Characterizations of H4 avian influenza viruses isolated from ducks in live poultry markets and farm in Shanghai

H4 avian influenza virus is one of the most prevalent influenza virus subtypes in birds. The evolution and pathogenicity of H4 AIV in domestic birds of China remain largely unclear. In the present study, a total of eight H4 AIV strains isolated in duck farm and live poultry markets (LPM) were characterized. Phylogenetic analysis indicated that these strains are divided into two groups in the Eurasian lineage. Eight genes of MH-2/H4N6 isolated from a duck farm were closely related to three H4N6 viruses from LPM, suggesting a potential AIV link between farms and LPMs. Additionally, the HA, NA, PB2, NP, and NS genes of two other H4N6 viruses isolated in LPM clustered with that of MH-2/H4N6. However, the remaining genes were more closely related to other sublineages, suggesting that MH-2/H4N6-originated viruses reassorted with other viruses in LPM. All H4 viruses replicated in mouse lungs without prior adaptation and all viruses replicated and transmitted among ducks. 29-1/H4N2, MH-2/H4N6, and 420-2/H4N6 viruses caused systemic infection in infected ducks. However, most of the viruses were not adapted in chickens. The present results indicate a potential correlation of AIV between LPMs and farms and suggest that active surveillance of AIV in LPM is warranted in China.