Thousands of dead Adélie penguins on the remote Danger Islands off the Antarctic Peninsula puzzled researchers. Initially, the highly contagious bird flu was suspected. A new study now shows that another pathogen was apparently responsible for the mass die-off: bacterial avian cholera.
When researchers discovered numerous dead Adélie penguins on the Danger Islands in 2024, avian influenza was the obvious suspect. The H5N1 virus had already spread to parts of Antarctica and caused deaths amongst seabirds and marine mammals.
An international research team therefore investigated the cause of death in more detail. As reported by the University of California, Davis, on 14 October 2026, the bacterium Pasteurella multocida was detected in 46 out of 49 penguins examined. The pathogen causes what is known as avian cholera, an infectious disease that can lead to severe illness and sudden death in seabirds.
An underestimated threat to penguin colonies
The discovery is remarkable because avian cholera has so far received significantly less attention than avian influenza. Both diseases can cause major losses in densely populated seabird colonies. Whilst avian influenza is caused by a virus, avian cholera is caused by a bacterium.
The Danger Islands lie in the Weddell Sea, north-east of the Antarctic Peninsula, and are home to significant breeding colonies of Adélie penguins. During the breeding season, the birds live in close proximity to one another, which can facilitate the transmission of pathogens.
It is still unclear how the bacterium reached the colonies and how it spread amongst the birds. Nor can it be concluded from the samples analysed that all the deaths were caused by avian cholera.
The study highlights the importance of carefully analysing disease outbreaks in Antarctica. Not every mass die-off of seabirds is attributable to avian influenza. In order to better protect penguin colonies, researchers must also investigate other pathogens and their routes of transmission.
Rosamaria Kubny, PolarJournal