The high pathogenicity avian influenza virus (HPAIV) subtype H5N1 (clade 2.3.4.4b) has caused a panzootic, with unprecedented massive outbreaks in wild mammals and birds. The rapid spread of the virus in South America and the Subantarctic islands raised concerns about HPAIV reaching Antarctica. With the purpose of monitoring HPAIV spread to Antarctica, an international expedition aboard the sailing vessel Australis was organized in March 2024, looking for signs of the virus in seabirds and marine mammals in the area of the Antarctic Peninsula. A molecular laboratory was set up in the vessel allowing to use state-of-the-art diagnostic methods for samples to be tested by RT-qPCR for presence of influenza A virus (gene M) and the H5 subtype. HPAIV was confirmed by Oxford Nanopore sequencing of the multibasic cleavage site of the hemagglutinin gene. RT-qPCR and sequencing protocols were further optimized back in the laboratory detecting more positive samples. HPAIV H5 was detected in seven locations (Beagle Island, Beak Island, Devil Island, D’Urville Monument, Heroina Island, Hope Bay, and Paulet Island), infecting five species: skua, snowy sheathbill, Antarctic fur seal, Adelie penguin and gentoo penguin. Prevalence of HPAIV H5 positive animals was identified in unusual mortality events of Adelie penguins (9 positive/12 total in Beagle Island and 9 positive/21 total in Heroina Island) and skuas (10 positive/10 total in Beak Island), suggesting the virus had significant population impacts on Antarctic fauna. The results show that HPAIV spread more extensively than previously appreciated in the Antarctic Peninsula during the austral summer 2023-2024.

Begoña Aguado(1,a), Angela Vázquez(1,a), Ana Moraga(1,a), Lineke Begeman(2), Florencia Soto(3), Anne Günther(4), Matteo Iervolino(2), Rafael González-Serrano(1), Paula Martínez-García(1), Sandra González de la Fuente(1), Alice Reade(5), Adam Coerper(5), Ben Wallis(5), Martin Beer(4), Thijs Kuiken(2), Javier Negrete(6), Ralph E. T. Vanstreels(7), Meagan Dewar(8,b) and Antonio Alcamí(1,b) (1) Centro de Biología Molecular Severo Ochoa, CSIC, Spain (2) Department of Viroscience, Erasmus University Medical Center, the Netherlands (3) Instituto de Biología de Organismos Marinos (IBIOMAR-CONICET), Argentina (4) Institute of Diagnostic Virology, Friedrich-Loeffler-Institut, Federal Research Institute for Animal Health, Germany (5) Ocean Expeditions Support Vessel S/V Australis (6) Departamento de Biología de Predadores Tope, Instituto Antártico Argentino, Argentina (7) Karen C. Drayer Wildlife Health Center Univ. of California - Davis, USA (8) Future Regions Research Centre, Federation University Australia, Australia (a) Shared-first authors (b) Shared-senior authors

Notable outbreaks, field and molecular epidemiology, and surveillance in wild birds