Background
H9N2 avian influenza virus (AIV) has been widely spreading among poultry populations for decades. The decline in poultry production performance and the cross-species adaptability it causes will continue to pose threats to the poultry industry and public health. To investigate the prevalence of H9N2 AIV in southeastern China, this study conducted an epidemiological investigation of H9N2 AIV circulating in live poultry markets in Fujian Province, selecting three representative H9N2 virus strains for molecular evolutionary and biological characterization.

Results
Analysis of the three strains revealed that their HA genes all belonged to the typical B-like lineage, and the NA genes of strains FZ26 and FZ83 originated from the H6N2 subtype, indicating frequent natural reassortment events, and molecular marker analysis revealed mutations at key receptor-binding sites that enhance affinity for human-type receptors. Animal experiments demonstrated that these three H9N2 viruses exhibited certain pathogenicity in chickens but did not cause mortality, suggesting they are low pathogenic viruses. Infected chickens displayed mild clinical symptoms, mainly respiratory-related signs such as head shaking and sneezing. Viral load detection revealed that all three viruses replicated efficiently in the respiratory tissues of chickens, especially in the trachea and lungs. The virus persisted in oropharyngeal and cloacal secretions, with oropharyngeal swabs showing significantly higher viral loads than cloacal swabs, and virus shedding lasted for an extended period. Additionally, histopathological examination found varying degrees of tissue damage in the trachea and lungs of infected chickens.

Conclusions
In summary, although these three H9N2 virus strains exhibited low pathogenicity, their stable replication ability in the respiratory tract and significant viral shedding characteristics indicate strong transmissibility and potential epidemic risk. These findings highlight the necessity of strengthening virus surveillance and prevention measures in live poultry markets and provide a scientific basis for avian influenza control strategies.