Phylogenetic and Molecular Characteristics of Wild Bird-Origin Avian Influenza Viruses Circulating in Poland in 2018-2022: Reassortment, Multiple Introductions, and Wild Bird-Poultry Epidemiological Links.

Since 2020, a significant increase in the severity of H5N highly pathogenic avian influenza (HPAI) epidemics in poultry and wild birds has been observed in Poland. To further investigate the genetic diversity of HPAI H5N viruses of clade 2.3.4.4b, HPAIV-positive samples collected from dead wild birds in 2020-2022 were phylogenetically characterized. In addition, zoonotic potential and possible reassortment between HPAIVs and LPAIVs circulating in the wild avifauna in Poland have been examined. The genome-wide phylogenetic analysis revealed the presence of three different avian influenza virus (AIV) subtypes (H5N8, H5N5, and H5N1) during the HPAI 2020/2021 season, while in the next HPAI 2021/2022 epidemic only one H5N1 subtype encompassing seven various genotypes (G1-G7) was confirmed. No reassortment events between LPAIVs (detected in the framework of active surveillance) and HPAIVs circulating in Poland have been captured, but instead, epidemiological links between wild birds and poultry due to bidirectional, i.e., wild bird-to-poultry and poultry-to-wild bird HPAIV transmission were evident. Furthermore, at least five independent H5N8 HPAIV introductions into the Baltic Sea region related to unprecedented mass mortality among swans in February-March 2021 in Poland, as well as a general tendency of current H5N viruses to accumulate specific mutations associated with the ability to break the interspecies barrier were identified. These results highlight the importance of continuous active and passive surveillance for AI to allow a rapid response to emerging viruses.