Thousands of Adélie penguins found dead in Antarctica in 2024 were primarily victims of avian cholera, not high pathogenicity avian influenza (H5N1) as researchers initially suspected, according to a study published today in Proceedings of the Royal Society B. The findings highlight an underrecognized threat to Antarctic wildlife and the importance of comprehensive disease investigations, particularly as H5N1 continues to raise concerns for seabird and marine mammal populations.
In March 2024, an international research expedition visited Antarctica's remote Danger Islands, home to more than 1.2 million breeding pairs of Adélie penguins. Researchers discovered thousands of dead penguins across two islands, along with carcasses of several other seabird species. With H5N1 having recently reached the Antarctic Peninsula, bird flu was the initial suspected cause.
However, laboratory testing and pathological examinations revealed that most of the investigated deaths were caused by avian cholera, a disease caused by the bacterium Pasteurella multocida. The bacterium was detected in 46 of 49 animals sampled, while H5N1 was present in only a few.
Ralph Vanstreels, a wildlife veterinarian with the UC Davis One Health Institute's Latin America Program and one of the senior authors of the study, has worked extensively on the frontlines of wildlife disease outbreaks and mass mortality events throughout Latin America and Antarctica.
"We are very concerned with the impacts of bird flu on Antarctic wildlife, as H5N1 viruses have already been linked to seabird mortalities in Antarctica," said Vanstreels. "However, this case is an important reminder that other pathogens continue to circulate and cause mortality in these populations."
Genetic analysis revealed that the bacterial strain responsible for the outbreak was closely related to one identified in Antarctic seabirds more than two decades earlier, suggesting that avian cholera has circulated in the region for at least 20 years. Researchers also found close genetic links to strains detected more than 8,000 kilometers away in the Southern Ocean.
"The findings suggest avian cholera may be a more widespread and underestimated cause of seabird mortality in Antarctica than previously recognized," said lead author Matteo Iervolino from Erasmus University. "They also underscore the importance of sustained wildlife health surveillance in remote ecosystems, where outbreaks can go undetected or uninvestigated."
The HPAI Australis Expedition was funded by the International Association of Antarctica Tour Operators and Ocean Expeditions. The European Union provided additional funding.
