18 June 2026
Drone and ground surveys conducted by the Australian Antarctic Program have revealed a mass mortality event among southern elephant seals on sub-Antarctic Heard Island, marking a critical and deeply concerning eastward expansion of the virus across the Southern Ocean.
Since 2021, HPAI H5N1 clade 2.3.4.4b has caused unprecedented wildlife mortality on a global scale. After devastating South American coastlines and spreading to South Georgia and the Antarctic Peninsula, the virus continued its eastward progression across the Indian Ocean basin during late 2024, reaching remote islands including Gough, Marion, Crozet, and Kerguelen. Until recently, mainland Australia and its nearby sub-Antarctic territories had remained free of the panzootic strain. Because these remote islands support exceptionally dense breeding colonies of immunologically naïve marine mammals and seabirds, scientists have long feared that viral introduction could result in rapid and irreversible ecological damage.
To determine whether the virus had crossed from neighboring territories, researchers deployed aerial drones and ground teams to the Heard and McDonald Islands during expeditions conducted in October 2025 and January 2026.
The findings paint a grim picture of the outbreak's scale:
Mass Seal Mortality: Drone imagery revealed catastrophic losses among southern elephant seal (Mirounga leonina) pups. By the final surveys, 8,573 dead pups had been recorded, representing a staggering 62% mortality rate among counted pups. Total estimated pup mortality reached 13,359 individuals - nearly 77% of the island's seasonal cohort.
Using Bayesian phylogeographic modeling, the team traced the origin of the virus to the Crozet Islands, estimating that it was introduced to Heard Island around August 2025, coinciding with the late-winter return of migratory wildlife. Genetic sequencing confirmed that all positive samples belonged to genotype B3.2. Although the viruses lacked the well-recognized mammalian-adaptive PB2 E627K mutation, they carried several other widespread mutations, including PB1-F2 N66S and PA S224P/N383D, which have been associated with enhanced viral replication and increased pathogenicity in mammals.
The loss of such a large proportion of a single cohort threatens severe long-term consequences for these long-lived, slow-reproducing K-selected species (organisms that prioritize quality over quantity when it comes to reproduction). Southern elephant seals breed in dense aggregations, creating ideal conditions for both direct-contact and environmental transmission. Moreover, their strong site fidelity makes abandonment of infected breeding grounds unlikely, potentially exposing future generations to recurrent outbreaks.
Preprint: McInnes et al. (2026). Mass mortality of southern elephant seals during a multi-species outbreak of HPAI H5N1 on sub-Antarctic Heard Island. bioRxiv.
