Evolved avian influenza virus (H7N9) isolated from human cases in a middle Yangtze River city in China, from February to April 2017.

Seven cases of avian influenza A H7N9 virus infection were reported from February to April 2017 in Changsha City. Viral genome was acquired by RT-PCR, aligned with other H7N9 viruses using Clustal W, and phylogenetic trees were constructed using the neighbor-joining method. Our results showed the representativeness of H7N9 virus infections in Middle Yangtze River City. The hemagglutinin segment contained Thr160Ala, Gly186Val and Gln226Leu substitutions, which are associated with increased binding affinity in humans. Phylogenetic analysis indicated that H7N9 viruses had an avian origin, and belonged to the Yangtze River Delta lineage. The proportion of PB2 Ala588Val substitutions in viruses revealed a significantly increasing in recent years, from 0.8 % (1 of 128 cases) to 84.9 % (275 of 324 cases). The data indicate that H7N9 viruses may be more capable of infecting mammals, even though they are still considered low pathogenic avian influenza virus. Hence, the prevalence and genetic evolution of this virus should be closely monitored to prevent more severe human pandemics.