In December 2022 and January 2023, we isolated clade 2.3.4.4b H5N1 high-pathogenicity avian influenza (HPAI) viruses from six American crows (Corvus brachyrhynchos) from Prince Edward Island and a red fox (Vulpes vulpes) from Newfoundland, Canada. Using full-genome sequencing and phylogenetic analysis, these viruses were found to fall into two distinct phylogenetic clusters: one group containing H5N1 viruses that had been circulating in North and South America since late 2021, and the other one containing European H5N1 viruses reported in late 2022. The transatlantic re-introduction for the second time by pelagic/Icelandic bird migration via the same route used during the 2021 incursion of Eurasian origin H5N1 viruses into North America demonstrates that migratory birds continue to be the driving force for transcontinental dissemination of the virus. This new detection further demonstrates the continual long-term threat of H5N1 viruses for poultry and mammals and the subsequent impact on various wild bird populations wherever these viruses emerge. The continual emergence of clade 2.3.4.4b H5Nx viruses requires vigilant surveillance in wild birds, particularly in areas of the Americas, which lie within the migratory corridors for long-distance migratory birds originating from Europe and Asia. Although H5Nx viruses have been detected at higher rates in North America since 2021, a bidirectional flow of H5Nx genes of American origin viruses to Europe has never been reported. In the future, coordinated and systematic surveillance programs for HPAI viruses need to be launched between European and North American agencies.

Seabirds are frequently infected by avian influenza virus (AIV), which prior to 2021 primarily consisted of low-pathogenic AIV with limited reports of disease during infection. However, since highly pathogenic AIV (HPAIV) H5N1 clade 2.3.4.4b was introduced to North America in late 2021, HPAIV outbreaks in seabirds have occurred in multiple regions, with high levels of morbidity and mortality in many species. While monitoring active viral infections is critical for tracking disease burden, monitoring prior viral exposure via antibody detection in species that experienced large outbreaks is important for identifying individual- and population-level impacts of AIV on immunity and survival. We capitalized on ongoing egg collection programmes to assess the prevalence of antibodies against AIV nucleoprotein (NP) and hemagglutinin subtype 5 (H5) in 523 eggs collected in 2022 and 2023 from 11 seabird species that breed in the northwestern Atlantic, including primarily samples from eastern Canada and two from western Iceland. The prevalence of AIV antibodies in eggs varied across regions, species and years. American common eider (Somateria mollissima dresseri) eggs had the highest AIV antibody prevalence compared to sympatric species in 2023. Longitudinal samples were available for northern gannets (Morus bassanus) and American herring gulls (Larus argentatus smithsoniansus) at several sites, where the prevalence of anti-NP and anti-H5 antibodies increased from 2022 to 2023. Examining AIV antibody prevalence in seabird eggs can be a useful tool to investigate population-level AIV exposure, while we acknowledge our limited understanding of differential antibody waning rates and the relationship between titre and susceptibility.