Thirteen prevailed Newcastle-disease viruses (NDV) isolated in China during 2001-2004 were purified by chick embryo fibroblast (CEF) plaque assay and characterized pathotypically and genotypically. The biological tests showed that these viruses were highly virulent. Sequence analysis based on the variable region (nucleotide 47-420) of the F gene indicated that of the 13 NDV isolates 2 belonged to genotype II, 2 to genotype IX and 9 to genotype VII. Isolates with genotype VII shared 94.6%-99.3% nucleotide (nt) homology with the F gene, whereas for genotype VII and La Sota was only 82.7%-84.1%. In addition, these NDV isolates all shared 95.2%-100% nt homology with the hemagglutinin-neuraminidase (HN) gene, whereas only 79.1%-84.3% compared these viruses with La Sota. The cross neutralization assays were done using positive serums in specific pathogen free (SPF) chicken embryos respectively. Correlation of the neutralization index in chicken embryo with the homologies of F and HN gene of different NDV isolates were analyzed by SPSS8.0 software. The result showed that the neutralization index was closely correlated with nt sequence (P < 0.01, r = 0.35) or deduced amino acid sequence (P < 0.01, r = 0.34) of the HN gene, whereas weekly correlated (P < 0.05, r = 0.20 or 0.19) with the F gene, and non-correlated with 374 nt segment. This implied that the genetic mutations of HN resulted in antigenic variations of these viruses and the search for new vaccines would be necessary.
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