Scientists Trace Mass Penguin Deaths to Avian Cholera

More than 537 penguin carcasses discovered on a landing ramp in Antarctica led researchers to identify avian cholera, rather than bird flu, as the primary cause of a major wildlife die-off in early 2024. Wildlife veterinarian Dr. Ralph Vanstreels and his colleagues investigated the deaths on the Danger Islands, near the Antarctic Peninsula, where thousands of Adélie penguins and other birds may have died.

The researchers found the carcasses in March 2024 while investigating an outbreak initially suspected to involve highly pathogenic avian influenza, or H5N1. The landing ramp alone contained more than 537 dead penguins, and the team estimated that many more carcasses were scattered across the islands, which host two mega-colonies of penguins.

Vanstreels, a wildlife veterinarian affiliated with the University of California, Davis, said the team could not complete a comprehensive count because conditions limited the time researchers could spend ashore. The scientists estimated that thousands of Adélie penguins and other birds had died.

Vanstreels described the uncertainty confronting the team as they searched through the snow. "Was each bump a rock or was it a carcass? We had to do quite a bit of intensive searching [through the snow]," said Vanstreels, who is based in Argentina.

The remote location also created logistical risks that prevented an extended investigation. "You cannot stay there very long because you might be locked down with an iceberg blocking your way out," Vanstreels said.

The investigation began amid growing concern over the global spread of H5N1, which reached mainland Australia in June 2026 after the continent had remained free of the virus for years. According to the World Animal Health Information System, the virus has contributed to nearly 300 million poultry deaths since late 2021, including birds killed through preventive culling.

Scientists have also estimated that bird flu has killed millions of wild animals and marine mammals worldwide, although the total remains difficult to establish. Researchers had expected the virus to reach Australia through bird movements from Southeast Asia, but evidence pointed to an arrival from Antarctica, likely carried by migrating birds.

Dr. Amandine Gamble, an assistant professor in Cornell University's Department of Public and Ecosystem Health, explained why researchers initially focused on migration from Southeast Asia. "We have a lot of migration of birds between Australia and Southeast Asia, so for the first few years, we're like: That's where the virus is coming from," Gamble said.

Michelle Wille, a senior research fellow at the Center for Pathogen Genomics at the University of Melbourne, had previously described the virus's spread through repeated wildlife outbreaks. "This is a virus that's not assisted by airplanes," Wille told NPR in 2025.

She continued by emphasizing the scale of the mortality associated with its spread. "This is a virus that's traveling by mass mortality after mass mortality after mass mortality after mass mortality," Wille said.

Given the arrival of H5N1 in Antarctica in late 2023 and early 2024, Vanstreels and his colleagues organized their expedition primarily to investigate suspected bird flu. The team collected samples while wearing protective equipment, including double gloves, coveralls, glasses and masks, and followed strict biosafety procedures.

Vanstreels recalled the difficulties of working in the protective gear. "We looked a little bit stiff when walking — like a penguin walking almost — because of all the gear," he said.

The researchers transported the samples to a small laboratory aboard their vessel and obtained preliminary results that evening. Tests detected bird flu, but the amount of virus in the samples was lower than expected for a disease responsible for the deaths.

"There is bird flu here, but the virus loads are low," Vanstreels said.

The findings prompted the team to investigate other possible causes. The carcasses appeared relatively fresh, making the low viral levels difficult to reconcile with bird flu as the principal cause of death.

"We were unsure what was happening," Vanstreels said.

Several months later, further examinations by Vanstreels' colleagues in the Netherlands revealed bacterial infection in the birds' tissues. Microscopic analysis identified avian cholera, a bacterial disease that affects birds and is distinct from human cholera.

"Under the microscope, you can see that there's bacteria in the tissues," Vanstreels said.

The team's findings were published in October 2026 in the journal Proceedings of the Royal Society B. Researchers noted that the bird-specific strain of the disease differs from strains that infect mammals, making the risk to humans relatively low.

"The risk for human infection is not really that high when you see an outbreak on birds," Vanstreels said.

Avian cholera can cause substantial bird mortality worldwide, particularly in cold environments where the bacteria thrive. However, researchers still lack sufficient evidence to determine how widespread the disease is in Antarctica.

The last investigation into avian cholera on the Antarctic Peninsula took place 25 years ago, leaving a substantial gap in surveillance. Vanstreels cautioned that the absence of recent investigations should not be mistaken for evidence that the disease disappeared.

"One could be misled to think that: Well, that was a unique thing that happened back in 2001, and nothing's happened ever since," Vanstreels said.