Influenza A(H7N9) virus emerged and resulted in human infections in Chongqing, southwestern China since 2017

Objectives: Influenza A(H7N9) virus has emerged and resulted in human infections in Chongqing, southwestern China since 2017. This study aimed to describe the epidemiological characteristics of the first epidemic in this region.

Methods: The epidemiological data of patients were collected. Live poultry markets (LPMs), commercial poultry farms (CPFs) and backyard poultry farms (BPFs) were monitored, and poultry sources were registered. Samples derived from the patients, their close contacts, and the environments were tested for influenza A(H7N9) virus by real-time reverse transcriptase polymerase chain reaction. Genetic sequencing and phylogenetic analysis were also conducted.

Results: Since the confirmation of the first patient infected with influenza A(H7N9) virus on March 5, 2017, nine patients had been identified within four months in Chongqing. Their mean age was 45 years, 77.8% were male, 66.7% were urban residents and 55.6% were of poultry related occupation. All patients became infected after exposure to live chickens. The median time interval from initial detection of influenza A(H7N9) virus in Chongqing to the patients' onset was 75 days. Since initial detection in February 2017, influenza A(H7N9) virus was detected in 21 (53.8%) counties within four months. The proportion of positive samples was 2.94% (337/11,451) from February 2017 to May 2018, and was higher (χ²=75.78, P<0.001) in LPMs (3.66%, 329/8979) than that in CPFs (0.41%, 5/1229) and BPFs (0.24%, 3/1243). The proportion of positive samples (34.4%, 22/64) at the premises to which the patients were exposed was significantly higher than that (5.7%, 257/4474) in premises with no patients. Phylogenetic analysis indicated that the viruses isolated in Chongqing belonged to the Yangtze River Delta lineage and resembled those circulated in Jiangsu and Anhui provinces between late 2016 and early 2017.

Conclusion: Influenza A(H7N9) virus was newly introduced into Chongqing most likely between late 2016 and early 2017, which swept across half of Chongqing territory and resulted in human infections within months. The most impacted premises and population were LPMs and poultry related workers respectively in the epidemic.