Skuas as sentinels of high pathogenicity avian influenza H5N1 on the Antarctic Peninsula in the 2024/2025 austral summer.

Despite Antarctica's geographic isolation, the first incursion of high pathogenicity avian influenza (HPAI) H5N1 was detected in the 2023/2024 austral summer. Surveillance for HPAI H5N1 in Antarctica remains patchy due to logistical, financial and infrastructure challenges, with many suspected cases remaining unconfirmed and few viral genome sequences available to date. Through the 2024/2025 austral summer, we undertook five sampling expeditions to the South Shetland Islands and Antarctic Peninsula facilitated by cruise ships/operators. Across more than 500 faecal environmental samples collected from apparently healthy penguins and marine mammals, we found no detectable evidence of HPAI H5N1. However, HPAI H5N1 was detected in all but one of the skua carcasses sampled, which, in most cases, were found within metres of penguin sub-colonies. All HPAI H5N1 viral genome sequences from skuas on the Antarctic Peninsula fell within a single lineage, which included those genomes from skuas sampled in the 2024/2025 season from the South Shetland Islands. Genomes were in a different clade to those from the Antarctic Peninsula collected in the 2023/2024 austral summer. Our results confirm that although the prevalence may be low, HPAI H5N1 is recurring in Antarctica, emphasizing the need for ongoing surveillance to monitor and mitigate threats to wildlife, even in the planet's most isolated regions.