sulfonamide administration in 4 dogs. J Vet in water. Other antibacterial diaminopyrimidines have
Intern Med 11:20.

Weiss DJ, Klansner JS. 1990. Drug-associated aplastic anemia in
dogs: eight cases (1984–1988). J Am Vet Med Assoc 196:472.

Wilson RC, et al. 1989. Bioavailability and pharmacokinetics
of sulfamethazine in the pony. J Vet Pharm Ther 12:99.

Wu S, et al. 2010. Prevalence and characterization of plasmids
carrying sulfonamide resistance genes among Escherichia
coli from pigs, pig carcasses and human. Acta Vet Scan 52:47.

YounSok C, et al. 2008. Trimethoprrm-sulfamethoxazole for the
treatment of tear staining syndrome in dogs. J Vet Clin 25:115.

Antibacterial Diaminopyrimidines:
Aditoprim, Baquiloprim, Ormetoprim,
and Trimethoprim

Diaminopyrimidines interfere with folic acid pro-
duction by inhibition of dihydrofolate reductase.
Some diaminopyrimidines have marked specificity

Chapter 17. Sulfonamides, Diaminopyrimidines, and Their Combinations 287

similar antibacterial activities to trimethoprim but offer Chlamydophila spp., Mycobacterium spp., and P. aerugi-
significant pharmacokinetic advantages, particularly nosa is negligible. Activity of aditoprim, baquiloprom,
those of greater half-lives and tissue distribution. and ormetoprim is similar to or very slightly less than
that of trimethoprim.
Mechanism of Action
Resistance
Diaminopyrimidines interfere with the synthesis of tet-
rahydrofolic acid from dihydrofolate by combining with High-level resistance to trimethoprim and other diami-
the enzyme dihydrofolate reductase. Selective antibacterial nopyrimidines is usually the result of transposon- or
activity occurs because of greater affinity for the bacterial integron-encoded plasmid or chromosomal synthesis of
rather than the mammalian enzyme. Diaminopyrimidines a resistant dihydrofolate reductase enzyme (Skold, 2001).
thus inhibit the same metabolic sequence as the sulfona- Changes that affect bacterial permeability or efflux
mides, preventing bacterial synthesis of purines and thus pumps can result in moderate resistance. Resistance
of DNA. A synergistic and bactericidal effect occurs when is increasingly reported, particularly among Entero-
the diaminopyrimidines are combined with sulfonamides bacteriaceae. Resistance to trimethoprim is extensively
(see sulfonamide-diaminopyrimidine combinations), and documented as widespread in bacteria isolated from
for this reason these drugs are invariably used with a sul- animals, particularly in enteric bacteria isolated from
fonamide in veterinary medicine. farmed animals of all types exposed to trimethoprim. At
least 30 phylogenetically different dfr resistance genes
Interestingly, in the United Kingdom trimethoprim expressing dihydrofolate reductases have been character-
alone rather than the combination is now generally used ized. Isolates with plasmid- or integron-mediated resist-
in human medicine (Hughes, 1997). The reasons for the ance commonly show multiple resistance, which includes
abandoned use of trimethoprim-sulfonamide combina- sulfonamide resistance. Examples include multidrug-
tion in favor of trimethoprim alone are (1) bacteriostatic resistant Salmonella such as S. typhimurium DT104 and
synergy is only demonstrable when the concentration of S. Newport. The apparent spread of a trimethoprim
each drug is less than bacteriostatic, but the bacteriostatic resistance gene from porcine to human E. coli has been
effect of trimethoprim in urinary tract infections, for described (Jansson et al., 1992).
which the drug is most commonly used, is often detecta-
ble in urine for several days; (2) diaminopyrimidines are Pharmacokinetic Properties
more widely distributed into tissues than sulfonamides,
reaching sites, such as cells, which sulfonamides do not Diaminopyrimidines including trimethoprim are lipid-
penetrate well; (3) most of the adverse effects of the com- soluble organic bases that are approximately 60% bound
bination are the result of the sulfonamide component; to plasma proteins. They are rapidly absorbed from the
and (4) the original claim that the combination prevented intestine after oral administration. The drugs distribute
the emergence of resistance is dubious because sulfona- widely, penetrating cellular barriers by non-ionic diffu-
mide resistance is widespread and because plasmids con- sion and attaining effective concentrations in most body
ferring resistance to sulfonamides often also confer tissues and fluids. The drug may concentrate in fluids,
resistance to trimethoprim (Hughes, 1997). The licensed such as the prostate, that are acidic relative to plasma.
medical use in the United Kingdom of the combination is The average milk-to-plasma equilibrium concentration
therefore restricted largely to the treatment of ratio is 3:1. The dose, systemic availability from the dos-
Pneumocystis jirovecii infection. age form, and route of administration determine the
plasma concentration profile and tissue levels of the
Antimicrobial Activity drug. Hepatic metabolism (oxidation followed by conju-
gation reactions) is the principal process for elimination.
Antibacterial diaminopyrimidines are generally bacte- Because of this, the half-life and fraction of the dose that
riostatic, broad-spectrum drugs active against Gram- is excreted unchanged in the urine vary widely among
positive and Gram-negative aerobic bacteria, but not species. In ruminants, the short half-life of trimethoprim
usually against anaerobes (Table  17.1). Bacteria with is due to rapid demethylation to produce inactive com-
an MIC ≤ 1 μg/ml are usually regarded as susceptible. pounds. Replacing the phenyl ring of trimethoprim with
Activity against Mycoplasma spp., Chlamydia and

