Newcastle disease and other avian paramyxoviruses

Rev. sci. tech. Off. int. Epiz., 2000,19 (2), 443-462

Newcastle disease and other avian

paramyxoviruses

D.J. Alexander

Central Veterinary Laboratory, Weybridge, New Haw, Addlestone, Surrey KT15 3NB, United Kingdom

Summary

Newcastle disease (ND), caused by avian paramyxovirus serotype 1 (APMV-1)
viruses, is included in List A of the Office International des Epizooties. Historically,
ND has been a devastating disease of poultry, and in many countries the disease
remains one of the major problems affecting existing or developing poultry
industries. Even in countries where ND may be considered to be controlled, an
economic burden is still associated with vaccination and/or maintaining strict
biosecurity measures. The variable nature of Newcastle disease virus strains in
terms of virulence for poultry and the different susceptibilities of the different
species of birds mean that for control and trade purposes, ND requires careful
definition. Confirmatory diagnosis of ND requires the isolation and
characterisation of the virus involved. Assessments of virulence conventionally
require in vivo testing. However, in vitro genetic characterisation of viruses is
being used increasingly now that the molecular basis of pathogenicity is more
fully understood. Control of ND is by prevention of introduction and spread, good
biosecurity practices and/or vaccination. Newcastle disease viruses may infect
humans, usually causing transient conjunctivitis, but human-to-human spread has
never been reported.
Eight other serotypes of avian paramyxoviruses are recognised, namely: APMV-2
to APMV-9. Most of these serotypes appear to be present in natural reservoirs of
specific feral avian species, although other host species are usually susceptible.
Only APMV-2 and APMV-3 viruses have made a significant disease and economic
impact on poultry production. Both types of viruses cause respiratory disease and
egg production losses which may be severe when exacerbated by other
infections or environmental stresses. No reports exist of natural infections of
chickens with APMV-3 viruses.

Keywords

Aetiology - Avian diseases - Avian paramyxoviruses - Control - Diagnosis -
Epidemiology - Newcastle disease - Public health.

Introduction represents a major limiting factor for increasing poultry
production in many countries.
Two poultry diseases are considered to be sufficiently serious
to be included in List A of the Office International des The greatest impact of ND may be on village or backyard
Epizooties (OIE), namely: highly pathogenic avian influenza chicken production. In developing countries throughout Asia,
(HPAI) and Newcastle disease (ND) (85). While HPAI occurs Africa, Central America and some parts of South America, the
relatively rarely, ND is enzootic in some areas of the world and village chicken is an extremely important asset representing a
a constant threat to most birds reared domestically. Very few significant source of protein in the form of eggs and meat.
commercial poultry flocks are not influenced in some way by However, ND is frequently responsible for devastating losses
measures aimed at controlling ND and spread of the virus (4). in village poultry. For example, Spradbrow estimated that
A large majority of the countries rearing poultry commercially 90% of the village chickens in Nepal die each year as a result
rely on vaccination to control ND, but ND nevertheless of ND (107). Social and financial restraints mean that the

444 Rev. sci. tech. Off. int. Epiz., 19

control of ND in village chickens in developing countries is - mesogenic: viruses causing clinical signs consisting of
extremely difficult, if not impossible, and this situation respiratory and neurological signs, with low mortality
impinges on the further development of commercial poultry
production and the establishment of trade links. - lentogenic: viruses causing mild infections of the
respiratory tract
Aetiology
- asymptomatic enteric: viruses causing avirulent infections
Viruses in which replication appears to occur primarily in the gut.

The three virus families Rhabdoviridae, Filoviridae and Although these categories are useful for descriptive purposes,
some overlapping does occur and some viruses are difficult to
Paramyxoviridae form the order Mononegavirales, i.e. viruses place.

with negative sense, single-stranded, non-segmented, Antigenic relationships

ribonucleic acid genomes. Newcastle disease is caused by Cross relationships in haemagglutination inhibition (HI) and
other tests have been detected between some of the APMV
avian paramyxovirus serotype 1 (APMV-1) viruses, which serotypes ( 7 , 8 ) . However, use of a variety of serological and
non-serological tests has tended to confirm the distinctiveness
together with viruses of the other eight APMV serotypes of the APMV serotypes. Antigenic relationships between
APMV-1 and other serotypes are important as these may affect
(APMV-2 to APMV-9), have been placed in the genus diagnosis of ND. Generally, such cross-reactions in serological
tests are low, but APMV-3 viruses may show sufficiently high
Rubulavirus, sub-family Paramyxovirinae, family levels of cross reactivity with conventional APMV-1 antisera to
cause problems. Apparent antibodies to APMV-3 virus
Paramyxoviridae, in the current taxonomy ( 9 5 ) . Recent detected in chickens have been attributed to high antibody
levels to NDV as a result of vaccination (33).
studies involving the sequencing of the whole ND virus
Antigenic variation between viruses within APMV serotypes
(NDV) genome have suggested that avian paramyxoviruses has been reported for most of the serotypes where more than a
few isolates have been obtained. For NDV (APMV-1),
are sufficiently different from other rubulaviruses to warrant differences detectable by conventional HI tests have been
reported, although only rarely (12, 2 6 , 54). One of the most
the creation of a separate genus (45). noted variations of this kind has been the virus responsible for
the panzootics in racing pigeons that occurred during the
The prototype strains of each of the nine serotypes of avian 1980s. The NDV responsible, referred to as pigeon APMV-1
paramyxoviruses are shown in Table I. (PPMV-1), was demonstrably different from standard strains
in HI tests, although not sufficiently different that
Strains of NDV have been distinguished on the basis of the
clinical signs produced in infected chickens. Beard and
Hanson (29) defined the following five groups or pathotypes:

- viscerotropic velogenic: viruses responsible for disease
characterised by acute lethal infections, usually with
haemorrhagic lesions in the intestines of dead birds

- neurotropic velogenic: viruses causing disease
characterised by high mortality which follows respiratory and
neurological disease, but in which gut lesions are usually
absent

Table I
Serotypes of avian paramyxovirus

Prototype virus strain Usual natural hosts Other hosts Disease produced in poultry

APMV-1 (Newcastle disease virus) Numerous Varies from extremely pathogenic to inapparent, depending on
strain and host infected
APMV-2/chicken/California/Yucaipa/56 Turkeys, passerines Chickens, psittacines, Mild respiratory disease or egg production problems: severe if
Turkeys rails exacerbation occurs
APMV-37turkey/Wisconsin/68
None Mild respiratory disease but severe egg production problems
APMV-3*/parakeet/Netherlands/449/75 worsened by exacerbating organisms or environment
APMV-4/duck/Hong Kong/D3/75 Psittacines, passerines None
APMV-5/budgerigar/Japan/Kunitachi/74 Ducks Geese No infections of poultry reported
APMV-6/duck/Hong Kong/199/77 Budgerigars Lorikeets None known
Ducks Geese, rails, turkeys
APMV-7/dove/Tennessee/4/75 No infections of poultry reported
APMV-8/goose/Delaware/1053/76 Pigeons, doves Turkeys, ostriches
APMV-9/domestic duck/New York/22/78 Ducks, geese Mild respiratory disease and slightly elevated mortality in turkeys:
Ducks no disease in ducks or geese

Mild respiratory disease in turkeys

No infections of poultry reported
None known

* Serological tests may distinguish between turkey and psittacine isolates
APMV: avian paramyxovirus

