Global Patterns of Geographic Distribution, Temporal Trends, Host Spectrum, and Molecular Variation of H9N2 Avian Influenza Virus, 1966–2023

The H9N2 avian influenza virus (AIV) is currently widespread globally and poses a serious threat to the poultry industry and public health; however, its global epidemiological characteristics and key molecular mutation patterns have not yet been systematically elucidated. Based on the NCBI Influenza Virus Resource Database, this study collected relevant data on the H9N2 AIV from around the world between 1966 and 2023. It conducted a comprehensive analysis of the geographic distribution of 11,933 strains, the temporal distribution of 11,824 strains, the host sources of 11,756 strains, and the genetic polymorphisms at key functional sites. The results showed that 90.14% of H9N2 AIV sequences in the dataset originated from Asia, with China contributing the largest number of sequences. The global sequence availability pattern underwent four phases: sporadic sequence availability, gradual increase, high sequence availability, and subsequent decline, with 2007–2018 representing a period of relatively high availability of publicly accessible sequences. Host analysis indicated that H9N2 AIV sequences were predominantly derived from poultry-related hosts, with chickens being the main host source in the dataset, and available sequences have also been reported from various mammals, including pigs and humans, as well as environmental media. Molecular analysis revealed that receptor-binding-related sites in the HA protein, such as Q226L and I155T, have become dominant variants, while mammalian-adaptive mutations such as PB2 E627K and D701N remain at low frequencies; M2 S31N is widely prevalent, whereas the detection rate of NA drug-resistance-associated mutations is low. This study systematically reveals the global distribution patterns of available H9N2 AIV sequences, host distribution characteristics, and key molecular mutation patterns of the H9N2 AIV, providing a scientific basis for cross-host transmission risk assessment, molecular surveillance, and targeted prevention and control.