An outbreak of H5 avian influenza killed an estimated 13,300 southern elephant seal pups on Heard Island and the McDonald Islands, according to findings reported by Australian scientists. the date is June 18, 2026, when the researchers announced the scale and significance of the outbreak.
Scientists encountered large numbers of seal carcasses after reaching the isolated sub-Antarctic territory during an expedition in October 2025. Ground observations and aerial surveys carried out in October 2025 and January 2026 produced the estimate of 13,300 dead pups. In some elephant seal breeding groups, known as harems, mortality reached as high as 97 percent. [Source 13725]
Genetic testing identified a pathogenic H5 bird flu strain in affected wildlife. The investigation linked the virus not only to the deaths of seals but also to infections among penguins and other birds inhabiting the rocky islands. Researchers described the findings as the first confirmed detection of H5 avian influenza in an Australian external territory. [Source 13725]
The scientists said the outbreak demonstrated the virus’s continued movement eastward through wildlife populations. Biologist Julie McInnes said the detection on Heard Island and the McDonald Islands marked an expansion of the virus into territory administered by Australia. The researchers considered it likely that infected wildlife introduced the virus in August 2025 after travelling from the Crozet Islands, a sub-Antarctic archipelago approximately 1,500 kilometres northwest of the Australian territory. [Source 13725]
Southern elephant seal pups suffered the heaviest documented losses. Their concentration in breeding colonies meant the surveys could reveal exceptionally severe mortality in particular groups, while laboratory analysis enabled researchers to associate the deaths with the H5 strain. The evidence also showed that the outbreak crossed species boundaries within the local ecosystem, affecting marine mammals as well as several bird populations. [Source 13725]
Heard Island and the McDonald Islands are uninhabited by people and lie about 4,000 kilometres southwest of mainland Australia. The volcanic, sub-Antarctic environment is difficult to reach, and entry requires Australian government permission. Those conditions make expeditions and systematic wildlife assessments challenging, so the two survey periods provided important measurements of the outbreak’s effects over time. [Source 13725]
The findings establish a confirmed presence of H5 bird flu in a remote Australian-administered ecosystem and document a major mortality event among elephant seal pups. They also indicate that the virus had reached multiple wildlife species by the time scientists conducted their fieldwork and testing. Researchers’ proposed route of introduction remains an assessment based on the movement of infected wildlife from elsewhere in the sub-Antarctic region. [Source 13725]
