| Newcastle disease virus (NDV), also known as avian paramyxovirus-1 (PPMV-1), is a member of the Avulavirus genus in the Paramyxoviridae family. The pathogenicity of NDV is categorized as velogenic, mesogenic, or lentogenic based on the disease severity in chickens. Although NDV has only one serotype, substantial genotypes and subgenotypes have been recognized. The genotype VI NDV could be classified into subgenotype Ⅵa to Ⅵi as reported recently. Subgenotype VI b strains were pigeon-origin isolates and exhibited host-specificity to pigeons, while most subgenotypes Ⅵa and Ⅵc to Ⅵi NDVs were isolated from chickens. In this study, the biological characteristics of six genotype VI isolates were examined, and four isolates belonging to different subgenotype were subjected to the pathogenicity tests in chickens. In addition, the phylogenic analysis of genotype VI NDVs were conducted in this paper and the characteristic genetic differences between the pigeon-origin and chicken-origin genotype VI strains were identified.1. Biological characterization of different subgenotype VI NDVsPathogenicity of six genotype VI isolates showed that the MDT and ICPI of chicken-origin isolates were under 60 h and over 1.7, which indicated that these five chicken-origin NDVs were the virulent ones. Among these viruses, Kuwait strain showed lower virulence in terms of its MDT and ICPI. In addition, the MDT, ICPI and IVPI values of JS/07/04/Pi were 88 h,1.2 and 0.89, respectively, indicating that this strain was mesogenic. The experimental infection with Kuwait (Ⅵa), JX-1/94 (VIf) and ZhJ3 (Ⅵg) in chickens could induce typical ND clinical signs and generate 100% morbidity and mortality. Severe gross lesions and histopathology changes were present in various visceral organs, and especially, the necrosis was occurred in the spleen, thymus and bursa. The ratio of virus shedding in oropharyngeal and cloacae reached 100% at 5 days post-infection, and the peak of viral titration in tissues also appeared at this point. However, no clinical signs or gross lesions were present in the chickens inoculated with pigeon strain JS/07/04/Pi. These results indicated that except the subgenotype VIb NDV of pigeon-origin, the other subgenotype NDVs belonging to genotype VI exhibited high pathogenicity in chickens, however, the virulence of earlier subgenotype Ⅵa strain Kuwait was relatively lower.2. Phylogenetic analysis of genotype VI NDVsThirty genotype VI NDVs were chosen for phylogenetic analysis based on the F gene sequences. The results showed that the NDVs isolated in the 1980s and before exhibited close phylogenetically, irrespective of their subgenotypes and origin, and the nucleotide and amino acid sequence idenitities among them were 92.5～100% and 92.7-100%, respectively. Both the pigeon-origin and chicken-origin strains displayed high homology with subgenotype Ⅵa strains isolated in the 1960s, which indicated that the genotype VI viruses isolated in the 1980s were likely derived from this subgenotype. However, the genetic distance of the viruses isolated after the 1980s was expanded. According to the above analysis, we inferred that the early genotype VI NDVs derived from the subgenotype Ⅵa strains and evolved into different subgenotype gradually. Also, the same conclusion can be drawn from phylogenetic analysis with complete genomes of 40 genotype VI NDVs. The results of phylogenetic analysis based on the whole genome sequences was consistent with those based on the coding regions of each gene, from which we concluded that the genotype VI NDV evolved on the whole genome level uniformly. Furthermore, to compare the genetic difference between pigeon-origin and chicken-origin strains, the nucleotide and residue substitution of 40 strains of genotype Ⅵ NDVs were analyzed. Compared with the chicken-origin strains, the nucleotide mutation A32G in the leader sequence together with several specific amino acids substitutions in the six coding proteins of pigeon-origin strains were identified. |
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