Rev. sci. tech. Off. int. Epiz., 19 (2) 445

conventional ND vaccines were not protective (16). Disease
Monoclonal antibodies (mAbs) raised against various strains
of NDV have been used to establish the uniqueness of the Newcastle disease
variant NDV responsible for the panzootic of ND in pigeons
and have proven particularly useful in identifying the spread The clinical signs seen in birds infected with NDV vary widely
of this virus around the world (14, 17, 91). Monoclonal and are dependent on factors such as the virus, host species,
antibodies to NDV have also been used to distinguish between age of host, infection with other organisms, environmental
specific viruses. For example, two groups have described stress and immune status. In some circumstances, infection
mAbs that distinguish between the common vaccine strains, with the extremely virulent viruses may result in sudden, high
Hitchner B l and La Sota (47, 77), while other mAbs can mortality with comparatively few clinical signs. Although the
separate vaccine viruses from epizootic virus in a given area clinical signs are variable and influenced by other factors, so
(108). that none can be regarded as pathognomonic, certain signs do
appear to be associated with particular viruses. This has
Antigenic variation detected by mAb typing has also been resulted in the grouping of viruses into five 'pathotypes' on the
used in epidemiological studies. In a large study of over 1,500 basis of the predominant signs in affected chickens (see
viruses, Alexander et al. (21) used the ability of viruses to react above). These groupings are by no means clear-cut, and even
with panels of mAbs, consisting initially of nine mAbs and in experimental infections of specific-pathogen-free (SPF)
later extended to twenty-six or twenty-eight mAbs, to place chickens considerable overlapping occurs (9). In addition, in
strains and isolates of NDV into groups on the basis of ability the field, exacerbating factors may result in the clinical signs
to react with the different mAbs. Viruses in the same mAb induced by the milder strains mimicking those of the more
group shared biological and epizootiological properties. Use pathogenic viruses.
of such panels, particularly the extended panel, has indicated
that viruses tend to remain fairly well conserved during In general terms, ND may consist of signs of depression,
outbreaks or epizootics and this often allows valuable diarrhoea, prostration, oedema of the head and wattles,
assumptions to be made concerning the source and the spread nervous signs, such as paralysis and torticollis, and respiratory
of ND. signs (75). Decline in egg production, perhaps leading to
complete cessation of egg laying, may precede more overt
Variations within other serotypes have been reviewed by signs of disease and deaths in egg-laying birds. Virulent ND
Alexander (7). Marked variations have been recorded between strains may replicate in vaccinated birds, but the clinical signs
viruses placed in APMV-2, APMV-3 and APMV-7 serotypes. will be greatly diminished in relationship to the antibody level
The variations recorded often showed two viruses with little achieved (24).
relationship to each other, but each showing strong similarity
with a third virus. Ozdemir et al. produced mAbs against As with clinical signs, no gross or microscopic lesions can be
APMV-2 virus and were able to class isolates from a wide considered pathognomonic for any form of ND (75).
range of hosts and geographical locations into four groups Carcasses of birds dying as a result of virulent ND usually
(88). The antigenic variations in the APMV-3 group seem to have a fevered, dehydrated appearance. Gross lesions vary
have greater significance, as these appear to separate turkey according to the infecting virus. Virulent panzootic NDVs
isolates from psittacine isolates. Anderson et al. produced typically cause haemorrhagic lesions of the intestinal tract.
mAbs to APMV-3 virus and the use of these suggested that These are most easily seen if the intestine is opened and may
antigenic variation within the APMV-3 serogroup may be vary considerably in size. Some authors have reported lesions
more complicated than suggested using polyclonal antisera most typically in the proventriculus, while others consider
(25). lesions to be most prominent in the duodenum, jejunum and
ileum. Even in birds showing neurological signs prior to
Phylogenetic studies death, little evidence is found of gross lesions in the central
nervous system. Lesions are usually present in the respiratory
Development of improved techniques for nucleotide tract when clinical signs indicate involvement. These
sequencing, the availability of sequence data of NDV placed in generally appear as haemorrhagic lesions and congestion;
computer databases and the demonstration that even airsacculitis may be evident. Egg peritonitis is often seen in
relatively short sequence lengths could give meaningful laying hens infected with virulent NDV.
results in phylogenetic analyses has led to a considerable
increase in such studies in recent years. Considerable genetic Microscopic lesions are not considered to have any diagnostic
diversity has been detected, but viruses sharing temporal, significance. In most tissues and organs, changes consist of
geographical, antigenic or epidemiological parameters tend to hyperaemia, necrosis, cellular infiltration and oedema.
fall into specific lineages or clades and this has proven Changes in the central nervous system are those of
valuable in assessing both the global epidemiology and local nonpurulent encephalomyelitis.
spread of ND ( 2 3 , 4 0 , 57, 67, 6 8 , 73, 1 0 2 , 1 0 9 ) .

446 Rev. sci. teck Off. int. Epiz., 19

Other avian paramyxoviruses Saif et al. reported APMV-7 virus as the primary pathogen in
natural outbreaks of respiratory disease with elevated
As summarised in Table I, of the other APMVs, only APMV-2 mortality in turkeys. Furthermore, mild respiratory disease
and APMV-3 viruses have been consistently shown to was demonstrated in turkeys infected experimentally (100).
infect and cause disease in poultry, although APMV-6 and
APMV-7 viruses have been associated with clinical disease in Molecular basis of pathogenicity of Newcastle
turkeys. disease virus

Uncomplicated infections of chickens or turkeys with During replication, NDV particles are produced with a
APMV-2 viruses probably result in only mild respiratory precursor fusion glycoprotein, F0, which has to be cleaved to
disease. However, much more serious disease may ensue due F l and F2 for the virus particles to be infectious ( 9 8 ) . This
to exacerbation by other organisms. Avian paramyxovirus post translation cleavage is mediated by host cell proteases
serotype 2 viruses have been associated with respiratory and (79). Trypsin is capable of cleaving F 0 for all NDV strains and
egg production problems in chickens and turkeys ranging in vitro treatment of non-infectious virus will induce
from mild to severe with elevated mortality. Associated infectivity (80).
disease has usually been more severe in turkeys than chickens
(7). Virus isolation and/or the presence of antibodies have also The importance of F 0 cleavage can be demonstrated by the
confirmed APMV-2 virus infections in chicken and turkey ability of viruses normally unable to replicate or produce
flocks that have experienced no clinical disease (34). The plaques in cell culture systems to do both if trypsin is added to
clinical outcome of infections of domestic poultry with the agar overlay or culture fluid. The cleavability of the FO
APMV-2 viruses is probably dependent on exacerbation by molecule was shown to be related directly to the virulence of
other organisms or environmental conditions. Experimental viruses in vivo. Viruses pathogenic for chickens could
infections of laying turkeys showed that egg production and replicate in a wide range of cell types in vitro with or without
hatchability were adversely affected although fertility was not added trypsin, whereas strains of low virulence could
(28). replicate only when trypsin was added (96, 9 7 ) . The FO
molecules of viruses virulent for chickens can apparently be
Avian paramyxovirus serotype 3 viruses have been associated cleaved by a host protease or proteases found in a wide range
with respiratory disease and egg production problems in of cells and tissues. This allows these viruses to spread
turkeys (27). To date, no natural infections have been throughout the host, damaging vital organs. In contrast, FO
recorded in chickens, although experimental infections have molecules in viruses of low virulence appear to have restricted
demonstrated that these birds are susceptible to the serotype. sensitivity to host proteases, resulting in restriction of these
In uncomplicated infections of turkeys, the first sign is often a viruses to growth only in certain host cell types.
decline in egg production. However, early mild respiratory
signs have been reported, which suggest the respiratory tract Collins et al. compared the deduced amino acid sequences at
may be the initial site of infection (10). The decline in egg the cleavage site of the FO precursor of twenty-six APMV-1
production varies considerably according to the age of the isolates (38). All fourteen viruses that were virulent for
birds and the presence of secondary infections. In chickens had the sequence 112R/K-R-Q-K/R-R116 at the
uncomplicated infections of birds a few weeks after beginning C-terminus of the F2 protein and F (phenylalanine) at residue
to produce eggs, the effective loss may be around 1-2 eggs per 117, the N-terminus of the F l protein. The eleven viruses of
bird per week for five or six weeks, after which the production low virulence had sequences in the same region of
returns to expected levels (32). Production problems are 112G/E-K/R-Q-G/E-R116 and L (leucine) at residue 117. Thus,
associated with a high level of white-shelled eggs, and the a double pair of basic amino acids was apparently required at
hatchability and fertility of eggs are also reduced. Infection at, residues 112 and 113, and 115 and 116, plus a phenylalanine
or just before, the point of lay may result in more serious at residue 117, if the virus was to show virulence for chickens.
losses, with the flock failing to reach target production The exception was the single pigeon variant virus examined
throughout the laying period. Far more serious respiratory (PPMV-1) which had the sequence m G - R - Q - K - R - F 1 1 7 ; in
disease and egg production problems have been recorded tests in vivo, this virus fell within the definition of viruses
when dual infection with NDV (including live vaccines), virulent for chickens (see below). Jestin and Cherbonnel
influenza viruses, chlamydiae, mycoplasmas or other bacteria demonstrated a similar motif at the F2/F1 cleavage site for two
has occurred. No studies have reported on the lesions viruses isolated from chickens (61), which they grouped
associated with APMV-3 infections of turkeys. antigenically with PPMV-1 viruses (60). Further studies have
indicated that this variation is usual for PPMV-1 viruses, but
Avian paramyxovirus serotype 6 has been isolated on one has no significance in the variability of pathogenicity for
occasion from turkeys with egg production and mild chickens recorded with these viruses (39).
respiratory problems.
The major influence on the pathogenicity of NDV is therefore
the amino acid motif at the F 0 cleavage site, the presence of

