Recurring incursions and dissemination of novel Eurasian-origin H5Nx avian influenza viruses in Atlantic Canada.

Wild birds are important hosts of influenza A viruses (IAVs) and play an important role in their ecology. The emergence of the A/goose/Guangdong/1/1996 H5N1 (Gs/GD) lineage marked a shift in IAV ecology, leading to recurrent outbreaks and mortality in wild birds from 2002 onwards. This lineage has evolved and diversified over time, with a recent important derivative being the 2.3.4.4b sub-lineage, which has caused significant mortality events in wild bird populations. An H5N1 clade 2.3.4.4b virus was transmitted into North America from Eurasia in 2021, with the first detection being in Newfoundland and Labrador in Atlantic Canada, and this virus and its reassortants then spread broadly throughout North America and beyond. Following the first 2021 detection, there have been three additional known incursions of Eurasian-origin strains into Atlantic Canada, a second H5N1 strain in 2022 and two H5N5 strains in 2023. In this study, we document a fifth incursion in Atlantic Canada that occurred in 2023 by another H5N5 strain. This strain spread throughout Atlantic Canada and into Quebec, infecting numerous species of wild birds and mammals. Genomic analysis revealed mammalian-adaptive mutations in some of the detected viruses (PB2-E627K and PB2-D701N) and mutations in the hemagglutinin (HA) and neuraminidase (NA) genes that are associated with enhanced viral fitness and avian transmission capabilities. Our findings indicate that this virus is continuing to circulate in wildlife, and confirms Atlantic Canada is an important North American entry point for Eurasian IAVs. Continued surveillance and genomic analysis of IAVs detected in the region is crucial to monitor the evolution of these viruses and assess potential risks to wildlife and public health.