Sydney, Australia - A pathogenic strain of bird flu killed more than 13,000 elephant seal pups after infecting a breeding colony on a sub-Antarctic volcanic island, Australian scientists said Thursday.

Researchers found the remote Heard and McDonald Islands littered with seal carcasses when they arrived on a research expedition in October 2025.

Genetic testing confirmed the contagious H5 bird flu strain had killed seals, penguins, and birds living on the rocky outcrop – the first time it had been detected in one of Australia's external territories.

Southern elephant seal pups were the worst hit by the outbreak, scientists from the Australian Antarctic Program said, with mortality in some seal "harems" as high as 97%.

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Ground and aerial surveys conducted in October 2025 and January 2026 counted 13,300 dead elephant seal pups.

"These observations of H5 bird flu at Heard Island and McDonald Island are the first detection in an Australian external territory and show the continued eastward movement of the virus," biologist Julie McInnes said.

The researchers said the virus had likely been introduced in August 2025 by infected wildlife that travelled from the Crozet Islands, a sub-Antarctic archipelago about 930 miles to the northwest.

The Heard and McDonald Islands – uninhabited by humans – unexpectedly hit the headlines when they were included on President Donald Trump's list of international tariffs in April 2025.

Lying 2,500 miles southwest of mainland Australia, the islands are fearsomely wild.

Access is allowed only with permission from the Australian government.