Characterization of virulent Newcastle disease viruses from vaccinated chicken flocks in Eastern China

BACKGROUND Newcastle disease (ND) is one of the most contagious and devastating diseases to poultry in the world. The causative agents are virulent strains of Newcastle disease virus (NDV), which belong to the genus Avulavirus, sub-family Paramyxoviridae, family Paramyxovirinae. Knowing the genomic and antigenic characteristics of virulent NDVs might contribute to ND control in China. RESULTS The results showed that all of the virulent strains belonged sub-genotype VIId shared the same cleavage site (112)RRQKR/F(117) in the fusion protein. At least 69 % (38 of 55) of the NDV strains possessed E347K variation in the hemagglutinin-neuraminidase protein. The cross-neutralization tests confirmed that the strains harboring 347 K showed lower antigenic relatedness with LaSota. Furthermore, the immune-challenge experiment indicated that LaSota could not provide complete protection against infection with the E347K variant NDVs as the vaccinated birds were still able to be infected and shed virulent challenge viruses. CONCLUSIONS Currently, sub-genotype VIId NDVs are the prevalent virulent strains circulating among vaccinated chicken flocks in Eastern China. Our findings indicated that the E347K variation in HN gene would expand the antigenic difference with LaSota, which may be responsible for the increasing isolation rate of these strains from vaccinated chickens.