H5N1 was not the only suspect
In March 2024, researchers found thousands of dead Adelie penguins and other seabirds at the Danger Islands off the Antarctic Peninsula. The timing, during a wave of highly pathogenic H5N1 bird flu across the region, led scientists to suspect avian influenza.
A detailed investigation of the carcasses pointed elsewhere. An international team of researchers, including scientists from the University of California, Davis, swabbed and collected tissue samples from 49 carcasses. Avian cholera was the most likely cause of death for most of the adults they examined, including 10 of 11 Adelie penguins at Heroina Island and every adult sampled at a second colony.
A strain that has circulated for decades
Genetic analysis showed the bacterial strain behind the outbreak has been present in Antarctica for more than two decades. The findings were published by the Royal Society.
Avian cholera is caused by the bacterium Pasteurella multocida. It spreads quickly in dense bird colonies, where birds crowd together while breeding. Penguins at these megacolonies nest shoulder to shoulder, which makes outbreaks hard to contain once they start.
Surveillance needs to widen
The authors say the result matters for how wildlife disease is monitored. With bird flu dominating attention and testing, cholera can be missed, and both diseases can strike the same colonies in the same season.
Antarctic wildlife is already under pressure from shrinking sea ice, changing prey availability and warming water. Adding a bacterial pathogen to that list complicates conservation planning for species that breed in just a few large, tightly packed groups, where a single outbreak can reach a big share of a population.
Bird flu surveillance in the region has been intense since H5N1 reached Antarctica, and field teams now routinely test carcasses for the virus. This study suggests testing should also cover bacterial causes, which need different laboratory methods and can be missed when attention is fixed on influenza.