Evidence for possible H5N1 reported from Heard Island

Eric J Woehler, ASG

Heard Island and the nearby McDonald Islands are located c.4100km southwest of Perth in the Southern Indian Ocean at 53°S 73°30’E. The islands are south of the Antarctic Polar Front and approximately 70% of the islands are permanently glaciated. The islands are a listed World Heritage property, and support globally significant populations of seabirds and marine mammals.

Media reports in late October 2025 suggested an Australian Antarctic Division expedition to Heard Island had identified unusual mortality in Southern Elephant Seals that were deemed to be consistent with H5N1 bird flu. The reports also noted there was no evidence of unusual mortality in any seabirds. With reports of H5N1 identified at the nearby French Iles Kerguelen 400km to the northwest of Heard Island, the suspected presence of H5N1 at Heard Island is warranted.

One unusual aspect of Southern Elephant Seal pups on Heard Island is that they can, and do, disperse widely from the foreshore harems once they are weaned. One of the challenges in censuses of elephant seals on Heard is that the weaners can be 100s of metres (and occasionally farther) from the foreshores. This behaviour is much more pronounced than on other Subantarctic islands where the coastal/foreshore real estate is much more constrained.

At western Heard Island, in an area known as the 'Four Bays', there is a large sandy plain (the "Nullarbor") that Southern Elephant Seal weaners use to move around and disperse. The plain is unvegetated, and the weaners that venture there are very conspicuous targets for predation events by Southern Giant Petrels and Subantarctic Skuas. It is not unusual to see weaners, alive and dead, subjected to predation/scavenging by giant petrels and skuas from mid-October (peak pup counts) onwards as weaners leave the harems and disperse.

Part of the Nullarbor, Heard Island. Image © Eric J Woehler

Vision aired on news media showed a scattered elephant seal pup carcasses on the Nullarbor being sampled, and the content was consistent with normal elephant seal weaner mortality at Heard Island. The samples collected will be returned to Australia for testing. Until the results are known, there remains no evidence for H5N1 at Heard Island. The absence of any reports of seabirds showing symptoms of H5N1 there are welcome. It may well be that the "unusual" mortality of elephant seals at Heard Island is normal post-weaning mortality of pups.

It is critical that the most stringent quarantine protocols are implemented for the return of the expeditioners, their equipment and samples etc. If H5N1 is present on Heard Island, the return to Australia of material/gear from the expedition may present an opportunity for H5N1 or variant/s to enter Australia.

Scientists visited the remote Heard Island as part of a seven-week research voyage on the icebreaker RSV Nuyina (Australian Antarctic Division: Rowena Hannaford). Photograph from ABC [https://www.abc.net.au/news/2025-10-25/bird-flu-h5n1-likely-present-in-heard-island-elephant-seals/105931840]