Rev. sci. tech. Off. int. Epiz., 19 (2) 447

basic amino acids at positions 113, 115 and 116 and Caged 'pet birds'
phenylalanine at 117 in virulent strains means that cleavage Virulent NDV isolates have often been obtained from captive
can be effected by protease or proteases present in a wide caged birds (103). Kaleta and Baldauf suggested that
range of host tissues and organs. For viruses of low virulence, infections of recently imported caged birds were unlikely to
cleavage can occur only in the presence of proteases have resulted from enzootic infections in feral birds in the
recognising a single arginine, i.e. trypsin-like enzymes. The countries of origin (64). The infections were considered likely
replication of such viruses is therefore restricted to areas to have originated at holding stations before export, either as a
where trypsin-like enzymes are present, such as the result of enzootic NDV at those stations or of spread from
respiratory and intestinal tracts, whereas virulent viruses can nearby poultry such as backyard chicken flocks. Panigrahy e£
replicate in a range of tissues and organs, resulting in a fatal al. described outbreaks of severe ND in pet birds in six States
systemic infection (96). of the USA in 1991 (89). Illegal importations were assumed to
be responsible for the introductions of the virus.

Epidemiology Domestic poultry
Virulent NDV strains have been isolated from all types of
Host range commercially reared poultry, ranging from pigeons to
ostriches.
The natural and occasional hosts of the different APMV
serotypes are summarised in Table I. Racing and show pigeons
In the late 1970s, an NDV strain named PPMV-1, showing
Host range of Newcastle disease some antigenic differences from classical strains, appeared in
pigeons, probably arising in the Middle East. In Europe, the
Newcastle disease viruses have been reported to infect animals strain was first reported in racing pigeons in Italy in 1981 (30)
other than birds, ranging from reptiles to humans (71). Kaleta and subsequently produced a true panzootic, spreading in
and Baldauf concluded that NDV infections have been racing and show pigeons to all parts of the world (8).
established in at least 241 species of birds representing 27 of
the 5 0 orders of the class (64). All birds are probably Host range of other avian paramyxoviruses
susceptible to infection, but, as stressed by Kaleta and Baldauf,
the disease observed with any given virus may vary The known host range of each of the other serotypes of APMV
enormously from one species to another. is more restricted than that of NDV (Table I). Viruses of
serotypes APMV-4, APMV-8 and APMV-9 appear to be
Wild birds restricted to infections of ducks and geese. Similarly, APMV-6
Newcastle disease virus isolates have been obtained frequently viruses have been isolated frequently from feral ducks and
from migratory feral waterfowl and other aquatic birds. Most geese, in which viruses of this serotype appear to be enzootic.
of these isolates have been of low virulence for chickens and Avian paramyxovirus serotype 6 viruses have also been
similar to viruses of the 'asymptomatic enteric' pathotype. The isolated from rails that presumably may have had contact with
potential of such viruses to become virulent and the role of ducks and geese; spread to turkeys has also been recorded.
feral birds in the spread of ND are described below. The most
significant outbreaks of ND in feral birds were those reported Avian paramyxovirus serotype 5 viruses were restricted to
in double-crested cormorants (Phalacrocorax auritus) in isolations made during a single epizootic in budgerigars in
North America during the 1990s. Newcastle disease in Japan between 1974 and 1976 (81), but more recently,
cormorants and related species had been reported earlier, in similar viruses have been isolated from budgerigars in Great
the late 1940s in Scotland (31) and in Quebec in 1975 (37). Britain (52). Species of psittacines closely related to
Recent outbreaks in cormorants in North America were first budgerigars may also be infected.
reponed in 1990 in Alberta, Saskatchewan and Manitoba in
Canada (114). In 1992, the disease re-appeared in cormorants The naturat hosts for APMV-7 viruses appear to be pigeons
in western Canada, around the Great Lakes, and in the and doves (species of the order Columbiformes). However,
northern Midwest of the United States of America (USA), in infections of turkeys (100) and ostriches (115) have recently
the latter case spreading to domestic turkeys ( 5 6 , 7 8 ) . been reported. Presumably, these infections resulted from
Antigenic and genetic analyses of the viruses suggested that all contact with pigeons or doves.
the 1990 and 1992 viruses were very closely related, despite
the geographical separation of the hosts. Since these outbreaks Avian paramyxovirus serotype 2 viruses have been isolated
affected birds that would follow different migratory routes, from both chickens and turkeys in association with
the initial infection is thought to have occurred at a mutual respiratory disease. However, the primary natural host
wintering area in Central America. Disease in double crested appears to be small perching birds of the order Passeriformes.
cormorants was observed again in Canada, in 1995 and in Other species have been reported to be infected, including
California in 1997. In both instances, NDV was isolated from captive caged psittacines that have usually been in contact
dead birds, and as previously, the viruses appeared to be with caged passerines.
closely related (70).

448 Rev. sci. tech. Off. int. Epiz., 19

Avian paramyxovirus serotype 3 viruses can be divided into caged birds in all countries that monitor such birds. Avian
two antigenically distinguishable groups. The first has only paramyxovirus serotype 2 viruses have been isolated in
been reported to infect turkeys. In experiments, chickens have surveys of passerine birds in countries of Eastern and Western
been demonstrated to be susceptible to infection, but no Europe, the Middle East, South-East Asia, Africa and Central
natural infections have been reported. Viruses of the second America, essentially wherever surveillance of passerine species
group have been isolated frequently from captive caged has been undertaken. In poultry, APMV-2 viruses have been
psittacines and passerines. The evidence suggests that these isolated from chickens and/or turkeys in countries of Europe
APMV-3 viruses primarily infect psittacine species but will (Eastern and Western), Asia, the Middle East, North and
spread to passerines placed in contact. No isolations of Central America. The APMV-3 subserotype infecting turkeys
APMV-3 viruses have been reported from feral birds. has been isolated only in countries of Western Europe and
North America, but these countries represent the primary
Geographical distribution producers of turkeys.

Newcastle disease virus Current distribution

In many respects, estimations of the geographical distribution Newcastle disease
of NDV in domestic poultry are confused by the use of live
vaccines in all but a few countries throughout the world. In Assessment of the prevalence of ND in the world at any given
addition, in many countries, a distinction is made between the time is extremely difficult. In some countries or areas, disease
disease situation in large scale commercial poultry and in is not reported at all or only reported if outbreaks occur in
village chickens. When countries or areas are declared free of commercial poultry, while the presence of disease in village
ND, further complications are caused by the definition of the chickens or backyard flocks is ignored. Even in commercially
type of ND viruses described within the claim of 'freedom'. reared poultry, estimations of the geographical distribution of
Even in Australia, which was free of the virulent virus between NDV are confused by the use of live vaccines in all but a few
the 1932 outbreak (2) and the 1998 outbreaks, commercial countries throughout the world. In some countries, the
poultry flocks were frequently found to be infected with distribution is especially confused by the use as live vaccines
viruses similar to those placed in the 'asymptomatic enteric' of viruses that would be considered sufficiently virulent in
pathotype group (105, 113). Similarly, on the island of other countries to be defined as ND when infecting poultry.
Ireland, which had long been recognised as free of ND,
monitoring surveys often reveal symptomless infections, In Western Europe, reported outbreaks increased markedly
presumably due to spread from waterfowl, and occasional during the early 1990s, peaking at 2 3 9 outbreaks in countries
epizootics of virulent infections occur (87). of the European Union (EU) in 1994. The distribution with
time suggested a single epidemic during the early to
The severe form of ND is undoubtably a serious problem mid-1990s, but in fact, antigenic and phylogenetic evidence
either as an enzootic disease or as a cause of regular, frequent indicates that several strains of virus were responsible for
epizootics throughout Africa, Asia, Central America and parts these outbreaks.
of South America (43, 9 9 , 1 0 6 ) .

