Virulent strains of Newcastle disease virus (NDV) cause Newcastle disease (ND), a devastating disease of poultry and wild birds. Phylogenetic analyses clearly distinguish historical isolates (obtained prior to 1960) from currently circulating viruses of class II genotypes V, VI, VII, and XII through XVIII. Here, partial and complete genomic sequences of recent virulent isolates of genotypes II and IX from China, Egypt, and India were found to be nearly identical to those of historical viruses isolated in the 1940s. Phylogenetic analysis, nucleotide distances, and rates of change demonstrate that these recent isolates have not evolved significantly from the most closely related ancestors from the 1940s. The low rates of change for these virulent viruses (7.05 ؋ 10 ؊5 and 2.05 ؋ 10 ؊5 per year, respectively) and the minimal genetic distances existing between these and historical viruses (0.3 to 1.2%) of the same genotypes indicate an unnatural origin. As with any other RNA virus, Newcastle disease virus is expected to evolve naturally; thus, these findings suggest that some recent field isolates should be excluded from evolutionary studies. Furthermore, phylogenetic analyses show that these recent virulent isolates are more closely related to virulent strains isolated during the 1940s, which have been and continue to be used in laboratory and experimental challenge studies. Since the preservation of viable viruses in the environment for over 6 decades is highly unlikely, it is possible that the source of some of the recent virulent viruses isolated from poultry and wild birds might be laboratory viruses. Infections with virulent strains of Avian paramyxovirus 1 (APMV-1, synonymous with Newcastle disease virus [NDV]), a member of the family Paramyxoviridae, subfamily Paramyxovirinae, and genus Avulavirus (1), cause Newcastle disease (ND) in birds. NDV is a pathogen capable of producing a devastating disease in domestic fowl, with vast social and economic consequences (2). Chickens infected with NDV show a wide spectrum of clinical signs that vary with different virus strains (3). Hanson and Brandly (4) categorize ND viruses into three main pathological groups: lentogens are avirulent and cause mild enteric, respiratory, or subclinical disease (5); mesogens cause disease and death primarily for chickens younger than 8 weeks and produce mainly respiratory disease (6); velogens induce severe systemic infections with high mortality rates (7). According to the World Organisation for Animal Health (OIE) (8), virulent NDV strains, which include both mesogenic and velogenic strains, must meet one of the following criteria: (i) have an intracerebral pathogenicity index (ICPI) in day-old chicks (Gallus gallus) of 0.7 or greater; or (ii) have multiple basic amino acids at the C terminus of the F2 protein and phenylalanine at residue 117, which is the N terminus of the F1 protein. The term "multiple basic amino acids" refers to the presence of at least three arginine or lysine residues from positions 113 through 116. More info...

Newcastle disease (ND), caused by virulent strains of Newcastle disease virus (NDV), is a devastating disease of poultry worldwide. The pathogenesis of ND in quail is poorly documented. To characterize the ability of virulent NDV strains to replicate and cause disease in quail, groups of 14 two-week-old Japanese quail ( Coturnix japonica) were experimentally inoculated with 10 EID (embryo infectious dose 50%) units of 1 of 4 virulent NDV strains: 2 isolated from quail ( N2, N23) and 2 from chickens ( Israel, Pakistan). At day 2 postinfection, noninfected quail (contact group) were added to each infection group to assess the efficacy of virus transmission. Tested NDV strains showed moderate pathogenicity, with highest mortality being 28% for the N2 strain and below 10% for the others. Two N2-inoculated birds showed neurological signs, such as head tremor and ataxia. Microscopic lesions were present in N2-, Israel-, and Pakistan-inoculated birds and consisted of nonsuppurative encephalitis. Contact birds showed no clinical signs or lesions. In both inoculated and contact birds, virus replication was moderate to minimal, respectively, as observed by immunohistochemistry in tissues and virus isolation from oropharyngeal and cloacal swabs. Strains originally isolated from quail resulted in higher numbers of birds shedding in the inoculation group; however, transmission appeared slightly more efficient with chicken-derived isolates. This study shows that virulent NDV strains have limited replicative potential and mild to moderate disease-inducing ability in Japanese quail.

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