Characterization of genotype IX Newcastle disease virus strains isolated from wild birds in the northern Qinling Mountains, China

This study was conducted to evaluate the virulence and evolution of genotype IX Newcastle disease virus (NDV) isolates obtained from wild birds in the northern Qinling Mountains of China. Five isolates were obtained from 374 larynx and cloacae swabs, which were collected from multiple asymptomatic wild bird species from August 2008 to July 2011, and were subsequently characterized by pathotype and genotype. Deduced amino acid sequences revealed that all five NDV isolates exhibited velogenic fusion protein cleavage sites motif ¹¹²R-R-Q-R-R-F¹¹⁷, shared as high as 99.8–99.9 % homology with each other, and varied in pathotype by intracerebral pathogenicity indices (ICPI) of 0.425–1.638. Phylogenetic analysis showed that all five isolates were clustered to genotype IX NDV. This is the first study to confirm multiple asymptomatic wild bird species as natural carriers of virulent genotype IX NDV. A novel NDV isolate from the Spotted-necked Dove (family Columbidae) exhibited discordance between its lentogenic ICPI and its virulent proteolytic cleavage site motif ¹¹²R-R-Q-R-R-F¹¹⁷. Although the five isolates underwent several amino acid mutations in the fusion protein, evidence of continuous evolutionary divergence did exist in the genotype IX NDV, which was always regarded as a conservative genotype.