In other areas such as Europe, the situation appears to be one Between 1991 and 1995, the majority of outbreaks in the EU
of sporadic epizootics occurring despite vaccination occurred in the Benelux countries and Germany,
programmes (65). predominantly in backyard poultry. Most of the outbreaks
since 1995 have also been in backyard poultry.
Other avian paramyxoviruses
One notable aspect of the outbreaks in Western Europe
The countries in which each of the APMV serotypes has been during the 1990s was the occurrence of outbreaks in
isolated from the various types of avian hosts have been listed countries which had been free of the disease for many years.
by Alexander (7). Viruses of the serotypes isolated regularly Since 1995, eighteen outbreaks have been reported in
from migratory ducks and geese (i.e. APMV-4 and APMV-6) Denmark, one in Sweden, two in Finland, one in Norway, one
appear to be ubiquitous and distributed world-wide. Fewer in the Republic of Ireland and twenty-six in Northern Ireland,
isolations of APMV-8 viruses have been recorded, but the all areas of Western Europe that had been declared free of ND
presence of this serotype in migratory waterfowl in the USA and which were monitored regularly by serological testing
and Japan is an indication that these too may have a with no evidence of ND virus infections.
world-wide distribution. Isolation of APMV-7 viruses from
feral and/or captive pigeons and doves in Japan, the USA and Although voluntary vaccination was permitted, Great Britain
England (18) again suggests that infections of viruses of this was also essentially free of ND, and the outbreak confirmed in
serotype may occur throughout the world. pheasants in 1 9 9 6 was the first in this country since 1984
(20). That outbreak and those in Great Britain in 1984 (15)
Avian paramyxovirus serotype 2 and APMV-3 viruses both were shown to b e caused by the variant NDV responsible for
appear to have a world-wide distribution in varying hosts. the panzootic in racing pigeons (PPMV-1) that had reached
Viruses of both serotypes have been isolated from captive

Rev. sci. tech. Off. int. Epiz., 19 (2) 449

Great Britain in 1983 (13). This virus is still detected The first explanation remains a possibility. Some consider that
occasionally in racing and feral pigeons. However, in 1 9 9 7 , the disease is unlikely to have gone unreported if it was
eleven outbreaks of ND were confirmed in Great Britain in enzootic in village chickens, but even at present, village
commercial poultry, four in broiler chickens and seven in chickens throughout Africa, Asia and the Americas often show
turkeys (22). Although the viruses isolated were highly high levels of mortality, either regularly or as large die-offs
virulent for chickens infected in the laboratory, the clinical every few years which largely go undiagnosed. Similarly,
disease signs seen in field infections were variable and not occasional descriptions of disease outbreaks prior to 1926 are
always associated with high mortality, especially in turkeys. very similar to ND.
Epidemiological investigations indicated that the majority of
the outbreaks occurred as a result of secondary spread by Until recent years, the second explanation has been the one
human agency from two or more primary infected flocks. generally accepted to be most likely. This was fed by the
Outbreaks were caused by similar viruses in countries of finding, during the panzootic of 1 9 7 0 - 1 9 7 3 , that movement
Scandinavia in 1996 (23). These outbreaks, linked to the of captive caged birds, particularly psittacines, which may be
unusual patterns of movement of migratory birds at the end of resistant excreters of NDV, was to some extent responsible for
1996 and beginning of 1997, suggested that migratory birds the introduction and spread of NDV in some countries, and
may have been responsible for the primary introduction of the most noticeably in California ( 5 0 , 1 1 2 ) . However, as
causative virus into Great Britain (22). discussed above, viruses isolated from feral birds are usually
of low virulence and caged birds are most probably infected
Until 1998, Australia had been free of virulent NDV since the after trapping. Maintenance of virus in any species of feral bird
1932 outbreak (2). Since 1966, viruses similar to those placed that is at all affected by the virus seems unlikely due to the
in the 'asymptomatic enteric' pathotype group ( 1 0 5 , 1 1 3 ) had likely effect of infection on the survival of the bird.
been recognised in wild birds in Australia, and on occasions
had spread to commercial poultry flocks. Two outbreaks of The third explanation has usually been dismissed as
virulent ND occurred in Australia in 1998 and further representing too large a mutation to be within the bounds of
outbreaks were reported in 1999. probability of occurring, especially without any apparent
evolutionary advantage that would result in selection.
Other avian paramyxoviruses However, viruses isolated from ND outbreaks in Ireland and
Australia during the 1990s have suggested that some virulent
As discussed above, the most recent developments NDVs may emerge this way.
concerning other APMVs were the isolation of APMV-7
viruses from ostriches ( 1 1 5 ) and turkeys (100). Until these In Ireland in 1990, two outbreaks of ND occurred in egg
reports, natural infections with APMV-7 viruses had been laying birds. The viruses isolated were highly virulent and
detected only in pigeons and doves. Recent reports of other apparently identical (19). Interestingly, these viruses were
APMVs are scarce, although investigations of poultry and very closely related antigenically and genetically (40) to
other birds for other reasons suggest that the distribution and viruses of tow virulence normally isolated from feral
host range of the other APMVs remains constant. waterfowl, but known to have infected chickens on the island
of Ireland in 1987 (76). The group formed by these viruses
Origins of virulent Newcastle disease viruses was both antigenically and genetically distant from all other
ND viruses. Collins et al. had shown that the virulent 3 4 / 9 0
The emergence of ND as a highly pathogenic disease of virus had four nucleotide differences at the site coding for the
poultry in 1 9 2 6 , initially predominantly in South-East Asia, F 0 cleavage site compared to the related viruses of low
suggested that a sudden major change had occurred either in virulence (Table II), which would explain the higher virulence
the virus or hosts, although earlier outbreaks may have been for chickens ( 3 8 ) . However, while the distinctiveness of this
confused with HPAI. Hanson (55) considered that the various group of viruses from other NDVs supports the theory that
hypotheses proposed could be grouped in three categories, as the virulent viruses originally arose by mutation f r o m those of
follows: low virulence, the mechanism by which this may have
occurred is not at all clear.
a) the virulent virus has always existed in poultry in
South-East Asia, but the disease with its enormous economic Phylogenetic studies have shown all the virulent viruses
impact was only recognised as a significant problem when the responsible for the outbreaks in Australia in 1998 and 1999 to
commercialisation of the poultry industry commenced in that be extremely closely related to each other and to the endemic
part of the world virus of low virulence, suggesting emergence by mutation,
which in this instance required only two point mutations
b) the virus was enzootic in different species, possibly (Table III).
inhabiting tropical rain forests, and spread to domestic
poultry due to the incursion of man into that habitat

c) a major mutation occurred in a precursor virus of low
virulence.

