Thousands of Adélie penguins found dead on two remote Antarctic islands in 2024 were mostly killed by avian cholera, a bacterial disease, rather than the H5N1 bird flu that scientists first suspected. The findings were published Oct. 14 in Proceedings of the Royal Society B by an international team led by Erasmus University Medical Center in the Netherlands.

The bacterium behind avian cholera, Pasteurella multocida, turned up in 46 of the 49 animals the team sampled, according to UC Davis, whose One Health Institute took part. Bird flu was detected in only a few, and at levels the researchers concluded were not the main cause of death.

The result matters well beyond Antarctica. It shows that a wildlife die-off can look like a bird flu outbreak while being driven by something else, a lesson for disease surveillance that also applies to the waterfowl and seabird die-offs American wildlife agencies investigate every year.

Hundreds of Carcasses on Heroína and Beagle Islands

The die-off was found in March 2024 at the Danger Islands, a small archipelago in the northern Weddell Sea off the tip of the Antarctic Peninsula. The islands host two Adélie penguin mega-colonies with more than 1.2 million breeding pairs, according to UC Davis, and are protected as an Antarctic Specially Protected Area.

In a preprint version of the study, the team reported counting 537 Adélie carcasses, including 172 adults and 365 chicks, on Heroína Island's main landing area alone. They estimated several hundred, possibly thousands, more on the island's plateau and found hundreds more on nearby Beagle Island.

Dead skuas, snowy sheathbills, a southern giant petrel and an Antarctic fur seal were also found. Most of the carcasses were frozen and partly buried in snow, suggesting the animals had died weeks before the researchers arrived. Based on breeding timing, the team estimated the outbreak happened between January and early February 2024.

Lab Tests Pointed Away From H5N1

When the deaths were first reported in 2024, bird flu was the leading suspect. H5N1 had just reached the Antarctic Peninsula region, and field tests at the time were inconclusive, as Reuters reported via CTV News when samples were sent to laboratories.

The laboratory work told a different story. Carcasses carried heavy loads of avian cholera bacteria across multiple organs, and tissue examinations found lesions typical of the disease in the liver and blood vessels. Where bird flu genetic material appeared, it was at low levels, with no viral protein or flu-type damage in the tissue. The team said H5N1 may have played a secondary role but was not the primary cause.

The bacterial strain also had a history. Genetic analysis showed it was closely related to one found in Antarctic seabirds more than two decades earlier and to strains detected more than 8,000 kilometers (about 5,000 miles) away in the Southern Ocean, according to UC Davis. That suggests avian cholera has circulated in the region for at least 20 years.

Lead author Matteo Iervolino of Erasmus University said the findings suggest avian cholera may be "a more widespread and underestimated cause of seabird mortality in Antarctica" than previously recognized.

Limits of a Single Field Visit

The study has clear boundaries. The researchers visited each island only once, sampled a subset of carcasses, and could not culture live bacteria because samples were preserved for biosafety. Causes of death among chicks were less clear and may have included starvation or a mix of factors.

The findings also do not show a long-term population decline at the Danger Islands. Adélie penguins are not currently threatened with extinction globally, although populations on the western side of the Antarctic Peninsula have fallen. The researchers say repeated outbreaks at this site could still matter regionally because it shelters a large share of the peninsula's Adélies.

H5N1 remains a real threat in the region. Separate research published earlier this year by members of the same expedition confirmed bird flu as the cause of a skua die-off at Beak Island in 2024, according to E&E News. The new study does not lower that concern. It shows the two diseases can circulate at the same time.

A Surveillance Lesson for U.S. Bird Die-Offs

Avian cholera is not exotic to Americans. The study notes that large outbreaks have struck North American waterfowl nearly every year since the 1940s, especially in wetlands where ducks and geese crowd together. With H5N1 also active in U.S. birds and livestock, state wildlife agencies often face the same question the Antarctic team did: which disease is actually killing the animals?

The researchers argue that testing only for bird flu can miss the real cause. They recommend combining laboratory testing for several pathogens with detailed tissue examination, an approach that costs more but gives managers accurate information for decisions on closures, culling or monitoring.

For readers who hunt, birdwatch or live near wetlands, the practical advice is consistent with existing guidance. Do not handle sick or dead wild birds with bare hands, keep pets away from carcasses, and report unusual bird deaths to your state wildlife agency or the U.S. Department of Agriculture rather than moving the animals yourself.

Antarctic research teams return to the peninsula during the coming austral summer, which runs roughly from November through February. Follow-up monitoring at the Danger Islands could show whether avian cholera returns each breeding season. Nature World News will report new findings as they are published.

The bottom line is that the mystery has a mostly bacterial answer. Bird flu remains a serious threat to Antarctic wildlife, but this die-off shows that assuming the most familiar culprit can hide another disease that has quietly persisted in the region for decades.

What Readers Want to Know

What killed the Antarctic penguins in 2024?

A new study found that avian cholera, caused by the bacterium Pasteurella multocida, was the main cause of death for most of the animals examined.

Was bird flu involved at all?

H5N1 was detected in a few animals at low levels. The researchers say it may have played a secondary role but was not the primary cause.

Where did the die-off happen?

On Heroína and Beagle islands in the Danger Islands, off the northern tip of the Antarctic Peninsula in the Weddell Sea.

How many penguins died?

Researchers counted 537 Adélie carcasses at one site on Heroína Island and estimated hundreds to possibly thousands more across both islands.

Is avian cholera found in the United States?

Yes. Avian cholera outbreaks have affected North American waterfowl nearly every year since the 1940s.

What should I do if I find dead wild birds?

Do not touch them with bare hands, keep pets away, and report the deaths to your state wildlife agency or the USDA.