Molecular evolution and amino acid characteristics of newly isolated H9N2 avian influenza viruses from Liaoning Province, China

H9N2 is widespread among poultry and humans. Though this subtype is not lethal to either species, it can cause considerable financial losses for farmers and threaten human health. In this study, 10 new H9N2 avian influenza viruses (AIVs) produced by reassortment were isolated from domestic birds in Liaoning Province between March 2012 and October 2014. Nucleotide sequence comparisons indicate that the internal genes of one of these strains are highly similar to those of human H7N9 viruses. Amino acid substitutions and deletions occurred in the HA and NA proteins separately, indicating that all 10 of these isolates may have an enhanced ability to infect mammals. A cross-hemagglutinin inhibition assay conducted with two vaccine strains that are broadly used in China suggests that antisera against vaccine candidates cannot completely inhibit the new isolates. Two of the 10 newly isolated viruses could replicate in respiratory organs of infected BALB/c mice without adaption, suggesting that these isolates can potentially infect mammals. The continued surveillance of poultry is important to provide early warning and control of AIV outbreaks. Our results highlight the high genetic diversity of AIV and the need for more extensive AIV surveillance.