450 Rev. sci. tech. Off. int. Epiz., 19

Table II susceptible birds in this way, even over short distances. The
Nucleotide/amino acid sequences at FO cleavage site of a virus of high success of this route of transmission will depend on many
virulence (34/90) isolated from poultry in Ireland compared to an environmental factors, such as temperature, humidity and
antigenically and genetically closely related virus of low virulence stocking density. In contrast, transmission of virus infection
isolated from ducks from one bird to another via contaminated faeces can be easily
demonstrated. The pigeon variant virus, the 'asymptomatic
Virus Virulence Nucleotide/amino acid sequence at FO enteric' viruses, and other viruses which fail to induce
cleavage site significant respiratory signs in infected birds, are likely to be
MC110 Low transmitted primarily in this way (11, 13).
GAA CGG CAG GAG CGT CTG
34/90 High 112ERQER*L117 Primary and secondary introduction
AAA CGG CAG AAG CGTTTT
112KRQKR*F117 Several reviews have dealt with the way in which NDV may be
introduced into a country or area and then subsequently
If mutations to virulence do occur, it is not clear whether spread from flock to flock (3, 8, 7 1 , 72). The main methods
these take place in feral birds and then pass to poultry, or by which virus can be spread are summarised below.
occur once the virus has been introduced in poultry.
However, the lack of virulent isolations from feral birds Movement of live birds
suggests that the latter is more likely. Migratory feral birds may be responsible for the primary
introduction of infection, but nearly all NDV isolates obtained
If virulent ND strains can emerge from those of low virulence from feral birds are of low virulence. A more significant role of
by mutation, this may have important repercussions on the such birds may be the transmission of virus within an area
current philosophy on control of ND, not least in view of the following NDV infection of poultry. Exceptions to the
enormous quantities of live vaccines used throughout the presence of virus of low virulence in migratory birds have
world. However, no evidence exists to suggest that the been discussed in the section entitled 'Host range'.
licensed lentogenic vaccines have mutated and become
virulent strains. World trade in captive caged birds is enormous, and in many
countries virulent NDV has been isolated frequently from
Table III such birds held in quarantine. For example, 147 virulent
Nucleotide/amino acid sequence at FO cleavage site of viruses of high NDV isolations were made from 2,274 lots of captive birds
and low virulence isolated in Australia in 1998 held in quarantine in the USA from 1974 to 1981 (103). Some
infected psittacines have been shown to excrete virulent virus
Virus Virulence Nucleotide/amino acid sequence at FO intermittently for extremely long periods, in some cases for
cleavage site more than one year (48), which further emphasises the
1154/98 Low potential role these birds may have in the introduction of
GGA AGG AGA CAG GGG CGT CTT NDV to a country or area.
1238/98 High 111GRRQGR*L117
GGA AGG AGA CAG AGG CGT TTT Game birds may also be implicated in the introduction of
1249/98 High 111GRRQRR*F117 NDV to a country, since considerable international trade
GGA AGG AGA CAG AGG CGT TTT occurs in these birds, which are often imported for immediate
111GRRQRR*F117 release.

Introduction and spread The potential for racing pigeons to carry and introduce NDV
into a country or area has been highlighted by the panzootic
Investigation into the epidemiology of avian paramyxoviruses in such birds since the early 1980s.
other than APMV-1 that infect poultry has been limited. The
similarities to APMV-1 in infection and replication suggest Trade in backyard flocks and other birds kept for recreational
that the same mechanisms and risks will apply. purposes (hobby birds) has been implicated in the
introduction and spread of ND in the outbreaks in countries
Transmission between birds of the EU from 1991 to 1994.

Excluding predatory birds or the feeding of untreated swill Modern methods of slaughter of commercial poultry,
containing poultry meat to poultry, spread from bird to bird marketing of poultry meat and veterinary inspection, have
appears to occur as the result of either inhalation of excreted reduced the movement of live commercial poultry (excluding
droplet particles or the ingestion of infective material such as day-old chicks) in many developed countries. However, in
faeces. Although the administration of live vaccines by aerosol many countries, the normal method of trade is by five poultry
demonstrates clearly that infection may be established via the markets. Such markets, where birds of many different species
respiratory route, remarkably little experimental evidence
exists to suggest that infected birds will pass on the virus to

Rev. sci. tech. Off. int. Epiz., 19 (2) 451

may be placed in close contact, represent ideal reservoirs of airborne spread has played á major role and in recent years
virus from which disease may be disseminated. airborne spread has not been an issue in reported outbreaks
and an alternative, more likely, cause has nearly always
Movement of people and equipment existed, particularly the movement of poultry and the agency
of humans.
Secondary spread during most epizootics of ND in recent
years has been the result of the movements of personnel or Vaccines
equipment. Humans may be infected with NDV, but the most
likely role of personnel is in the transfer of infective faeces Good manufacturing practices should ensure that vaccines are
from one site to another on hair, clothing, footwear, crates, highly unlikely to be carriers of virulent NDV. However, in
feed sacks, egg trays or vehicles. the past, birds have become infected when vaccines against
other diseases have been contaminated with NDV, or as a
Movement of poultry products result of failure to inactivate killed vaccines prepared from
virulent NDV. In 1996 and 1997, a series of NDV isolates of
In the past, poultry meat has been incriminated as the main low virulence were obtained from poultry flocks in Denmark,
vehicle for the introduction and spread of NDV. For example, a country which pursues a non-vaccinating policy for ND.
in 1947, one-third of the first 5 4 2 outbreaks in England and These viruses were demonstrated to be the result of
Wales were considered to be directly attributable to feeding contamination of avian virus vaccines with vaccinal NDVs
poultry waste to chickens (51). Sampling of batches of frozen (63). This episode further emphasises the potential for spread
poultry imported into Great Britain in the same year produced of ND in this way.
an isolation rate of up to 6 6 % (94). Modem methods of
poultry carcass preparation and legislation on the feeding of Non-avian hosts
untreated swill to poultry have greatly diminished the risk
from poultry products, but the possibility of spread in this Non-avian species are likely to introduce NDV by mechanical
way nevertheless remains. transfer of infective faeces, e.g. by insects, rodents or
scavenging animals. In hot countries, reptiles which may
Contaminated poultry food or water enter poultry houses should not be ignored as potential
transmitters of NDV, as susceptibility to infection has been
In countries of the British Isles, outbreaks of ND in reported in reptiles.
commercial poultry have been associated with feed
contaminated with infective faeces from feral pigeons infected Diagnosis
with NDV (15, 8 7 ) . Similarly, water contaminated with
infective faeces may introduce NDV to a flock. The diagnostic methods described in this section are primarily
those recommended by the OIE for ND. Diagnosis of other
Airborne spread APMVs is essentially similar. Avian paramyxovirus infections
have usually been diagnosed by serology or virus isolation. In
In recent years, the significance of airborne transfer of virus common with ND, antibodies to APMVs may be detected by
has been the subject of some debate. During the 1960s and HI tests using the relevant antigens and controls. Avian
1970s, this was considered a major method of spread and paramyxoviruses can be isolated from tracheal or faecal swabs
Smith considered it the most logical explanation of spread in or tissue samples from infected birds by inoculation of eight-
outbreaks occurring in 1960 and 1962 in Great Britain (104). to ten-day-old embryonating chicken eggs via the allantoic
In the same country, Dawson considered windborne spread cavity. Confirmation of the virus as belonging to the APMV
to be of major significance during the outbreaks of 1 9 7 0 - 1 9 7 2 serotype can be performed by HI tests with specific antiserum.
that were noted for severe respiratory signs and unusual
patterns of spread (44). However, in the epizootic of Definition of Newcastle disease
1971-1973 in California, with ostensibly the same virus,
respiratory signs were not especially prominent and Utterback Although the vast majority of birds are likely to be susceptible
and Schwartz considered airborne spread to be of low to infection with NDVs of both high and low virulence for
significance (110). chickens, the disease seen with any given virus may vary
enormously from one species to another.
Few studies have attempted to assess the survival of airborne
virus, but Hugh-Jones et al. were able to detect virus 6 4 m but The clinical signs seen in birds infected with NDV vary widely
not 165 m downwind of an infected premises (58). These and are dependent on factors such as the virus, host species,
authors stressed the importance of environmental conditions, age of host, infection with other organisms, environmental
particularly relative humidity, on the likelihood of airborne stress and immune status. In some circumstances, infection
spread. with the extremely virulent viruses may result in sudden, high
mortality with comparatively few clinical signs. Thus, the
When climatic conditions are favourable and poultry farms clinical signs are variable and influenced by other factors so
sufficiently concentrated, as in Northern Ireland in 1973, it is that none can be regarded as pathognomonic. As discussed
difficult not to conclude that airborne spread may play a
significant role in epidemics of ND (74). However, in the
majority of outbreaks there has been no evidence that

452 Rev. sci. tech. Off. int. Epiz., 19 (2)

above, virulent viruses have been divided into those in which Samples from live birds should include both tracheal and
the predominant signs produced in infected chickens are cloacal swabs, the latter should be visibly coated with faecal
intestinal lesions (termed viscerotropic) and those that induce material. Swabbing may harm small delicate birds, but the
mainly neurological signs (neurotropic). However, in practice collection of fresh faeces may serve as an adequate alternative.
such distinctions are not clear cut and may be affected by
other factors. Some viruses produce only mild respiratory Where opportunities of obtaining samples are limited, it is
disease (lentogenic), and others replicate in the intestine with important that cloacal swabs (or faeces) and tracheal swabs
little or no clinical signs (asymptomatic enteric). Viruses in (or tracheal tissue) are examined in addition to organs or
these last two categories are widely used as live vaccines tissues that are grossly affected or associated with clinical
against ND. disease.

