Australia has confirmed its first detection of H5N1 bird flu in a marine mammal after a dead long-nosed fur seal tested positive in South Australia on Saturday. A second suspected case was identified in another long-nosed fur seal found on the Pages Islands off Kangaroo Island, with testing results pending. The discovery has prompted warnings from leading scientists about potential widespread mortality in the country’s pinniped populations.
Three types of pinnipeds inhabit Australian waters: long-nosed fur seals, Australian fur seals, and endangered Australian sea lions. Approximately 12,000 Australian sea lions remain in existence, with roughly three-quarters located in South Australia. Experts have assessed all three species as facing very high risk from H5 bird flu. Scientists expressed particular concern about the small population size of Australian sea lions, which could face existential threats if the virus spreads rapidly through colonies.
H5N1 has already caused severe impacts on marine mammal populations elsewhere. In 2023, the virus killed more than 17,000 southern elephant seal pups in Argentina. Drone surveys detected more than 13,000 dead southern elephant seal pups on Australia’s Heard Island in 2025 and 2026. South America and the Southern Ocean have experienced astronomical mortality rates in sea lions, fur seals, and elephant seals infected with the virus.
Vulnerability stems from the concentrated nature of pinniped colonies that overlap with infected seabird populations. Virus contamination in water and on beaches facilitates transmission between species and individuals. While mammal-to-mammal transmission of H5N1 typically occurs following environmental contamination from birds, such transmission could accelerate spread through seal and sea lion populations. No registered vaccines currently exist for treating the virus in mammals, though the United States has conducted experimental vaccine trials on a limited number of seals.
South Australia announced it is investigating vaccination opportunities for high-priority populations, particularly Australian sea lions at Seal Bay on Kangaroo Island. Environmental advocates noted that recent algal blooms had already weakened these populations by affecting food sources, potentially increasing vulnerability to infection.
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