288 Section II. Classes of Antimicrobial Agents

the bicyclic ring of baquiloprim resulted in an increase in Jansson C, et al. 1992. Spread of a newly found trimethoprim
half-life from 1 hour (trimethoprim) to 10 hours resistance gene, dhfrIX, among porcine isolates and human
(baquiloprim) in cattle and from about 2 to 5 hours in pathogens. Antimicrob Agents Chemother 36:2704.
pigs, while replacement of a methyl group in trimetho-
prim by the dimethylamino group of aditoprim increased Skold O. 2001. Resistance to trimethoprim and sulfonamides.
its half-life in cattle to 4–7 hours, in horses to 9–14 hours, Vet Res 32:261.
and in pigs to 8–9 hours, or greater. Greater tissue distri-
bution may be one factor responsible for prolonged half- Van Miert ASJPAM. 1994. The sulfonamide-diaminopyrimidine
life compared to trimethoprim. story. J Vet Pharm Ther 17:309.

Toxicity and Adverse Effects Antibacterial Diaminopyrimidine-
Sulfonamide Combinations
The antibacterial diaminopyrimidines are relatively non-
toxic drugs. Their main, though clinically unimportant, Antibacterial diaminopyrimidines are combined with a
potential toxic effect is to induce folic acid deficiency at variety of sulfonamides (sulfadiazine, sulfamethoxazole,
high doses, so care should be used in pregnant animals. and sulfadoxine) in a fixed (1:5) ratio, which in people
Rarely, aseptic meningitis related to trimethoprim ther- produces a 1:20 ratio of drug concentrations in the
apy has been reported in humans. Hyperkalemia may plasma after oral or parenteral administration. This
occur under unusual circumstances. ratio is desirable since maximum synergy occurs when
the drugs are present in the ratio of their MICs; diami-
Clinical Applications nopyridines are 20–100 times more active than the sul-
fonamides, so that combinations are formulated to give
Antibacterial diaminopyrimidines are currently used a 1:20 ratio in human serum. This ratio occurs because
only in combination with sulfonamides in animals, diaminopyrimidines (lipid-soluble organic bases) are
although there may be a need to reassess the benefits of concentrated in tissues whereas sulfonamides (weak
the combination. Alone or in combination they may be organic acids) remain largely in extracellular fluids. At
a drug of choice for treating prostatic infections caused these MICs and in this ratio, the combination produces
by Gram-negative bacteria, since prostatic concentra- a bactericidal effect against a wide range of bacteria,
tions may reach 10 times those of plasma, at which con- with some important exceptions, and also inhibits cer-
centration the drug may be bactericidal. Nevertheless, tain other microorganisms. Since the combinations of
clinical results in treating chronic prostatitis with tri- different diaminopyridines, with sulfonamides, give
methoprim may be disappointing, probably because of essentially similar antibacterial effects, comments will
the nature of the disease process. Trimethoprim admin- relate largely to trimethoprim-sulfonamide combina-
istered orally has been used to prevent relapse after tions but can be extrapolated to other combinations.
treatment of L. monocytogenes meningitis in humans.
Antibacterial diaminopyrimidines, including trimetho- Veterinary preparations follow medical usage in that
prim, combined with sulfonamides or dapsone may be they contain diaminopyridines combined with a sulfon-
the prophylactic drugs of choice for Pneumocystis carinii amide in the 1:5 ratio. For trimethoprim, the half-lives
(jirovecii) pneumonia (Hughes, 1988). of the components (sulfadiazine, sulfadoxine, or sulfa-
methoxazole) do not coincide in any species (except
Bibliography humans) whereas they are more similar for baquiloprim
(sulfadimidine, sulfadimethoxine) and ormetoprim
Brown MP, et al. 1989. Pharmacokinetics and body fluid and (sulfadimethoxine). The dosage aims at maintaining
endometrial concentrations of ormetoprim-sulfadimeth- bacteriostatic concentrations of the sulfonamide, which,
oxine in mares. Can J Vet Res 53:12. for a time after each dose, is enhanced by the synergistic
bactericidal action of the combination.
Davies AM, MacKenzie NM. 1994. Pharmacokinetics of
baquiloprim and sulphadimidine in pigs after intramuscu- Mechanism of Action
lar injection. Res Vet Sci 57:69.
The combination of a diaminopyrimidine with a
Hughes WT. 1988. Comparison of dosages, intervals, and sulfonamide inhibits sequential steps in the synthesis of
drugs in the prevention of Pneumocystis carinii pneumo-
nia. Antimicrob Agents Chemother 32:623.