Newcastle disease viruses show a considerable range of The samples should be placed in phosphate buffered isotonic
virulence for susceptible hosts such as chickens. In tests used saline (PBS) at pH 7.0-7.4 containing antibiotics. The
to assess virulence, variation generally consists of clusters antibiotics can be varied according to local conditions but
around the two extremes, but for a variety of reasons, some could contain penicillin (2,000 units/ml), streptomycin
viruses may show intermediate virulence (mesogenic). (2 mg/ml), gentamycin (0.05 mg/ml) and mycostatin
Equally, the very virulent viruses may infect and replicate in (1,000 units/ml) for tissues and tracheal swabs, but at
vaccinated birds without causing clinical disease (35, 53, 90). concentrations 5-fold higher for faeces and cloacal swabs. The
pH of the solution must be adjusted to pH 7.0-7.4 following
This enormous variation in virulence and clinical signs means the addition of the antibiotics. Faeces and finely minced
that careful definition of what constitutes ND is necessary for tissues should be prepared as 10%-20% (w/v) suspensions in
the purposes of trade, control measures and policies. the antibiotic solution. Suspensions should be processed as
soon as possible after 1 h-2 h at room temperature. Where this
At the 67th General Session of the OIE held in Paris in May is impracticable, samples may be stored for up to several days
1999, Resolution No. XIII was to amend the definition of ND at 4°C.
in the Manual of Standards for Diagnostic Tests and Vaccines,
Chapter 2.1.15. on Newcastle disease, as follows: The supernatant fluids of faeces or tissue suspensions
obtained through clarification by centrifugation at 1,000 g are
'Newcastle disease issdefined as an infection of birds caused by inoculated into the allantoic sac of at least five embryonated
a virus of avian paramyxovirus serotype 1 (APMV-1) that SPF chicken eggs of nine to eleven days incubation. After
meets one of the following criteria for virulence: inoculation, these eggs are incubated at 35°C-37°C for four to
seven days. Eggs containing dead or dying embryos as they
a) The virus has an intracerebral pathogenicity index (ICPI) arise, and all eggs remaining at the end of the incubation
in day-old chicks (Gallus gallus) of 0.7 or greater. period, should first be chilled at 4°C and the allantoic fluids
tested for haemagglutination (HA) activity. Fluids that give a
Or negative reaction should be passaged into at least (one further
batch of eggs.
b) Multiple basic amino acids have been demonstrated in the
virus (either directly or by deduction) at the C-terminus of the Haemagglutination activity detected in bacteriologically sterile
F2 protein and phenylalanine at residue 117, which is the fluids harvested from inoculated eggs may be due to the
N-terminus of the F1 protein. The term 'multiple basic amino presence of any of the fourteen haemagglutinin subtypes of
acids' refers to at least three arginine or lysine residues influenza A viruses or of the eight other APMV serotypes.
between residues 113 to 116. Failure to demonstrate the Newcastle disease virus can be confirmed by the use of
characteristic pattern of amino acid residues as described specific antiserum in an HI test. Usually chicken antiserum is
above would require characterisation of the isolated virus by used, prepared against one of the strains of NDV.
an ICPI test.

In this definition, amino acid residues are numbered from the Cross-reactions in HI tests between NDV and APMVs,
N-terminus of the amino acid sequence deduced from the particularly APMV-3 may cause some problems which can be
nucleotide sequence of the F0 gene, 113-116 corresponds to resolved by the use of suitable antigen and antiserum controls
residues - 4 to - 1 from the cleavage site' (86).
Pathogenicity indices
Diagnostic tests
The extreme variation in virulence of different NDV isolates
Identification of the agent and the widespread use of live vaccines means that the
identification of an isolate of NDV from birds showing clinical
Samples from dead birds should consist of oro-nasal swabs, as signs does not confirm a diagnosis of ND. As indicated in the
well as samples collected from lung, air sac, intestine
(including contents), spleen, brain, liver and heart tissues.
These may be collected separately or as a pool.

Rev. sci. tech. Off. int. Epiz., 19 (2) 453

definition, an assessment of the virulence of the isolate by ICPI - place 0.025 ml of virus suspension (i.e. infective allantoic
test or amino acid sequencing is also required. fluid) in the first well. For accurate determination of the HA
content, this should be from a close range of an initial series of
The intracerebral pathogenicity index (ICPI) test should be dilutions, i.e. 1/3, 1/4,1/5,1/6, etc.
done as follows:
- two-fold dilutions of 0.025 ml amounts of the virus
- fresh infective allantoic fluid with an HA titre of greater suspension are made across the plate
than 2 4 (1/16) is diluted 1/10 in sterile isotonic saline, with no
additives - dispense a further 0.025 ml of PBS to each well

- 0.05 ml of the diluted virus is injected intracerebrally into - dispense 0.025 ml of 1% v/v chicken red blood cells to each
each of ten one-day-old chicks hatched from an SPF flock well

- the birds are examined every 2 4 h for eight days - mix by tapping the plate gently and allow RBC to settle for
approximately 4 0 minutes (either at room temperature or 4°C
- at each observation the birds are scored: 0 if normal, 1 if if ambient temperatures are high) when control RBC should
be settled to a distinct button
A
- haemagglutination is determined by tilting the plate and
sick, and 2 if dead observing the presence or absence of tear-shaped streaming of
- the ICPI is the mean score per bird per observation over the the red blood cells. The titration should be read to the highest
eight-day period. dilution giving complete haemagglutination (no streaming);
this represents 1 HA unit (HAU) and can be calculated
The most virulent viruses will give indices that approach the accurately from the initial range of dilutions.
maximum score of 2.0, whereas lentogenic strains will give
values close to 0.0. The HI test should be done as follows:

Serological tests - dispense 0.025 ml of PBS into each well of a.plastic
microtitre plate (V-bottomed)
Newcastle disease virus may be employed in a wide variety of
serological tests as an. antigen, enabling neutralisation or - place 0.025 ml of serum into first well of plate
enzyme-linked immunosorbent assays (ELISA) to be used for
diagnosis. At present, the HI test is most widely used. Chicken - make two-fold dilutions of 0.025 ml amounts of the serum
sera rarely give non-specific positive reactions in this test and across the plate-
any pre-treatment o f sera is unnecessary. Sera from species
other than chickens may sometimes cause agglutination of - add 4 HAU of virus/antigen in 0.025 ml to each well and
chicken red cells, so this property should first be determined leave for a minimum of 3 0 minutes at room temperature or
and then removed by absorption of the serum with chicken 60 minutes at 4°C
red cells.
- add 0.025 ml of 1% v/v chicken RBC to each well and after
Variations of the procedures for HA and HI tests are practised gentle mixing allow RBC to settle for approximately
in different laboratories. Examples are given in Chapter 4 0 minutes (either at room temperature or 4°C if ambient
2.1.15. of the Manual of Standards for Diagnostic Tests and temperatures are high) when control RBC should be settled to
Vaccines, as presented below (85). •a distinct button

The following examples apply in the use of V-bottomed - the HI titre is the highest dilution o f serum causing
microwell plastic plates in which the final volume for both complete inhibition of 4 HAU of antigen. The agglutination is
fa types of test is 0.075 ml. The reagents required for these tests assessed more exactly by tilting the plates. Only those wells in
are isotonic saline buffered with phosphate (0.1M) to which the red cells 'stream' at the same rate as the control
pH 7.0-7.2 (PBS) and red blood cells (RBC) taken from a wells (containing 0.025 ml RBC and 0.05 ml PBS only) should
minimum of three SPF chickens and pooled into an equal be considered as showing inhibition
volume of Alsever's solution (if SPF chickens are not available,
blood may be taken from unvaccinated birds regularly - the validity of results should be assessed against a negative
monitored and shown to be free of antibodies to NDV). Cells control serum, which should not give a titre > 2 2 and a.positive
should be washed three times in PBS before use as a 1% control serum for which/ the titre should be within one
(packed cell v/v) suspension. Positive and negative control dilution of the known titre..
antigens and antisera should be run with each test, as
appropriate. The value of serology in diagnosis is clearly related to the
immune status of the affected birds. Haemagglutination
inhibition titres may be regarded as positive if inhibition
occurs at a serum dilution of 1/16 ( 2 4 ) or more against 4 HAU
of antigen.

