Heard & McDonald Islands - Credit Wikipedia
#19,209
Last October, in Australia DAFF & DCCEEW: Suspected H5 bird flu in Elephant Seals at Australian Sub-Antarctic Island, we learned of the first suspected incursion of HPAI into an Australian possession; Heard Island.
Australia, NZ, and the islands of Oceania are the only major regions of the earth that remain HPAI H5 free - and while Heard Island is far-removed from the Mainland (see map above) - this was a sobering reminder that the virus continues to increase its geographic range.
In late November we saw Confirmation of H5 Bird Flu on Sub-Antarctic Heard Island, but the full extent of the spillover was undetermined. Previously we've seen very large die-offs of of marine mammals, which have included signs the virus may be close to achieving sustained mammal-to-mammal transmission.
- Two years ago, in a research letter (see EID Journal: HPAI A(H5N1) Viruses from Multispecies Outbreak, Argentina, August 2023), researchers wrote: ". . . it seems likely that pinniped-to-pinniped transmission played a role in the spread of the mammal-adapted HPAI H5N1 viruses in the region . . ."
- In the summer of 2024, we looked at a Preprint: Massive outbreak of Influenza A H5N1 in elephant seals at Peninsula Valdes, Argentina: increased evidence for mammal-to-mammal transmission
- And a year ago, in Nature Comms: Cross-species and mammal-to-mammal transmission of clade 2.3.4.4b HPAI A/H5N1 with PB2 adaptations, the authors wrote:
- Several mutations were detected months later in sea lions in the Atlantic coast, indicating that the pinniped outbreaks on the west and east coasts of South America are genetically linked. These data support sustained mammal-to-mammal transmission of HPAIV in marine mammals over thousands of kilometers of Chile’s Pacific coastline, which subsequently continued through the Atlantic coastline.
Earlier this year California reported a limited die-off of Elephant seals from H5N1 for the very first time. Unfortunately, much of the impact of H5 on marine mammals likely goes unreported.
Yesterday, a team of researchers reported that the carnage from the Heard Island outbreak was worse than originally feared, with 76% mortality among Elephant Seal pups.
Due to its length (n=25 pages), I've only posted some brief excerpts from the preprint. Follow the link to read it in its entirety. I'll have a brief postscript after the break.
Mass mortality of southern elephant seals during multi-species outbreak of HPAI H5N1 on sub-Antarctic Heard Island
Julie C McInnes, Tristan Burgess, Georgia Mergard, Melanie R Wells, Clive R McMahon, Matthew J Neave, Andrea Polanowski, Aleks Terauds, Jérémy Tornos, Mathilde Lejeune, François-Xavier Briand, Guy Baele, Thierry Boulinier, Helen Achurch, Rachael Alderman, Anna Lashko, Barbara Wienecke, Louise P Wynen, Benjamin Viola, Patti Virtue, Jarrod C Hodgson
doi: https://doi.org/10.64898/2026.06.16.732752
This article is a preprint and has not been certified by peer review
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Abstract
High pathogenicity avian influenza (HPAI) has spread across the sub-Antarctic, causing significant wildlife impacts. We report its first detection in an Australian external territory, Heard Island and McDonald Islands, which supports over one million breeding seabirds and seals. Drone and ground surveys (October 2025, January 2026), combined with viral genome analysis, confirmed infection with Influenza A H5N1 clade 2.3.4.4b at Heard Island.Drone surveys revealed mass mortality in southern elephant seals, with 8,573 pups (62%) recorded dead across Heard Island by the final surveys. Mortality increased at an average rate of 5.6% per day in a subset of harems, and the highest observed mortality in a harem was 97%. Based on the average (76%) mortality in the final surveys, total estimated pup mortality at Heard Island was 13,359 (from a total population of 17,364 pups), though this may be an underestimate as mortality was ongoing at this time.HPAI was detected in six of nine species tested and, we suspect, led to elevated mortality in king and gentoo penguins. Phylogenetic analysis indicates the virus was introduced from Crozet Islands, with an estimated arrival around August 2025. These data show the continued easterly spread of HPAI around the sub-Antarctic, with severe but heterogeneous impacts across taxa. Our results demonstrate the value of drones for large scale monitoring, underscoring the need for continued and enhanced HPAI surveillance across the Southern Ocean.
(SNIP)
DISCUSSION
HPAI H5N1 clade 2.3.4.4b has spread rapidly and widely across the globe and has now been detected at the remote territory of Heard Island and McDonald Islands in the southern Indian Ocean. This is the first detection of the panzootic clade 2.3.4.4b HPAI in Australian sub-Antarctic territory.
We confirmed the presence of H5N1 in southern elephant seals, Antarctic fur seals, gentoo and king penguins, a brown skua and a South Georgia diving petrel, and observed mass mortality in elephant seals and elevated mortality in king and gentoo penguins.
Our results provide strong evidence that the virus was introduced from Crozet Islands(<1,700 km from HIMI) based on current availability of virus sequences. It is clear from our observations and those of others 2,12,36 that HPAI can and does spread across vast ocean basins.
While the transport host(s) cannot be confirmed, southern giant petrels are a plausible candidate and have been suspected as long-distance transport hosts elsewhere 6,12. As the virus progresses eastwards around the sub-Antarctic, including along circumpolar flyways, there is potential for further geographic spread to additional HPAI-free regions, including Macquarie Island, New Zealand, Australia and East Antarctica.
While the authors acknowledge that the Heard island viruses carried `new mutations' some of which were `likely to facilitate mammalian transmission', they stop short of declaring sustained mammal-to-mammal transmission.
They do suggest this outbreak likely began from a single introduction of the virus - probably in August of last year - and most likely from the Crozet Islands. Elephant seals were most affected, but exactly how the virus spread on the island isn't fully understood.
The authors wrote:
However, the HIMI genomes had lower-than-expected root-to-tip divergence compared with other H5N1 2.3.4.4b viruses. This is most consistent with a low-cadence transmission chain involving few hosts, either on land or at sea, though incomplete sampling of intermediate viruses may also contribute. Hotspots of infection likely generate many such chains of transmission as infected animals disperse, though most likely die out without seeding new, distant outbreaks.
Regardless of the method of transmission, HPAI H5 quickly burned through the seal population taking a deadly toll. Add in H5's ability to cross vast distances of open ocean, and the ongoing bird-flu free status of Australia and New Zealand is precarious at best.
