Epidemiological and clinical characterization of human infections with H9N2 avian influenza virus in Changsha city, China, 2015-2025.

H9N2 avian influenza virus poses a persistent public health concern due to its zoonotic potential. This study investigated the epidemiological and clinical characteristics of human H9N2 avian influenza infection in Changsha City, along with related environmental surveillance data, to provide a scientific basis for disease prevention and control. From January 2015 to December 2025, a total of 11 human H9N2 avian influenza cases were collected from historical surveillance data of the Changsha CDC in Changsha City. Cases occurred sporadically throughout the year; most (90.91%, 10/11) were mild, one severe case was co-infected with two types of respiratory pathogens, and the majority (72.73%, 8/11) were children under 5 years old. Among the 10 cases with available exposure history, 70.00% had been exposed to live poultry or LPMs, with the H9 and N2 subtypes detected in related environmental samples at rates of about 76%. Genetic analysis showed that the H9N2 viruses from human cases were highly homologous to H9N2 viruses found in the live poultry environment in Changsha City, while also exhibiting diverse mutations in key functional sites and domains. Environmental samples from large live poultry markets exhibited a significant year-on-year increase in positive rate (χ2=111.30, P<0.001), resulting in a substantially higher average rate in 2020-2024 versus 2014-2019 (77.11% vs. 47.43%; χ2=158.24, P<0.001). In conclusion, sporadic human H9N2 infections, predominantly affecting children, were mostly associated with live poultry exposure, with no evidence of human-to-human spread, while environmental data revealed a sharply increasing contamination trend in markets, pointing to a heightened risk.