Impacts of high pathogenicity avian influenza H5N1 2.3.4.4b south of the Antarctic Circle

ABSTRACT High pathogenicity avian influenza (HPAI) H5N1 2.3.4.4b poses a substantial conservation threat to ecosystems, populations, and species globally, with its continued spread into new regions increasing concern for potential ecological consequences. During surveys in February – March 2025, we confirmed the virus’s presence at the southern extent of its known range along the Western Antarctic Peninsula, with recorded mortalities in South Polar Skuas S tercorarius maccormicki on distinct islands in Marguerite Bay, as well as one confirmed and one suspected case in Kelp Gulls Larus dominicanus . At the time of sampling, no evidence of infection was observed in other seabird or mammal species. Consistent with previous global reports, skuas – here, South Polar Skuas – appear particularly vulnerable, yet broader impacts on the local seabird and mammal community remain unclear. Additionally, our use of rapid antigen tests (VDRG® AIV Ag Rapid kit 2.0 – Median Diagnostics) in the field demonstrated their potential utility for real-time surveillance, though false negatives (10%) highlight limitations in test sensitivity. These findings contribute to a growing understanding of the impacts of HPAI H5N1 2.3.4.4b outbreaks on Antarctic species and populations, and will inform continued monitoring, conservation strategies, and biosecurity measures in response to the virus’s ongoing spread.