537 dead penguins point to hidden bacterial threat

A team of researchers identified avian cholera as the primary cause of a mass die-off among Adélie penguins in the Danger Islands, a finding published in October in the Proceedings of the Royal Society B. The discovery corrects earlier assumptions that high pathogenicity avian influenza, or bird flu, was responsible for the deaths observed in March 2024.

Dr. Ralph Vanstreels, a wildlife veterinarian with the University of California, Davis, led the expedition to the islands located off the Antarctic Peninsula. The team initially arrived expecting to find evidence of a viral outbreak, given that bird flu had reached Antarctica in late 2023 and early 2024. However, Vanstreels and his colleagues found more than 537 penguin carcasses on a single landing ramp, an area roughly the size of an airport runway. They estimate that thousands of birds died across the two mega colonies in the region, though a complete count was not possible due to short sailing windows and the risk of icebergs blocking exit routes.

Laboratory analysis of samples taken from the deceased birds revealed low levels of bird flu virus, a result that did not align with the severity of the mortality. Microscopic examination of the tissues later identified bacteria, specifically the strain that causes avian cholera. This bacterial disease is distinct from human cholera and poses a limited risk to human infection, according to Vanstreels. The bacteria thrive in cold environments, which may explain the high death rates in the Antarctic setting.

The findings suggest that avian cholera may be a recurring, under-documented threat in the region. The last systematic search for the disease in the Antarctic Peninsula took place 25 years ago. Vanstreels notes that scientists may have been overlooking regular die-offs because attention has been focused heavily on the global spread of bird flu. Since late 2021, the avian influenza virus has killed or culled nearly 300 million poultry birds worldwide, a figure reported by the World Animal Health Information System. The virus recently reached mainland Australia in June 2026, ending a six-year period where the continent remained free of the pathogen.

Experts believe the strain of avian cholera found in the penguins is closely related to one detected 5,000 miles away on Île Amsterdam in the Southern Indian Ocean. Dr. Amandine Gamble, an assistant professor at Cornell University, states that Antarctica is more connected to the rest of the world than previously thought. The continent is governed by the Antarctic Treaty, which has 58 parties including the United States, France, Japan, and Russia. These nations collaborate on scientific efforts to track pathogens that travel across borders without regard for political lines.