Chapter 17. Sulfonamides, Diaminopyrimidines, and Their Combinations 289

folic acid and thus of the purines required for DNA in tissues and urine from that in plasma. Such variation is
synthesis. The interference by the diaminopyrimidine said not to be important, as the synergistic interaction
methoprim with recycling of tetrahydrofolic or dihydro- may occur over a wide range of concentration ratios of
folic acid is probably responsible for the synergistic the drugs but clearly it would not occur in some tissues,
interaction of the combination. since diaminopyrimidines are distributed more widely
than sulfonamides. Because of these variations in the
Antimicrobial Activity pharmacokinetics of diaminopyrimidines and sulfona-
mides, the length of effective action is difficult to assess
Diaminopyrimidine-sulfonamide combinations have a based on serum concentrations alone. This has given rise
generally broad and usually bactericidal action against to the suspicion that the manufacturer’s recommended
many Gram-positive and Gram-negative aerobic bacte- dosages are less than optimal, especially for trimethoprim
ria, and protozoa such as Toxoplasma. They are not combinations. A number of recent pharmacokinetic
active against anaerobic bacteria in vivo because thymi- studies have resulted in recommendations to increase
dine and PABA in the necrotic tissue antagonizes their dosage (Ensink et al., 2003, 2005).
antibacterial effect. Such an antagonistic effect is not
limited to anaerobes so that this combination may not Errors in laboratory testing are common because of
be fully effective in closed, non-draining, infections the presence of PABA or thymidine in media (Feary
where there is significant tissue debris. Pneumocystis et  al., 2005); in one study, half the strains reported as
carinii (jirovecii)and some malarial parasites are suscep- resistant in other laboratories were susceptible when
tible; Mycoplasma spp. and P. aeruginosa are resistant. tested in a reference laboratory. The use of lysed horse
blood, which contains thymidine phosphorylase, will
Synergism occurs when the microorganisms are sus- eliminate excess thymidine in the medium.
ceptible to both drugs in the combination. It may still
be  obtained, in up to 40% of cases, when bacteria are t Good susceptibility (MIC ≥ 0.5/9.5 mg/ml) is shown
resistant to sulfonamides. Synergy often occurs if the among the following Gram-positive aerobes: S. aureus,
organism is resistant to trimethoprim but sensitive to streptococci, Arcanobacterium spp., Corynebacterium
sulfonamides and in nearly 40% of cases in which the spp., E. rhusiopathiae, L. monocytogenes. Gram-
organism is resistant to each drug alone. Nevertheless, negative aerobes: Acinetobacter spp., Actinobacillus
many organisms described as susceptible to the combi- spp., Bordetella spp., Burkholderia cepacia, Brucella
nation are susceptible to the diaminopyrimidine com- spp., Dermatophilus congolense, Enterobacteriaceae
ponent only. Clinical response may sometimes be lower (E. coli, Klebsiella spp., Proteus spp., Salmonella spp.,
than expected from in vitro data, and better understand- Yersinia spp.), Haemophilus spp., Pasteurella spp.,
ing of the use of MIC data in prediction of clinical out- Stenotrophomonas maltophila. Anaerobes: Actinomyces
come is required. One element of such disappointing spp., Bacteroides spp., Fusobacterium spp., some
responses may also be the presence of thymidine and Clostridium spp., and Chlamydia spp.
PABA in infected tissue. Nevertheless, a more important
element may be widespread resistance to sulfonamides t Moderate activity (MIC ≥ 2/38 mg/ml) includes some
and consequently the lack of synergism in many cases, Mycobacterium spp. and some Nocardia spp.
so that only the diaminopyrimidine component is active.
For trimethoprim, the short half-life in some species t Resistance (MIC ≥ 4/76 mg/ml) is shown by Rickettsia,
may exacerbate a lack of synergism. Leptospira spp., P. aeruginosa, and Mycoplasma spp.
(Table 17.2).
Where synergistic interactions occur, a 10-fold increase
in activity of the trimethoprim component and a 100-fold Resistance
increase in activity of the sulfonamide component are
common. Synergism occurs at different drug concentra- Mechanisms of resistance were discussed under the
tion ratios with different bacterial species. Because of dif- individual components of the combination. Resistance
ferences between the diaminopyrimidine and sulfonamide to the combination has developed progressively with
in distribution and in the case of trimethoprim of elimi- use. Multiple integron-associated resistance, which
nation, the concentration ratios may differ considerably includes both sulfonamide and trimethoprim resistance,

