Characterization of recombinant H9N2 influenza viruses isolated from wild ducks in China.

Wild birds are considered to be the natural reservoirs for avian influenza A viruses (AIV). During active influenza surveillance in Poyang Lake of southeast China, we isolated and characterized 11 H9N2 viruses from two species of wild ducks. Phylogenetic analysis showed that the 11 isolates were almost identical with 99.3-100% nucleotide homology in their entire genome, and they all closely related in whole eight genes (95.6-99.4% homology) to human H9N2 isolates (HK/33982/2009) and clustered in the same sublineage. The isolates belonged to triple reassortant H9N2 genotype viruses containing Ck/Bei-like NA genes, Y439-like PA genes and six other G1-like genes. We also found that the subtype of virus replicated efficiently in the lungs and tracheas of BALB/c mice and caused mortality in 20-40% of infected groups after 3-6 days, which indicates that the subtype of virus is capable of establishing lethal mammalian infections. However, whether or not the virus has features transmittable from wild ducks to humans is not known. This study showed H9N2 subtype avian influenza virus for the first time in wild birds, and suggests that wild birds may carry the virus along migratory routes, highlighting the need for continued surveillance of wild birds.