Newcastle disease (ND) is one of the most important diseases of poultry, negatively affecting poultry production worldwide. The disease is caused by Newcastle disease virus (NDV) or avian paramyxovirus type 1 (APMV-1), a negative-sense single-stranded RNA virus of the genus Avulavirus , family Paramyxoviridae . Although all NDV isolates characterized to date belong to a single serotype of APMV-1, significant genetic diversity has been described between different NDV isolates. Here we present the complete genome sequence and the clinicopathological characterization of a virulent Newcastle disease virus isolate (NDV-Peru/08) obtained from poultry during an outbreak of ND in Peru in 2008. Phylogenetic reconstruction and analysis of the evolutionary distances between NDV-Peru/08 and other isolates representing established NDV genotypes revealed the existence of large genomic and amino differences that clearly distinguish this isolate from viruses of typical NDV genotypes. Although NDV-Peru/08 is a genetically distinct virus, pathogenesis studies conducted with chickens revealed that NDV-Peru/08 infection results in clinical signs characteristic of velogenic viscerotropic NDV strains. Additionally, vaccination studies have shown that an inactivated NDV-LaSota/46 vaccine conferred full protection from NDV-Peru/08-induced clinical disease and mortality. This represents the first complete characterization of a virulent NDV isolate from South America.

Newcastle disease, one of the most important health problems that affects the poultry industry around the world, is caused by virulent strains of Newcastle disease virus. Newcastle disease virus is considered to be endemic in several countries in the Americas, including Mexico. In order to control Newcastle disease outbreaks and spread, intensive vaccination programs, which include vaccines formulated with strains isolated at least 60 years ago, have been established. These vaccines are dissimilar in genotype to the virulent Newcastle disease viruses that had been circulating in Mexico until 2008. Here, 28 isolates obtained between 2008 and 2011 from different regions of Mexico from free-living wild birds, captive wild birds, and poultry were phylogenetically and biologically characterized in order to study the recent epidemiology of Newcastle disease viruses in Mexico. Here we demonstrate that, until recently, virulent viruses from genotype V continued to circulate and evolve in the country. All of the Newcastle disease viruses of low virulence, mostly isolated from nonvaccinated free-living wild birds and captive wild birds, were highly similar to LaSota (genotype II) and PHY-LMV42 (genotype I) vaccine strains. These findings, together with the discovery of two virulent viruses at the Mexican zoo, suggest that Newcastle disease viruses may be escaping from poultry into the environment.

To characterize the clinicopathologic features of recently described genotypes of Newcastle disease virus (NDV), 1 representative strain of genotype XIV and 2 of genotype XVII, all isolated from West Africa, were used to infect groups of ten 4-week-old specific pathogen-free chickens. The pathobiology of these 3 strains was compared to a South African NDV strain classified within genotype VII. All chickens infected with the 4 viruses died or were euthanized by day 4 postinfection due to the severity of clinical signs. Gross and histologic lesions in all infected chickens included extensive necrosis of lymphoid tissues (thymus, spleen, bursa of Fabricius, cecal tonsils, gut-associated lymphoid tissue), gastrointestinal necrosis and hemorrhages, and severe hemorrhagic conjunctivitis. Immunohistochemical staining revealed systemic viral distribution, and the most intense staining was in the lymphoid organs. Results demonstrate that the 3 West African strains from the previously uncharacterized genotypes XIV and XVII are typical velogenic viscerotropic NDV strains with lesions similar to the South African strain. Under experimental conditions, QV4 and LaSota NDV vaccine strains successfully protected chickens from morbidity and mortality against the genotype VII and one genotype XVII NDV strain, with no significant differences in the amount of virus shed when 2 vaccine schemes were compared.

Strains of Newcastle disease virus (NDV) have different abilities to elicit neurologic signs. To determine the capacity of different NDV strains to replicate and cause lesions in the brain, independently of their peripheral replication, 1-day-old chickens were inoculated in the subdural space with 7 NDV strains of different virulence (4 velogenic, 2 mesogenic, 1 lentogenic). Velogenic strains induced severe necrotizing and heterophilic ventriculitis and meningitis, as well as edema of the neuroparenchyma, and replicated extensively in the nervous tissue by day 2 postinfection, as demonstrated by immunohistochemistry, when all infected birds died. Clinical signs, microscopic lesions, and viral replication were delayed (days 3 and 4 postinfection) with mesogenic strains. Velogenic and mesogenic NDV strains replicated mainly in neurons, and immunolabeling was first detected in surface-oriented areas (periventricular and submeningeal), possibly as a reflection of the inoculation route. The lentogenic NDV strain did not cause death of infected birds; replication was confined to the epithelium of the ependyma and choroid plexuses; and lesions consisted of lymphoid aggregates limited to the choroid plexuses. Results show that extensive NDV replication in the brain is typical of velogenic and mesogenic, but not lentogenic, NDV strains. In addition, this study suggests that differences in the rate of NDV replication in nervous tissue, not differences in neurotropism, differentiate velogenic from mesogenic NDV strains. This study indicates that intracerebral inoculation might be used as an effective method to study the mechanisms of NDV neuropathogenesis.