290 Section II. Classes of Antimicrobial Agents

has been described in some Salmonella serovars and in preparation in horses, in some cases in anesthetized
pathogenic E. coli isolated from animals. horses. A 7% incidence of diarrhea was observed in a
study of the effect of twice-daily administration of oral
Pharmacokinetic Properties 30 mg/kg trimethoprim-sulfadiazine in horses. The
prevalence of diarrhea noted following trimethoprim-
In humans the half-lives of trimethoprim and sul- sulfonamide use in horses in another study was not
famethoxazole are similar, and maintenance dosage pro- significantly different from that observed in horses
vides continuous, therapeutic concentrations of both receiving other antibiotics, including penicillin (Wilson
drugs in plasma. In animals the half-lives of the drugs et al., 1996). Neurologic abnormalities in horses charac-
are not similar, but the combination is often clinically terized by reversible hypermetric gait, agitation and by
effective because of the relatively broad range of drug erratic behavior have been described as an unusual
ratio over which synergism occurs. The diaminopyrimi- adverse reaction (Stack et al., 2011).
dine component is concentrated in tissues whereas the
sulfonamide component moves only slowly from plasma Administration and Dosage
into tissues. The longer half-lives of newer diaminopyri-
midines (baquiloprim, ormetoprim) give the advantage Usual dosages are shown in Table 17.3. Dogs and cats can
of better maintenance of the 1:20 ratio said to be desir- be given the oral form (tablets) at the same dosage. Twice-
able, and of less frequent dosing. daily oral dosing of horses with 30 mg/kg trimethoprim-
sulfadiazine combination paste, rather than once daily
Following SC injection in cattle, trimethoprim seems is  recommended. Oral dosage with ormetoprim-sul-
to deposit in a slow-release form, so that serum concen- fadimethoxine paste in mares recommended for suscep-
trations remain below MIC. Because of this, the SC tible organisms was a loading dose of 9.2 mg ormetoprim
route cannot be recommended in cattle and perhaps in and 45.8 mg sulfadimethoxine/kg followed by half this
other species. dose every 24 hours (Brown et al., 1989).

Drug Interactions Clinical Applications

Trimethoprim-sulfonamide has sometimes been used in Diaminopyrimidine-sulfonamide combinations have
conjunction with ampicillin to provide “broad-spectrum the advantage of good distribution into tissues, safety, a
bactericidal antimicrobial coverage” before microbiol- relatively broad-spectrum bactericidal activity, and oral
ogy data are available. However, one study showed that administration. A disadvantage is antagonism of action
addition of ampicillin to trimethoprim-sulfonamide by infected tissue debris.
dosing regimens only marginally increased the spectrum
of activity. There is no known mechanism to suggest The combination can be recommended in the treat-
that such a combination might be synergistic. Rather, ment of urinary tract infections caused by common
such a combination may be effective in treating polymi- opportunist pathogens. The combination has a particu-
crobial infections involving aerobic bacteria susceptible lar place in the treatment of bacterial prostatitis because
to the trimethoprim-sulfonamide combination and of good tissue penetration. Other indications include
anaerobic bacteria susceptible to ampicillin. the treatment of enteric infections (E. coli, Salmonella,
Y. enterocolitica). The drug is of value in the treatment of
Toxicity and Adverse Effects brucellosis, often in combination with rifampin or doxy-
cycline. The combination is a drug of choice in the treat-
The combination has a wide margin of safety, and ment of Nocardia infections, but high oral dosage (3 mg
adverse effects can mainly be attributed to the sulfona- trimethoprim equivalent/kg every 6 hours) must be
mide. These effects are discussed in the general descrip- used for prolonged periods.
tion of the adverse effects of each drug class.
Other indications include the treatment of Pneumocystis
In horses, minor tissue damage and pain may occur carinii (jirovecii), Chlamydia and Chlamydophila infec-
after IM injection; transient pruritus has been reported tions, of listeriosis, of certain fast-growing mycobacterial
to follow the first but not subsequent doses. In isolated infections (M. kansasii, M. marinum), and of Coxiella
incidents a fatal adverse reaction (possibly respiratory infections. In human medicine, the combination is used
failure) followed IV injection of the combination

Chapter 17. Sulfonamides, Diaminopyrimidines, and Their Combinations 291

for the treatment of otherwise-resistant infections caused for acute mastitis. A beneficial effect of trimethoprim-
by Acinetobacter, Burkholderia and Stenotrophomonas sulfonamide on the treatment of coliform mastitis has
species, as well as of methicillin-resistant S. aureus been noted, particularly when combined with non-
in  humans (MRSA; Goldberg and Bishara, 2012). steroidal anti-inflammatory drugs (Shpigel et al., 1998).
Livestock-associated MRSA have, however, been associ-
ated with multiple drug resistance, including a novel tri- Other uses in cattle include the treatment of urinary
methoprim resistance gene (dfrK; Kadlec et al., 2012). tract infections and mixed aerobe-anaerobe infections
The drug is also used in the treatment of acute upper such as those occurring in post-parturient metritis. The
and lower respiratory tract infections caused by suscep- drug has potential but unproven use for the treatment of
tible organisms, as well as in infections in other sites. L. monocytogenes encephalitis in ruminants.

