Influenza A viruses belong to the Orthomyxoviridae family which comprises seven genera: Influenza virus A, B, C, and D, Thogotovirus, Isavirus and Quaranfilvirus [1, 2]. The influenza A viruses are further classified into subtypes based on the antigenicity divergence and sequence comparison of the two viral surface glycoproteins, HA and NA. Currently, 18 HA and 11 NA subtypes have been identified [3, 4]. According to epidemiological features, influenza in humans can be described as seasonal, pandemic or human-avian influenza. At present, seasonal A influenza is caused by the viruses of subtypes A(H1N1) and A(H3N2). Influenza pandemics occur when new strain/subtype of virus emerges. Large population will be infected causing high levels of mortality. The disease produced by the avian influenza viruses (AIV) that transmitted across species to humans is called human-avian influenza. The two most important AIV causing human threats are the H5N1 and H7N9 subtypes. The ongoing circulation of these viruses continues to pose a pandemic threat due to their rapid geographical expansion and genetic diversity, and may eventually the adaptation to humans which may result in human-to-human transmission [5].
Since March 2013, novel strains of H7N9 AIV have emerged and spread rapidly across mainland China. As of 4December 2019 H7N9 AIV has caused six epidemic waves with 1568 laboratory-confirmed cases. However, the H7N9 virus is still confined to China, with the exception of a few cases who had history of travelling to China [6]. In the first four waves, the geographic distribution of H7N9 outbreaks was much more limited to the southeast coastal area from near the Yangtze River delta (YRD) to farther south around the Pearl River delta (PRD). Few cases in inland areas were reported but the geographical distribution of the epidemic had clearly expanded [7, 8]. The fifth epidemic of H7N9 AIV infection in China broke out on October 1, 2016, and continued to spread during 2017. As of September 30, 2017, 766 people had been virologically confirmed, accounting for nearly half of all human cases reported since 2013 [9]. In wave five, there were eight provinces with newly emerged human H7N9 AIV infection, namely, including Chongqing, Gansu, Inner Mongolia, Shaanxi, Shanxi, Sichuan, Tibet, and Yunnan Province, which are areas of Western or Northern China [10].
Yunnan Province, southwest China (Fig. 1), did not experience of human infection during the first four epidemic waves of the H7N9 virus. However, two imported human H7N9 cases were detected in Kunming City, the capital of Yunnan, in February 2017 [11]. Four months later, indigenous human cases of H7N9 virus infection were demonstrated in the urban area of Wenshan City of the province. The abrupt emergence of human infection has attracted considerable attention on the current prevention and control strategies.
In this study, we investigated the epidemiological characteristics of patients infected with H7N9 AIV, compared the genetic features of local viral isolates with other viral strains in the 5th epidemic wave to determine the origins and evolution of the H7N9 virus, and discussed the effectiveness of current control measures in Wenshan City.