Four more Australian sea lions, including a pup, have tested positive for H5 bird flu at a key breeding site between the South Australian mainland and Kangaroo Island, bringing the species' confirmed total to five. South Australian Environment Minister Emily Bourke announced the results on Monday, Sept. 21, adding that about 900 greater crested terns had been found dead on the same islands.

The four positives came from 10 dead sea lions tested out of 12 found on North Page and South Page islands, ABC News reported. The state is now seeking federal approval for a vaccine trial targeting mammals, alongside a bird vaccine trial already underway at Cleland Wildlife Park, where more than 150 doses have been given since last month.

The development matters beyond Australia. The Australian sea lion is found nowhere else, and its first confirmed deaths from this virus show how a strain that has devastated wildlife across the Americas and the sub-Antarctic is now reaching one of the last regions it had spared.

Five Sea Lions and a Widening List of Species

The first sea lion case was confirmed on Sept. 13 at Seal Bay on Kangaroo Island. Bourke said that the animal had been recovering from a shark attack when the virus killed it, according to ABC's report on the first sea lion case. Kangaroo Island Mayor Michael Pengilly called the case "not unexpected," and said Seal Bay tours were being run from the boardwalk rather than the beach to help prevent spread.

The sea lions join a growing list of mammals. Australia's first mammal case, a long-nosed fur seal in South Australia's south-east, was announced on Aug. 23, and Bourke said on Sept. 13 that the state had since confirmed two more fur seal cases and that the virus had been found in four mammal species in South Australia. Officials have also confirmed H5N1 in a common dolphin and the nation's first pelican case on the Yorke Peninsula, while the virus has spread through wild birds including little penguins, magpies and crested terns, Mongabay reported. Mongabay also reported a detection in a red fox in Adelaide in late August.

Bourke said the magpie detections on Hindmarsh Island show the virus moving inland. She described the outbreak as a national issue rather than a coastal one.

A Declining Endangered Species with Little Margin

The Australian sea lion is listed as endangered on international threatened species lists. South Australia's National Parks and Wildlife Service estimates fewer than 12,000 remain, with about 85 percent living in the state and the rest in Western Australia. Bourke said the South Australian population was already declining by about 2 percent a year before bird flu arrived.

Scientists fear what could follow. The same broad strain has killed more than 13,000 seal pups on Heard Island, an Australian sub-Antarctic territory, more than three-quarters of that cohort, according to research reported by ABC News. Bill Bateman, an associate professor of behavioral ecology at Curtin University, wrote in The Conversation that the first confirmed sea lion death marks "a significant and worrying moment for Australian wildlife."

Marine mammals most likely catch the virus from infected birds by eating sick or dead seabirds or through contact with contaminated bodily fluids, water or ground at shared coastal sites, Bateman wrote. Victoria's chief biodiversity officer, James Todd, has said bird aggregations during breeding seasons will likely accelerate transmission to mammals.

Tracing the Strain from the Southern Ocean

Nature World News reported in July that South Australia was investigating suspected H5 bird flu cases in wild birds, raising fears the virus was spreading locally. At that point, detections were limited to birds. The spread into several mammal species changes the conservation picture for the region's seals, sea lions and dolphins.

The strain behind Australia's outbreak is high pathogenicity H5N1 clade 2.3.4.4b, first found in the country in two seabirds in an isolated area of southern Western Australia. This clade spread through the Americas from 2021 onward and reached sub-Antarctic islands by late 2023, causing mass deaths in wild birds and marine mammals along the way.

Human risk remains low. The Australian Centre for Disease Control reports no human cases of this clade in Australia and says it has not spread from person to person. Overseas, almost all human infections with the clade have followed exposure to poultry or, in the United States only, dairy cattle, and most cases detected in 2024 and 2025 were mild.

What Happens Next for Australian Wildlife

Several questions remain open. Officials have not said how many additional sea lions may be infected at other colonies, although two dead sea lions tested on Liguanea Island off the Eyre Peninsula returned negative results. The effect on overall breeding success will not be clear for months. Whether a mammal vaccine trial gains federal approval, and how quickly, is also unresolved.

For U.S. readers, the outbreak closes one of the last gaps in the strain's global reach. American travelers visiting Australian coasts should keep their distance from wildlife, follow closures and boardwalk rules, and never touch sick or dead birds or marine mammals. In Australia, sightings can be reported to the Emergency Animal Disease Hotline at 1800 675 888. In the United States, sick or dead wild birds and marine mammals should be reported to state wildlife agencies or NOAA's marine mammal stranding network.

Bateman has noted that safely removing carcasses is one of the few tools available, because it takes viral reservoirs away from scavengers. South Australia's next steps depend on surveillance results at breeding colonies and a federal decision on the vaccine trial. The bottom line is that a declining, endangered sea lion now faces a new disease threat, and the coming breeding season will show how far it spreads.

What Readers Want to Know

Which animals have tested positive in South Australia?

Officials have confirmed H5 bird flu in five Australian sea lions, long-nosed fur seals, a common dolphin, little penguins, magpies, greater crested terns and a pelican, among other species.

Why is the Australian sea lion case significant?

The species is endangered, found only in Australia, and already declining about 2 percent a year in South Australia, according to state officials.

How do sea lions catch bird flu?

Scientists believe most marine mammals are infected by eating sick or dead seabirds or by contact with contaminated fluids, water or ground at shared coastal sites.

Is there a vaccine for wildlife?

South Australia is trialing a bird vaccine at Cleland Wildlife Park and is seeking federal approval for a trial targeting mammals.

Is this strain dangerous to people?

Australian health authorities report no human infections with this clade in Australia and no person-to-person spread. The risk to the public is considered low.

What should travelers do?

Stay away from sick or dead wildlife, follow site closures and boardwalk rules, and report sightings to local authorities.