Cattle, Sheep, and Goats A special application in goats and sheep is in prevent-
The drug combination is widely used in dairy and beef ing Toxoplasma abortion; the drug is also potentially
cattle and has been used successfully in the treatment of useful in preventing chlamydial abortion in sheep. In
salmonellosis in calves, as well as in undifferentiated experimental Toxoplasma infections in mice, protection
diarrhea, in bacterial pneumonia, in foot rot, and in by trimethoprim-sulfonamide was inferior to pyrimeth-
septicemic colibacillosis. Baquiloprim-sulfadimidine amine-sulfadiazine, but clinical results in naturally
was not as efficacious as danofloxacin in the treatment occurring infections in humans have been excellent.
of experimentally induced E. coli diarrhea in calves
(White et al., 1998), presumably because the organism Swine
is less susceptible to the combination drug. The poten- Trimethoprim-sulfonamide combinations have been
tial for use in coliform septicemia and meningitis seems used successfully in controlling a wide variety of condi-
excellent but is increasingly limited by resistance. In tions in pigs, including neonatal and post-weaning
meningitis the drug should be administered IV 3 or 4 colibacillosis, salmonellosis, atrophic rhinitis, greasy
times daily at the usual dosage. The potential for use in pig disease, streptococcal meningitis, and pneumonia.
the treatment of Listeria meningoencephalitis appears Atrophic rhinitis may be controlled by incorporating
excellent. The susceptibility of Histophilus somni, the drug in feed or water, or by injecting piglets at vari-
Pasteurella multocida, some Mannheimia haemolytica, ous times such as the third day of life and again in the
and of Arcanobacterium pyogenes suggests a useful third and sixth weeks. The mastitis-metritis-agalactia
application in bovine respiratory disease that has been syndrome has been controlled by the prophylactic
borne out by field studies. The drug combination administration of 15 mg/kg PO for 3 days before and
should be administered parenterally (not orally). 2 days after parturition. The combination has been
Clinical trials with undifferentiated bovine respiratory used in the eradication of A. pleuropneumoniae infec-
disease have failed to demonstrate improvement when tion from herds by treating adults through the water
dosage of trimethoprim-sulfadoxine was increased for 3 weeks in combination with removal of serologi-
beyond that recommended or when the product was cally positive animals. Isolates of MRSA from clinical
administered IV compared to IM, although pharma- infections in Dutch swine were all found to be suscepti-
cokinetic studies suggest that manufacturer’s once-daily ble to the combination (Wolf et al., 2012), in marked
recommended dosage of 17 mg/kg is too low. A pre- contrast to nasal isolates from swine in Belgium
ferred minimum dosage is 30 mg/kg SID or 15 mg/kg (Crombé et al., 2012); the ST398 strain found in swine
BID. Experimental studies have confirmed the antago- appears to be able readily to acquire multiple resistance
nistic effect of infected tissue debris on the action of the genes (argudin et al., 011). Other diaminopyrimidine-
combination (Greko et al., 2002). sulfonamide combinations are available for swine for
similar purposes to trimethoprim-sulfonamide combi-
When used to treat acute mastitis, the drug should be nations (Table  17.3). Susceptibility testing is required
given IV at high dose because of poor bioavailability before instituting treatment in view of the variable
after IM injection and relatively poor udder penetration; reports of resistance of common swine pathogens to the
a dosage of 48–50 mg/kg every 12 hours is appropriate combination, including bacteria such as H. parasuis
that used to be highly susceptible.

