Circumpolar spread of avian influenza H5N1 to southern Indian Ocean islands

Augustin Clessin, François-Xavier Briand, Jérémy Tornos, Mathilde Lejeune, Camille Pasquale, Romain Fischer, Florent Souchaud, Edouard Hirchaud, Samuel L. Hong, Tristan Bralet, Christophe Guinet, Clive R. McMahon, Béatrice Grasland, Guy Baele and Thierry Boulinier ()
Additional contact information
Augustin Clessin: IRD
François-Xavier Briand: Aviaires et Cunicoles
Jérémy Tornos: IRD
Mathilde Lejeune: IRD
Camille Pasquale: IRD
Romain Fischer: IRD
Florent Souchaud: Aviaires et Cunicoles
Edouard Hirchaud: Aviaires et Cunicoles
Samuel L. Hong: KU Leuven
Tristan Bralet: IRD
Christophe Guinet: CNRS-La Rochelle Université
Clive R. McMahon: Sydney Institute of Marine Science
Béatrice Grasland: Aviaires et Cunicoles
Guy Baele: KU Leuven
Thierry Boulinier: IRD

Nature Communications, 2025, vol. 16, issue 1, 1-12

Abstract: Abstract Since 2020, the outbreak of high pathogenicity avian influenza (HPAI) H5N1 virus clade 2.3.4.4b has turned into the largest documented panzootic 1,3. Here, we describe its arrival into the Indian Ocean sub-Antarctic archipelagos of Crozet and Kerguelen, where we first detected the virus in October 2024 in dead southern elephant seals. While the panzootic is ongoing, it has already caused unprecedented mortalities of marine mammals and seabirds. We collected brain swabs from seal and seabird carcasses and obtained 25 novel HPAI H5N1 2.3.4.4b sequences. Using phylogeographic analyses, we show that there have been independent introductions of the virus to Crozet and Kerguelen islands, most likely from the distant South Georgia islands in the Southern Atlantic, and not from the more nearby coasts of South Africa. Our results point to a year-long gap in genomic surveillance in the sub-Antarctic region. Locally, our analyses show that the virus is transmitted between different species. Our serological analyses show that some southern elephant seal had mounted an anti-H5 antibody response. Through its circumpolar spread to the Indian Ocean, HPAI H5N1 2.3.4.4b moves closer to Australia, which remains free from infections with this strain, and represents a major threat to the sub-Antarctic wildlife.

Date: 2025
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DOI: 10.1038/s41467-025-64297-y

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