Abstract:[Background] H9N2 avian influenza viruses are widespread in chickens and cause great losses. [Objective] Elucidate the sequence characteristics and the variation of antigenicity of H9N2 subtype avian influenza virus (AIV) in Hebei province to understand heterologous protection of AIV. [Methods] seven strains of H9N2 subtype AIV were isolated and identified from farms in Hebei province in 2017. We sequenced the HA genome of 7 H9N2 isolated strains. Then, the phylogenetic trees, amino acid in key functional sites and antigenicity were analyzed. [Results] The homology of HA genes for 7 isolated strains was between 95.5% and 97.2%. Comparing with the epidemic strains before 2016, the cleavage sites and the glycosylation sites were not significantly different from those of the previous strains, except the receptor binding sites of the HA proteins. The antigenicity of the isolated strains changed and a new antigen group formed. The analysis of the HA amino acid sites related to the antigenicity shows that there was a significant mutation in 9 antigenic sites of the newly isolated strains, which may be the molecular basis of antigenic variation. [Conclusion] The prevalent strains of H9N2 subtype AIV in laying-hen farms in Hebei province have mutated in key functional areas and presented antigenic variation, reminding us the importance of continuous surveillance of the genetic evolution and antigenic variation of H9N2 AIV. Changing the candidate strain of vaccine in time is important for the prevention and control of H9N2 AIV.