292 Section II. Classes of Antimicrobial Agents

Horses are usually resistant to trimethoprim, as part of their
The combination of trimethoprim-sulfadiazine is popu- common multidrug resistance (Perreten et al., 2010).
lar in horses because it can be administered as an oral
antibiotic to horses with few adverse effects. It is painful Consideration should be given to twice-daily dosing
when administered IM. It is, therefore, used orally to with trimethoprim-sulfadiazine. A blinded comparison
treat acute respiratory infections including strangles, of once versus twice-daily dosing with 30 mg/kg tri-
acute urinary tract infections, and wounds and abscesses methoprim-sulfadiazine in the treatment of canine pyo-
and is a drug of choice in salmonellosis. In recent derma showed an advantage of twice-daily dosing,
years, however, resistance has apparently increased in although this was not statistically significant possibly
Streptococcus equi subsp. zooepidemicus, so that in some because of small numbers of animals in the trial
studies less than 90% of isolates are susceptible in vitro (Messinger and Beale, 1993). In one study, however,
(Peyrou et al., 2003), although Feary et al. (2005) have mean serum and skin concentrations using once-daily
shown that reports of resistance may represent labora- dosing were considered to achieve therapeutically effec-
tory error. The combination is ineffective in eradicating tive concentrations (Pohlenz-Zertuche et al., 1992).
S. equi subspecies zooepidemicus in a tissue chamber
model of infection despite in vitro susceptibility of the The combination drug is effective against Bordetella
isolate and high concentrations of the drugs in the tissue bronchiseptica, although relapses after treatment with
chamber fluid (Ensink et al., 2003). For these reasons, trimethoprim-sulfadiazine for 5 days were common in
and because it can be partially antagonized by tissue experimental kennel cough. The drug should probably
debris, it is a less desirable choice than procaine penicil- be administered for several weeks in the treatment of
lin G for treatment of streptococcal infections. In foals this infection. In one study, a significant number of
the combination is used in the treatment of Actinobacillus isolates were found to be resistant to the combination
and coliform infections, although the latter use may be drug (Speakman et al., 2000), so that doxycycline or
compromised by resistance. The drug may be used for amoxicillin-clavulanic acid may now be a better choice
coliform meningitis, in which high doses should be for treatment of kennel cough. The drug has been used
administered slowly IV 3 or 4 times daily. The drug may successfully in the treatment of canine actinomycosis,
otherwise be administered orally but oral dosage recom- often in conjunction with procaine penicillin; the com-
mended by the manufacturers may be low and there is bination may be particularly useful where Nocardia spp.
apparent advantage to twice-daily dosage (30 mg/kg) of and A. viscosus have not been distinguished properly.
oral preparations (Van Duijkeren et al., 1994). The com- The combination has been effective in treating coccidi-
bination of sulfadiazine with pyrimethamine is a drug of osis in dogs and cats.
choice in the treatment of protozoal encephalomyelitis
(see antiprotozoal diaminopyrimidines). It is a drug of The excellent penetration into the prostate makes the
choice for P. jiroveci infections in foals. Direct infusion combination a treatment of choice in Gram-negative
of the combination into the uterus may cause endome- prostatic infections in dogs, equal to or better than mino-
trial inflammation. cycline, although now challenged by the fluoroquin-
olones. Similarly, the excellent penetration (50% of
Dogs and Cats serum concentrations) of the aqueous and vitreous
Trimethoprim-sulfonamide or ormetoprim-sulfadimeth- humors of the eyes by both drugs makes the combination
oxine combinations have wide application in dogs and suitable in the parenteral treatment of panophthalmitis
cats against specific and non-specific infections. The caused by Gram-negative bacteria. The combination is
combination is highly effective against many oppor- used together with clindamycin and pyrimethamine in
tunist bacteria present in canine urinary tract, skin and the initial treatment of Hepatozoon infections in dogs.
ear infections (S. pseudintermedius, streptococci, and The combination is also used with clindamycin in the
Enterobacteriaceae including E. coli and Proteus). The treatment of Neospora caninum infection.
drug has the potential for use in prophylaxis of urinary
tract infections. Methicillin-resistant S. pseudintermedius Poultry
Trimethoprim-sulfaquinoxaline and sulfamethoxazole-
ormetoprim are used in the prophylaxis and treatment
of E. coli, Haemophilus, and Pasteurella infections, as

Chapter 17. Sulfonamides, Diaminopyrimidines, and Their Combinations 293

well as of coccidiosis, and of Reimerella anatipestifer in Reichel MP, et al. 2007. Neosporosis and hammondosis in
dogs. J Small Anim Pract 48:308.
ducks. The combination has been used successfully in
Shpigel NY, et al. 1998. Relationship between in vitro sensi-
the treatment of Plasmodidium gallinaceum malaria in tivity of coliform pathogens in the udder and the outcome
of treatment for clinical mastitis. Vet Rec 142:135.
chickens (Williams, 2005). Depending on the extent of
Speakman AJ, et al. 2000. Antibiotic susceptibility of canine
use in different countries, which varies, resistance can Bordetella bronchiseptica isolates. Vet Microbiol 71:193.

be widespread among E. coli isolated from broilers, Stack A, et al. 2011. Suspect novel adverse drug reactions to
trimethoprim-sulfonamide combinations in horses; a case
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sulfadimethoxine in mares. Can J Vet Res 53:12. peutic potential for repeated oral doses of trimethoprim/
sulphachlorpyridazine in horses. Vet Rec 137:483.
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Vet Pharm Therap 25:413. folate reductase and thus preventing purine synthesis.
These drugs are used in the treatment of systemic proto-
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ant Staphylococcus aureus. Clin Microbiol Infect 18:745. and are generally preferred over alternatives such
as azithromycin and trimethoprim-sulfamethoxazole.
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efficacy of daily and twice daily trimethoprim-sulfadiazine 4 g sulfadiazine PO/day in 4 divided doses, adminis-
and daily sulfadimethoxine-ormetoprim therapy in the tered for up to 4 weeks. Dapsone combined with
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294 Section II. Classes of Antimicrobial Agents

Pyrimethamine combined with trimethoprim-sulfadi- Pyrimethamine and diaveridine are commonly com-
azine or with an oral sulfonamide alone (20 mg/kg q 24 h) bined with sulfaquinoxaline for their synergistic effect
has become a standard treatment for equine protozoal against coccidia. Pyrimethamine (1 mg/kg daily) com-
myeloencephalitis (EPM). Current maintenance dosage bined with a sulfadoxine (20 mg/kg daily) or trimetho-
is 1 mg/kg daily given orally with trimethoprim-sulfadi- prim-sulfadiazine has been used successfully in the
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mum of 4 months (Fenger, 1997). The trimethoprim and Laanen, 1998).
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drugs for the treatment of EPM are required, since 82 cases (1976–1986). J Am Vet Med Assoc 196:632.
pyrimethamine is teratogenic for animals and may lead
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Fluoroquinolones 18