The HA test should be done as, follows: Haemagglutination inhibition titres may be used to assess the
immune status, of a flock. In vaccinated flocks that are being
- dispense 0.025 ml of PBS into each well of a plastic monitored serologically, anamnestic responses may be
microtitre plate (V-bottomed)

454 Rev. sci. tech. Off. int. Epiz., 19 (2)

observed as the result of a challenge infection with field virus policies. Vaccinal viruses are somewhat different, as some are
(24), but great care should be exercised as variations may sufficiently virulent to cause disease in fully susceptible birds.
occur from other causes. For example, APMV-3 virus
infections of NDV-vaccinated turkeys have been In some countries, legislation is designed to reduce the
demonstrated to result in substantially increased titres to NDV likelihood of outbreaks from specific sources. For example,
(10). on the island of Ireland, legislation requires that poultry feed
is heat-treated to reduce the possibility of introduction of
Other diagnostic methods NDV by this, route.

Nucleotide sequencing On farms, control measures should attempt to prevent virus
from infecting the flock. The practice of good hygiene and
The knowledge that the virulence of NDV strains is primarily biosecurity measures aimed at preventing the introduction of
virus by the routes described above is of paramount
dependent on the amino acid sequence at the FO cleavage site importance on poultry farms.

has led to several suggestions for in vitro tests based on Biosecurity aimed at preventing disease should commence at
the planning stage of commercial poultry farms. Farms and
detection of the nucleotide sequence coding for the FO flocks should be well separated, hatcheries should be isolated
from poultry farms, different species should be reared on
cleavage site to replace the in vivo pathogenicity tests. Studies different sites, and an adequate fresh water supply should be
available, preferably one that does not draw on surface water.
have been performed using molecular techniques to
On poultry farms, the following points should be observed:
determine the FO cleavage site sequence by reverse
a) bird houses, feed stores and water tanks should be
transcription polymerase chain reaction either on the isolated bird-proofed

virus or on tissues and faeces from infected birds (62, 66, 84, b) movements on and off the farm should be kept to a
minimum
102). In other studies, oligonucleotide probes tailored to the
c) all equipment, especially vehicles, should be disinfected
FO cleavage site have been used to detect NDV in tissues and before access to the site is permitted

differentiate between viruses of high or low virulence by d) movements between different farms for egg collection,
carcass collection, feed delivery and similar should be to and
hybridisation techniques (59, 83). With both these from a specified collection and delivery point away from the
poultry flocks.
approaches, problems relating to mismatching of the primers
Visits by personnel such as bleeding or vaccination teams,
or probes or mixtures of virulent and avirulent viruses mean inseminators and veterinarians are the most likely method of
introduction of ND. If such visits are unavoidable, regimens of
that, at present, these in vitro techniques cannot wholly clothing change, equipment disinfection and other basic
hygiene controls must be enforced.
replace the in vivo ICPI test.
Vaccination
Monoclonal antibodies
Vaccination must be regarded as complementing good
Mouse mAbs directed against strains of NDV have been used management, biosecurity and hygiene in rearing domestic
in HI tests to allow rapid identification of NDV without the poultry. Vaccination against ND must never be considered an
possible cross reactions with other PMV serotypes that may alternative to these other measures.
occur with polyclonal sera (6).
Vaccinated birds challenged with virulent NDV may become
Some workers have used mAbs to distinguish between infected and excrete virus, although in relatively small
specific viruses. For example, two groups have described amounts, while remaining apparently healthy (53, 90, 110).
mAbs which distinguish between the common vaccine This disadvantage must be fully understood when planning
strains, Hitchner B l and La Sota (47, 77), while other mAbs the role of vaccination in the control of ND.
can separate vaccine viruses from epizootic virus in a given
geographical area (108). Details of vaccination regimens, methods of administration
and interpretation of antibody titres can be found in the
Panels of mAbs have been used to establish antigenic profiles review by Kouwenhoven (69), the text that follows considers
of NDV isolates based on their ability to react or not with the
viruses. This has proven to be a valuable method for grouping
and differentiating isolates of NDV which has been
particularly valuable in understanding the epizootiology of
outbreaks (20, 21).

Control

Newcastle disease

Policies

In countries or areas that are free of ND, the primary aim
should be to prevent the introduction of the virus. As
migratory and other feral birds frequently carry NDV strains
of low virulence, which occasionally spread to domestic
poultry, such viruses are usually excluded from control

Rev. sci. tech. Off. int. Epiz., 19 (2| 455

the factors leading to the decision to vaccinate and to the presence of enzootic disease is usually linked to severe
choice of vaccine. economic restrictions, which exclude the use of oil emulsion
vaccine. Mesogenic vaccines may have serious clinical effects
Decision to vaccinate if given to birds which have not been immunised previously
National or international legislation or agreements that give and these vaccines are sufficiently virulent to fall within the
the farmer and advisers little choice may control vaccination new OIE definition of ND.
policies. For example, in the Netherlands vaccination is
compulsory, but in countries such as Finland, Sweden and The most popular live vaccines have evolved from field
Norway vaccination is banned. In countries where legislation isolates of low virulence. Most are based on the 'lentogenic'
does not enforce vaccination under normal circumstances, virases Hitchner B l or La Sota or similar viruses, although
'ring vaccination' may be required should an outbreak occur. some 'asymptomatic enteric' viruses have also been used as
live vaccines. A difficulty associated with live vaccines is that
In many countries, severe ND is enzootic, usually in village the immune response appears to be related to the virulence of
chickens at least, and in such high risk areas, vaccination of the virus strain (93); La Sota, for example, gives better
commercially reared birds is the only way to reduce the protective antibody titres than Hitchner B l , but is more likely
disease and losses resulting from the inevitable introduction of to cause reactions. The method of application is important.
virus. In countries where disease is not enzootic, a decision to Individual application methods such as the use of eye drops
include prophylactic vaccination, as part of the control of ND and beak dipping are costly; mass application of live vaccine
is not so clear cut, especially when the cost and the possibility in generated aerosols or sprays or in drinking water is cheaper
of vaccine reactions are considered. In addition, the existence and more convenient. However, sprays and aerosols in
of a slaughter policy based on infection rather than disease, as particular may result in severe respiratory reaction and,
in the EU (41), would seem a disincentive for routine especially with La Sota virus, even high mortality.
vaccination.
The choice of vaccine may also be restricted by control
Choice of vaccine policies, and the Commission of the EU has set criteria
banning the use of live ND vaccines with an ICPI of greater
Choice of vaccine is governed by three main considerations, than 0.4 and inactivated vaccine production from viruses with
namely: an ICPI exceeding 0.7 (42).

a) the immunogenicity of the vaccine Other avian paramyxoviruses

b) the type of vaccine (inactivated or live) The risk from APMV-2 virus may be minimised by prevention
of introduction of the virus by bird-proofing poultry houses
c) the virulence of live vaccine strains, not only for the and good general hygiene practices. Some success has been
recipient host species, but also for other species which may reported in treatment of exacerbating secondary bacterial
become infected due to lateral spread of the vaccine. infections with antibiotics. Samberg et al. reported good
antibody response in day-old turkeys to an inactivated oil
For live vaccines, the likely exacerbation of infections with emulsion APMV-2 vaccine (101).
other organisms or vaccines must also be considered.
Avian paramyxovirus serotype 3 virases appear to spread
Using modem techniques, considerable antigenic diversity slowly. However, good hygiene, including careful disinfection
has been detected in NDV isolates and strains (21). However, and allowing time between restocking, has not always
the vaccines commonly in use are capable of producing prevented infection in subsequent flocks. Inactivated,
antibodies that protect against even those NDV isolates oil-emulsion vaccines are available in the USA, the United
showing a high degree of antigenic divergence from the Kingdom (UK) and other countries of Europe for use in
vaccine strain (16, 19). Thus, antigenic variation does not turkey breeding flocks. These are usually injected twice, four
seem to be an important consideration in the choice of weeks apart, before the birds begin to lay ( 3 2 , 4 9 ) . Duchatel et
vaccine. al. showed that subcutaneous inoculation of parakeets with
APMV-3 vaccine afforded these birds protection from
Although inactivated oil emulsion vaccines can be efficacious challenge with APMV-3 virus (46).
if used as primary vaccine (111), the additional cost of these
vaccines, both direct and in administration, means that live Public health
vaccines are usually preferred. Vaccination programmes for
laying birds are usually based on the use of several doses of
live lentogenic vaccine followed by inactivated vaccine at
point of lay.

