Seabirds' mortalities in Antarctica caused by HPAIV: skuas as a sentinel species.

Seabirds are at high risk of extinction, and the emergence of highly pathogenic avian influenza virus (HPAIV) represents an additional threat. In 2024, HPAIV A/H5 clade 2.3.4.4b was detected for the first time in the Antarctic Continent in a dead Brown Skua (Stercorarius antarcticus), and further evidence suggests that not all species have been equally affected. We conducted a survey of HPAIV-related mortalities across 16 seabird nesting sites along the western Antarctic Peninsula (WAP) during the 2024-2025 breeding season. We report a mortality of 35 adult skuas along the WAP and beyond the Antarctic Polar Circle. Eleven skuas from six different geographic areas were sampled and all were positive to HPAIV A/H5 clade 2.3.4.4b. Phylogenetic analyses revealed that these sequences are closely related to the initial HPAIV A/H5 clade 2.3.4.4b reported in Antarctica, and post-mortem findings suggest that the viral infection was likely the cause of death. No other suspected HPAIV-related mortalities were observed in sympatric species. Our findings represent the southernmost record of skua mortality in Antarctica related to HPAIV A/H5 infection to date, demonstrating the dissemination of 2.3.4.4b clade across several localities. The virus is differentially affecting sympatric seabird species and the skuas may act as sentinel species for influenza surveillance.