Steeve Giguère and Patricia M. Dowling

Introduction gave the group the name “fluoroquinolones.” The first
fluoroquinolone approved for use in clinical medicine
The fluoroquinolones, also known as quinolones, was norfloxacin, followed shortly thereafter by cipro-
4-quinolones, pyridine-β-carboxylic acids, and qui- floxacin. The first fluoroquinolone approved for use in
nolone carboxylic acids, are a large and expanding group animals was enrofloxacin, which was approved for use
of synthetic antimicrobial agents. The first of these in the United States in companion animals in 1988.
compounds, nalidixic acid, was initially described in Since the approval of enrofloxacin, seven other fluoro-
1962, introduced into clinical practice in 1963, and then quinolones have been approved for use in companion
approved for clinical use in 1965. Nalidixic acid had lim- and/or food animals.
ited clinical application because of its poor absorption
following oral administration, its moderate antibacterial The fluoroquinolones that are marketed for use in vet-
activity (MICs of 4–16 μg/ml for Enterobacteriaceae), erinary medicine today are typically well absorbed orally,
high protein binding (92–97%), and poor patient toler- have a large volume of distribution, penetrate nearly
ance (Bryskier, 2005). Attempts to introduce an intrave- every tissue and cell in the body, and have extended
nous form of nalidixic acid administration were elimination half-lives, allowing for every 24- or 48-hour
unsuccessful, primarily because of limited antibacterial dosing. At appropriate drug concentration:MIC ratios,
activity and high protein binding. Between the mid- the fluoroquinolones are rapidly bactericidal, exhibit
1960s and the early 1980s there were several other qui- concentration-dependent killing, and may exhibit a pro-
nolones approved for clinical use, for example, oxolinic longed in vivo post-antibiotic effect (PAE) on certain
acid, pipemidic acid, piromidic acid, and flumaquine. bacteria. However, the potential for fairly rapid selection
These drugs exhibited increased antibacterial activity of resistance in some pathogens is a disadvantage of this
but still had limited absorption and distribution. In the class of drugs. This can be minimized by appropriate
1980s, the addition of both a fluorine molecule at the 6 dose selection directed against the right pathogen for the
position of the basic quinolone structure and a pipera- right infectious disease process.
zine substitution at the 7 position enhanced the antibac-
terial activity of these compounds, including activity The fluoroquinolones are classified into different
against organisms such as Pseudomonas aeruginosa and groups based on their chemical structure or their bio-
staphylococci. These modifications also increased the logical activities. Classification by chemical structure
oral absorption and tissue distribution (Ball, 2000). The is  dependent on the number of rings associated
quinolone nucleus possessing the fluorine molecule with  the  pyridine-β-carboxylic acid nucleus (Bryskier,
2005). Group I is composed of monocyclic derivatives.

Antimicrobial Therapy in Veterinary Medicine, Fifth Edition. Edited by Steeve Giguère, John F. Prescott and Patricia M. Dowling.
© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

