Madslien, K.; Fosse, J. H.; Aars, J.; Boe, C. A.; Andersen, M.; Buhler, K.; Fjeldheim, I.; Gjerset, B.; Jorgensen, T.; Myhrvold, I. K.; Rohringer, A.; Sturod, K.; Tryland, M.; Ytrehus, B.; Tonnessen, R.; Nymo, I. H.

Highly pathogenic avian influenza virus (HPAIV) subtype H5N5 was detected in a one-year-old polar bear (Ursus maritimus) and an adjacent adult Atlantic walrus (Odobenus rosmarus rosmarus), both found deceased in Raudfjorden, Svalbard. This represents the first confirmed case of HPAI in a European polar bear and the second in an Atlantic walrus. Viral genomes were nearly identical and harbored PB2-E627V, a marker associated with mammalian adaptation. Several polar bears, including the deceased individual, had previously been observed feeding on the walrus carcass. Antibodies against H5 were detected in 75% of polar bears in 2023 (n=36) and 97% in 2024-2025 (n=65), suggesting extensive circulation of HPAIV in the population following the first detections in birds in Svalbard in 2022, whereas no antibodies were detected in samples from 2014-2022 (n=243).