An outbreak of highly pathogenic H5N1 bird flu on Australia's Heard and McDonald Islands in the Southern Ocean has killed over 13,000 southern elephant seal pups.
According to a report by the Australian Antarctic Division, the total number of young elephant seals on Heard Island was approximately 17,000, of which nearly 13,000 died from the virus. The average mortality rate was 76%, reaching up to 97% in some colonies.
Mass mortality of elephant seal pups was recorded across all studied breeding areas, including McDonald Island, until January 2026.
Lead author Dr. Julie McInnes, a wildlife biologist, noted that the detection of H5 influenza on Heard and McDonald Islands marks the first time the virus has spread beyond Australian territory, indicating its eastward movement across the subantarctic region.
Researchers also recorded high mortality among king and gentoo penguins. Analysis of samples from nine species confirmed H5N1 in six: southern elephant seals, king penguins, gentoo penguins, Antarctic fur seals, and giant petrels.
Genetic analysis suggests the virus likely reached the islands via wildlife from the French Crozet Archipelago, about 1,800 km from Heard Island, possibly in August 2025.
To assess the epidemic's scale, scientists used 120 drones covering over 1,600 km of coastline and conducted ground surveys along approximately 300 km of shore.
Heard and McDonald Islands are a UNESCO World Heritage site and one of the most important breeding grounds for marine mammals and birds in the Southern Ocean.
