Since the first human infection case with H7N9 avian influenzavirus(AIVs)wasidentifiedinMarch2013,therehas six viral outbreak waves recorded in China as of September 2018. In total, there have been 1,567 confirmed cases of human H7N9 infection and 615 deaths, the fatality rate is around 40% (http://www.who.int/csr/don/05-september2018-ah7n9-china/en/).Ingeneral,theH7N9virushasposed a great threat in avian and human health. According to molecular characteristics of the virus and the ability to cause diseaseandmortalityinchickens,theAIVsaredesignatedas highly pathogenic avian influenza virus (HPAIV) or low pathogenicity avian influenza virus (LPAIV) (https://www. cdc.gov/flu/avianflu/influenza-a-virus-subtypes.htm). Initially, many studies indicated that these H7N9 viruses isolated from birds in 2013 were low pathogenic, especially formice,andbarelyreplicableinducks(Pantin-Jackwoodet al.,2014;Zhangetal.,2013).However,someH7N9variants isolated from humans during January 2017 in Guangdong Province, China, possessed an insertion of 4 amino acids at the cleavage site of the HA protein, regarded as a molecular marker of HPAIV (Zhang et al., 2017; Zhou et al., 2017b). Animalexperimentsupportedthatthesevariantswereindeed highly pathogenic in chickens and lethal to mice (Qi et al., 2018). As of 5th September, 2018, the H7N9 HPAIVs have been reported in birds or the environment from at least 14 provinces, municipalities or autonomous regions in China