A virulent Newcastle disease virus strain was isolated from diseased chickens in Shanghai, China. The isolated strain was initially characterized as highly virulent because of a short mean death time in embryonated chicken eggs and specific-pathogen-free chickens and was typed as neurotropic by intracloacal inoculation of chickens. The isolated strain had a dibasic amino acid motif in the fusion protein cleavage site sequence required for systemic replication in the host cell. The strain fell into subgenotype VIId by phylogenetic analysis of the fusion protein gene. Although these results demonstrated some sequence similarity between the isolated strain and strains responsible for outbreaks of Newcastle disease in China and Taiwan, the unusually high mortality (86.4%) set this strain aside from other VII strains. Finally, a cross-protection assay demonstrated that La Sota and clone 30 live vaccines could not protect chickens from infection with the isolated strain, with a zero survival rate being observed when chickens were challenged with a high dose of virulent VIId virus.
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Vol. 55 • No. 2