The highly pathogenic avian influenza virus (HPAIV) subtype H5N1 (clade 2.3.4.4b) has caused a panzootic in wild birds and mammals, and reached the Antarctic continent in the austral summer 2023-2024. CSIC-UNESPA Antarctic Expedition was organized to search for new cases of HPAIV in the Northern Weddell Sea and Western Antarctic Peninsula areas, aboard the sailing vessel Australis. The molecular diagnostic laboratory set up in the Australis vessel allowed the rapid testing of samples from dead and live animals by RT-qPCR specific for 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. Influenza virus H5+ samples were found in 24 of the 27 sites visited, covering a wide range of the Antarctic Peninsula from 63.3oS to 67.8oS. Influenza virus H5+ subtype was detected by RT-qPCR in 188 animals from 9 species of birds and 4 mammalian species. Sequencing of the multibasic cleavage site in 70 selected animals demonstrated that the H5+ virus was HPAIV in all positive animal species at every site visited. Fifty percent of the dead animals tested were positive, and HPAIV was detected in asymptomatic live animals, demonstrating that the virus is circulating in Antarctica. Influenza virus H5+ was detected in air samples in some penguin colonies. The results show a large expansion of HPAIV along the Antarctic Peninsula in the austral summer 2024-2025, infecting 13 animal species tested, and the active circulation in asymptomatic animals from 11 species of Antarctic fauna.

Angela Vázquez-Calvo1, Rafael González-Serrano1, Eduardo F. Machado2, Caroline F. Pessi2, Joana M. Ikeda3, Ana Moraga1, Paula Martínez-García1, Alice Reade4, Eloise Goodison4, Ben Wallis4, Marcela Uhart5, José Luiz Catão-Dias2, Ralph E. T. Vanstreels5, Begoña Aguado1 and Antonio Alcamí1 1 Centro de Biología Molecular Severo Ochoa, CSIC, Spain 2 Laboratório de Patologia Comparada de Animais Selvagensm, Faculdade de Medicina Veterinária e Zootecnia, Universidade de São Paulo, Brazil 3 Instituto de Mamíferos Aquáticos, Brazil 4 Ocean Expeditions Support Vessel S/V Australis 5 Karen C. Drayer Wildlife Health Center, One Health Institute, School of Veterinary Medicine, University of California-Davis, USA

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