Mesogenic live vaccines tend to be used only in countries Newcastle disease virus is a human pathogen. In the UK, the
where virulent NDV is widespread and maintenance of high virus is placed in Hazard group 2 of the Advisory Committee
antibody titres is important to prevent serious disease. The on Dangerous Pathogens (1). The virus is thus considered to

456 Rev. sci. tech. Off. int. Epiz., 19 (2

be: 'A biological agent that can cause human disease and may Human infections with NDV have usually resulted from direct
be a hazard to employees; it is unlikely to spread to the contact with the virus, infected birds or carcasses of diseased
community...'. birds. Human to human spread has never been reported,
although spread by contagion is theoretically possible. The
A review of ND as a zoonosis recorded thirty-five published types of people known to have been infected with NDV
reports of ND virus infections of humans between 1948 and include laboratory workers (usually as a result of accidental
1971 (36). Since that time, few additional reports have been splashing of infective material into the eye), veterinarians in
published, which probably reflects the lack of serious, lasting diagnostic laboratories (presumably as a result of contact with
effects resulting from such infections and the fact that such infective material during post-mortem examinations),
infections are commonplace. workers in broiler processing plants and vaccination crews,
especially when live vaccines are administrated as aerosols or
Reported infections have not been life threatening and usually fine dust. Pedersden et al. reported significantly higher
have not been debilitating for more than one or two days. The antibody titres to NDV in people who had an association with
most frequently reported and best substantiated clinical signs poultry (92).
in human infections have been eye infections, usually
consisting of unilateral or bilateral reddening, excessive No reports exist of other APMV serotypes infecting humans.
lachrymation, oedema of the eyelids, conjunctivitis and However, the potential may exist, and virus of APMV-2
sub-conjunctival haemorrhage (36). Although the effect on serotype has been isolated from cynomolgus monkeys
the eye may be quite severe, infections are usually transient (Macaca fascicularis) (82).
and the cornea is not affected.

Reports of other clinical symptoms in humans infected with
NDV are less well substantiated, but suggest that a more
generalised infection may sometimes occur, resulting in chills,
headaches and fever, with or without conjunctivitis (36). Both
vaccinal strains and strains virulent for poultry may infect and
cause clinical signs in humans.

La maladie de Newcastle et les autres paramyxoviroses aviaires

D.J. Alexander

Résumé

La maladie de Newcastle, conséquence d'une infection par des paramyxovirus de
sérotype 1 (APMV-1), est inscrite sur la Liste A de l'Office international des
épizooties. Très meurtrière dans le passé, la maladie de Newcastle pose toujours
d'importants problèmes à l'aviculture industrielle existante ou naissante de
nombreux pays. Elle a une incidence économique élevée, même dans les pays où
elle est pratiquement maîtrisée, en raison des coûts liés à la vaccination et/ou au
maintien de mesures rigoureuses de biosécurité. La virulence varie selon les
souches du virus de la maladie de Newcastle, et la sensibilité des oiseaux diffère
d'une espèce à l'autre, ce qui entraîne la nécessité de bien définir la maladie
avant toute mesure prophylactique liée aux échanges. Pour confirmer le
diagnostic, il faut isoler et caractériser le virus en cause. L'évaluation de la
virulence se fait habituellement par des r e c h e r c h e s in vivo. Cependant, depuis
que la base moléculaire du pouvoir pathogène du virus est mieux connue, on

Rev. sci. tech. Off. int. Epiz., 19 (2) 457

recourt de plus en plus à la caractérisation génétique in vitro. La prophylaxie de la
maladie repose sur des méthodes visant à empêcher l'introduction et la
propagation du virus ainsi que sur des bonnes pratiques de biosécurité,
associées ou non à la vaccination. Les virus de la maladie de Newcastle peuvent
infecter l'homme, provoquant le plus souvent des cas sporadiques de
conjonctivite, mais aucun cas de transmission entre êtres humains n'a jamais été
observé.
Huit autres sérotypes de paramyxovirus aviaires ont été reconnus (APMV-2 à
APMV-9). La plupart de ces sérotypes semblent spécifiques à des espèces
d'oiseaux sauvages qui leur servent de réservoir naturel, mais d'autres espèces
hôtes sont également sensibles. Seuls les virus APMV-2 et APMV-3 ont eu un
impact sanitaire et économique important sur la production aviaire. Ces deux
types de virus provoquent des maladies respiratoires et une chute de ponte qui
peuvent être graves lorsque d'autres infections ou des f a c t e u r s de stress lié à
l'environnement viennent s'y ajouter. Aucune infection naturelle par les virus
APMV-3 n'a été signalée chez les volailles.

Mots-clés

Diagnostic - Épidemiologie - Étiologie - Maladie - Maladie de Newcastle - Maladies
aviaires - Paramyxovirus aviaires - Prophylaxie - Santé publique.

•

Enfermedad de Newcastle y otros paramixovirus aviares

D.J. Alexander

Resumen

La enfermedad de Newcastle, causada por paramixovirus aviares pertenecientes
al serotipo 1 (APMV-1), figura en la Lista A de la Oficina Internacional de
Epizootias. Esta enfermedad, que históricamente ha tenido efectos devastadores
sobre las poblaciones de aves de corral, sigue constituyendo en muchos países
uno de los principales problemas que afectan a las explotaciones avícolas, tanto
jóvenes como consolidadas. Incluso en países donde la enfermedad puede
considerarse bajo control, las campañas de vacunación y/o el mantenimiento de
estrictas medidas de seguridad biológica siguen suponiendo una pesada carga
económica. La desigual virulencia que presentan las cepas víricas para las aves
de corral, junto con la distinta susceptibilidad de las diversas especies de aves,
hace necesaria una definición muy cuidadosa de la enfermedad a efectos de
control sanitario y de actividad comercial. Para el diagnóstico de confirmación de
la enfermedad de Newcastle es preciso aislar y caracterizar el virus de que se
trate. Para evaluar su virulencia suelen ser necesarias pruebas in vivo. Sin
embargo, ahora que se comprenden más cabalmente los mecanismos
moleculares en los que se funda el poder patógeno del virus, se viene usando
cada vez más su caracterización genética in vitro. La lucha contra la enfermedad
integra desde medidas para prevenir la introducción y propagación del virus
hasta buenas prácticas en materia de seguridad biológica y/o vacunaciones.
Aunque el virus de la enfermedad de Newcastle puede afectar al hombre,
causando en general episodios pasajeros de conjuntivitis, no se ha descrito
ningún caso de transmisión entre seres humanos.
Se conocen otros ocho serotipos de paramixovirus aviares, numerados del
APMV-2 al APMV-9. Buena parte de esos serotipos parecen estar presentes en
reservorios naturales de determinadas especies aviares silvestres, aunque hay

458 Rev. sci. tech. Off. int. Epiz., 19

otras especies que suelen ser susceptibles y pueden ejercer de huéspedes. Sólo
los serotipos APMV-2 y APMV-3 han tenido efectos sanitarios y económicos de
cierta importancia para la producción avícola. Ambos tipos de virus provocan
afecciones respiratorias y caídas de la producción de huevos, que pueden
revestir cierta importancia cuando a sus efectos se suman los de otras
infecciones o presiones ambientales. No se ha descrito ningún caso de infección
natural de pollos por virus del tipo APMV-3.

Palabras clave

Control - Diagnóstico - Enfermedad - Enfermedad de Newcastle - Enfermedades aviares
- Epidemiología - Etiología - Paramixovirus aviares - Salud pública.

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