Co-circulation of H5N1 and H5N6 High Pathogenicity Avian Influenza Viruses in Migratory Birds at a Coastal Wintering Site in China.

Since 2020, H5Nx high Pathogenicity avian influenza viruses (HPAIVs) belonging to hemagglutinin (HA) clade 2.3.4.4b have undergone widespread dispersal in wild birds, posing substantial threats to global poultry populations and mammals worldwide. In this study, we report the co-circulation of clade 2.3.4.4b H5N1 and H5N6 HPAIVs associated with mortality events in Anseriformes and Charadriiformes at a coastal wintering site in Fujian Province, China. H5N1 and H5N6 viruses were identified in 4 species of waterfowl (Bean Goose, Eastern Spot-billed Duck, Eurasian Wigeon, and Eurasian Curlew). Phylogenetic analysis indicated that these H5N1 and H5N6 isolates were clade 2.3.4.4b strains circulating within Eurasia and lacked known mammalian adaptation markers in key proteins. The infected birds exhibited severe systemic multi-organ pathology, with the most pronounced damage observed in the lungs. H5N6 and H5N1 viruses caused lethal injury in BALB/c mice, with H5N1 leading to systemic infection. Environmental sampling also revealed the presence of H5N1 HPAIVs alongside low-pathogenicity avian influenza viruses (LPAIVs) such as H3N1 and H6N1. Our findings highlight the complex and dynamic transmission patterns of H5 HPAIV in wild bird populations and the substantial threat they pose to mammals, underscoring the critical importance of sustained surveillance for high pathogenicity avian influenza in wild birds.