Molecular Evolutionary and Amino Acid Characteristics of Newly Isolated H9N2 Avian Influenza Viruses in Liaoning Province, China

H9N2 is widespread in nature and is sporadically detected in many poultry and even human beings. Though this subtype could not lethality to poultry or human, it caused great financial losses for farmers and threaten to human health. In this study, ten new-reassortment H9N2 avian influenza viruses (AIVs) were isolated from domestic birds in Liaoning Province between March 2012 and October 2014. Nucleotide sequence comparisons indicated that the internal genes of one strain were highly similar to those of human H7N9 viruses, which was another proof on H9N2 as H7N9 donor’ gene. Amino acid substitutions and deletions found in the HA and NA proteins separately, which indicated that all ten of these isolates may have an enhanced ability to infect humans and other mammals. A cross-hemagglutinin inhibition assay conducted with two vaccine strains that are broadly used in China suggested that antisera against vaccine candidates could not provide complete inhibition of the new isolates. Three of the ten viruses tested could replicated in respiratory organs of infective BALB/c mice without adaption, which suggested their potential capacity for directly infecting 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.