Page 71 - Avian Virology: Current Research and Future Trends

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not show any clinical signs. Very little is known about the host Pigeon paramyxovirus type 1
specificity of NDV strains circulating in wild bird populations. PPMV-1 is a host range variant of NDV. It is closely related genet-
It is likely that an NDV strain circulating in one wild bird spe- ically and antigenically to NDV. However, PPMV-1 and NDV
cies has evolved genetically to grow better in that species versus can be readily distinguishable using NDV monoclonal antibod-
another. ies. The genome of PPMV-1 was found to be 15,192 nt in length
Most NDV infections in wild birds do not result in disease, (Ujvari, 2006). All PPMV-1 strains contain 6-nt insertion in the
but infected birds support viral replication, mount an immune 5′ non-coding region of the N gene. These viruses have a recog-
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response and shed virus in faeces and body fluids. Among wild nized virulence pattern (G/R)-R-Q-K-RF at the F protein
birds, double-crested cormorants and rock pigeons are the only cleavage site. The F and HN genes of PPMV-1 strains are 83–84%
known reservoirs of virulent NDV strains. They maintain virus similar at nt level with those of NDV strain LaSota. According
strains pathogenic to themselves and to other avian species. to phylogenetic analysis, the majority of PPMV-1 isolates cluster
Mortality in juvenile cormorants by NDV was first recognized into class II genotype VI, and most belong to subgenotypes VIa
in Canada in 1990 (Wobeser et al., 1993). Since then outbreaks and VIb (Dimitrov et al., 2016). The disease caused by PPMV-1
in cormorants have occurred in Canada and the USA (Heck- in pigeons was first reported in the Middle East in late 1970s
ert et al., 1996; Kuiken et al., 1998; Glaser et al., 1999). To (Kaleta et al., 1985). An epizootic of ND caused by PPMV-1
date, cormorants is the only wild bird species in which large- occurred in racing pigeons in 1980s in Europe, which reached
scale mortality ranging from < 1% to 92% has been reported panzootic proportions (Vindevogel and Duchatel, 1988). It was
(Kuiken, 1999). Pigeon is another species in which a natural found that a single strain of PPMV-1 was responsible for the
variant of NDV, known as PPMV-1, has been shown to cause world-wide epizootic among racing pigeons (Pearson et al., 1987).
high morbidity varying from 30% to 70% and low mortality PPMV-1 was first reported in UK in 1983 and the virus has been
rarely exceeding 10% (Vindevogel and Duchatel, 1988). isolated every year since then. All APMV-1 strains isolated in
Highly virulent NDV strains have also been isolated from UK are genetically closely related (Aldous et al., 2014). In North
dead, sick and asymptomatic psittacines and other pet bird spe- America, PPMV-1 was first reported in Florida in the 1980s and
cies imported into various countries (Panigrahy et al., 1993; now is present enzootically in pigeons and doves (Chong et al.,
Clavijo et al., 2000). Although it is not known if these viruses 2013). PPMV-1 causes high mortality in Columbiformes but can
were transmitted from poultry or natural infection in the wild, be transmitted to other avian species including poultry and cause
it is assumed that wild bird population of these species can fatal disease. The disease in pigeons is characterized by neuro-
carry virulent NDV strains. logical signs including head tremor, ataxia and paralysis of the leg
The role of wild birds in NDV outbreaks in poultry is not fully and wings, circling and in some cases blindness (Vindevogel and
understood but there is an epizootiological link between wild Duchatel, 1988; Kuiken, 1999). Although the disease is enzootic
birds and poultry originated viruses. Lentogenic NDV strains in domestic pigeons, wild pigeons, doves and exotic birds are
isolated from poultry in live bird markets were phylogenetically also affected. There is regular exchange of PPMV-1 strains among
related to NDV strains isolated from wild birds, suggesting that domestic pigeons, wild pigeons and doves. It has been suggested
they may have been transmitted from wild birds (Kim et al., that PPMV-1 strains isolated from doves are genetically distin-
2007b; Mia Kim et al., 2008). There is also circumstantial evi- guishable from those isolated from racing pigeons indicating
dence implicating wild birds in the spread of virulent NDV to host specificity in PPMV-1 strains (Terregino et al., 2003). The
poultry (Heckert et al., 1996; Alexander, 2001). Two outbreaks pathogenicity of PPMV-1 strains has been reported to be vari-
of ND on poultry farms in Ireland in 1990 (Alexander et al., able in chickens (Alexander and Parsons, 1984; Kommers et al.,
1992) were caused by velogenic isolates, that were genetically 2001). Although the fusion (F) proteins of all PPMV-1 strains
and antigenically very similar to low virulence viruses isolated contain a polybasic cleavage site motif, most strains cause mild
from wild waterfowl (Collins et al., 1998). Moreover, virulent or no disease in chickens and have low intracerebral pathogenic-
viruses isolated during outbreaks in 1998 to 2000 in Australia ity index (ICPI) values, indicating viral factors other than the F
were genetically very similar to viruses isolated from wild birds protein cleavage site sequence are necessary for pathogenicity in
(Gould et al., 2001). Infected wild birds were suspected to be chickens. Increased pathogenicity in chickens has been found
the source of virus in two ND outbreaks in gamebird flocks in when PPMV-1 is passaged in chickens or in embryonated eggs
the UK in 2005 and 2006 (Irvine et al., 2011). NDV vaccine (Alexander and Parsons, 1984; Kommers et al., 2001; Fuller et al.,
strains used in poultry production have been isolated from wild 2007). Several ND outbreaks in chickens have been attributed to
birds (Cardenas Garcia et al., 2013), suggesting that there is PPMV-1 (Pearson et al., 1987; Alexander et al., 1998). Therefore,
regular exchange of NDV between wild birds and poultry. The PPMV-1 is a constant threat to poultry populations, including
potential of wild waterfowl-derived low-virulence NDV strains gamebirds.
to become highly virulent has also been shown experimentally
by passaging in chickens (Shengqing et al., 2002; Tsunekuni
et al., 2010). These findings suggest that wild birds are a con- Molecular basis of NDV virulence
stant threat to domestic poultry and efforts should be made to NDV isolates cause a broad range of clinical disease in chickens,
restrict contact between wild birds and poultry. varying from asymptomatic to 100% mortality. Historically,