295

296 Section II. Classes of Antimicrobial Agents

Group II, which is the majority of fluoroquinolones on Table 18.1. Fluoroquinolones used in veterinary
the market today, is composed of bicyclic derivatives. medicine.*
This group is divided into two subgroups based on
substitutions at position 8 of the quinolone nucleus. Fluoroquinolone Comments
Group III is composed of tricyclic derivatives and
includes marbofloxacin. Group IV is comprised of those Enrofloxacin Available as tablets and injectable
molecules that are quadricyclic, of which only a few formulation for dogs and cats and as an
have been synthesized and none are marketed for use in Ciprofloxacin injectable solution for cattle. Only
veterinary medicine. The biological classification places Danofloxacin approved for treatment and control of
the 4-quinolones in three groups or generations. Difloxacin respiratory disease in cattle in the United
First-generation quinolones are those with antibacterial States and Canada.* Approved uses vary
activity restricted to the Enterobacteriaceae (e.g., nalidixic Ibafloxacin widely between countries, with some
acid and flumequine). Second-generation quinolones Marbofloxacin approvals for lactating dairy cows, swine,
have an extended spectrum of antibacterial activity. and poultry. Used extra-label in horses and
Most fluoroquinolones approved for use in people Pradofloxacin exotic animals.
(including ciprofloxacin, norfloxacin, and ofloxacin) Orbifloxacin
and all but one of the fluoroquinolones approved for use Only approved for humans, but used
in veterinary medicine are second-generation fluoro- extra-label in small animals.
quinolones. Third-generation fluoroquinolones have
considerably improved activity against streptococci and Only approved for treatment of respiratory
obligate anaerobes. Examples of third-generation fluo- disease in cattle in the United States and
roquinolones approved for use in people include trova- Canada, but approved for use in cattle,
floxacin, gatifloxacin, and moxifloxacin. Pradofloxacin swine, and poultry in Europe.
is the only third-generation fluoroquinolone approved
for use in animals. The fluoroquinolones can also be Only available as small animal oral
grouped according to their physiochemical properties formulations in the United States and
(Bryskier, 2005). Newer compounds are being explored Canada, but cattle and dog injectable
that optimize the various substitutions and allow for the formulations and poultry oral solution are
fluorine atom at position 6 to be replaced, which may available in Europe. Used extra-label in
reduce side effects, decrease metabolism and decrease horses.
interactions with other drugs. The emergence of resist-
ant bacterial strains, however, remains problematic. Oral formulation available for small animals
in Europe.
To date there have been eight fluoroquinolones
approved for use in veterinary medicine (danofloxacin, Only available as small animal oral
difloxacin, enrofloxacin, ibafloxacin [Europe only at this formulations in the United States and
time], marbofloxacin, orbifloxacin, pradofloxacin, and Canada, but large animal injectable
sarafloxacin). These fluoroquinolones and their cur- formulations are available in Europe. Used
rent  clinical uses in veterinary medicine are listed in extra-label in horses.
Table  18.1. Of these products, sarafloxacin has been
voluntarily withdrawn from the market in the United Oral formulations for use in dogs and cats.
States following a request by the Food and Drug
Administration’s Center for Veterinary Medicine. The Only available as small animal oral
use of enrofloxacin in poultry in the United States has formulations. Used extra-label in horses.
been withdrawn following a Judicial Review (Federal
Register, 2000). This chapter reviews chemical, micro- *Off-label use of fluoroquinolones in food-producing animal species is
biological, pharmacokinetic, pharmacodynamic, and illegal in the United States.
clinical aspects of the fluoroquinolone antibacterial
agents, with specific attention to those agents approved
for use in animals (Table 18.1).

Chemistry

The fluoroquinolones, like sulfonamide and nitro-
furans, are synthetic compounds (Grohe, 1998). The
first clinically approved 4-quinolone-type com-
pound was nalidixic acid. Nalidixic acid lacked sev-
eral of the characteristics associated with the
fluoroquinolones. For example, nalidixic acid has a
nitrogen atom at position 8 instead of a carbon atom.
With a nitrogen atom at position 1, nalidixic acid has

Chapter 18. Fluoroquinolones 297

H 5 O O F OF O
CH3 6 COOH COOH
7 4 COOH F COOH
N N N
8 3 NN N
N Enrofloxacin
N 2 N
1 C2H5 O
COOH
N

C2H5 N Ciprofloxacin
Nalidixic Acid O
C2H5
Norfloxacin COOH

F O F F
F COOH

CH3 N N N N
N N
N N CH3 F N Danofloxacin
F O Difloxacin CH3 O
COOH
CH3 Orbifloxacin F COOH

F

N N
HN
NN
HCI
ON
N CH3
CH3
Marbofloxacin
F
Sarafloxacin

Figure 18.1. Structures of fluoroquinolones used in veterinary medicine.

two nitrogen atoms in its basic nucleus making it a increased activity is attributed to increased penetration
naphthyridone molecule rather than a quinolone of the bacterial cell membrane (Petersen and Schenke,
molecule. In addition, nalidixic acid is not halogen- 1998). Substituting a piperazinyl ring for the methyl
ated like other quinolones. Since the discovery of group at position 7 increased Gram-negative activity
nalidixic acid’s antibacterial activities, more than including antipseudomonal activity. These modifica-
10,000 compounds have been designed from the par- tions led to the development of the first broad-spectrum
ent bicyclic 4-quinolone molecule. Today the majority fluoroquinolone, norfloxacin, which was marketed in
of fluoroquinolones marketed for clinical use in 1986. Additional studies demonstrated that substantial
veterinary medicine are bicyclic derivatives. One changes in potency could be obtained by variations at
exception is marbofloxacin, which is a tricyclic mol- the N-1 and C-7 positions. For example, ciprofloxacin is
ecule (Figure 18.1). similar in structure to norfloxacin but has a cyclopropyl
group in place of the ethyl group at N-1. This substitu-
Clinically, nalidixic acid has several limitations. These tion enhances ciprofloxacin’s Gram-positive and Gram-
include a narrow spectrum of activity, poor pharma- negative activity. This cyclopropyl group is also found
cokinetic properties, toxic effects, and a tendency to on enrofloxacin, danofloxacin, pradofloxacin and
select for resistant organisms. Replacing the hydrogen orbifloxacin. Difloxacin has a phenyl ring at position
atom at position 6 of the 4-quinolone molecule with a N-1 that reportedly gives it enhanced activity against
fluorine atom resulted in increased activity against Gram-positive bacteria, relative to enrofloxacin activity.
both  Gram-positive and Gram-negative bacteria. The