Scientists have discovered what killed thousands of penguins on remote Antarctic islands. An analysis of the remains of 49 deceased birds revealed that the culprit was avian cholera, rather than the H5N1 avian influenza virus, which had been previously suspected.
In early 2024, the H5N1 avian influenza virus reached the Antarctic region for the first time; it was confirmed in birds found dead on the sub-Antarctic island of South Georgia. Having already caused mass mortality among seabirds and marine mammals in other parts of the world, the virus raised fears of a potential catastrophe on this highly vulnerable continent. Penguins were a particular concern, as they live in massive, densely packed colonies and have no prior exposure to such novel infections.
Adélie penguins are the most numerous species of Antarctic penguin. The Danger Islands, located in the Weddell Sea, are home to two massive colonies comprising over a million breeding pairs. These islands were considered a final stronghold for the species—while populations elsewhere, such as on the western side of the Antarctic Peninsula, were in decline, the colonies here remained stable.
Avian cholera is an infection caused by the bacterium Pasteurella multocida. It affects more than 190 bird species worldwide. In birds, the disease progresses rapidly; an infected individual often dies within a matter of hours, frequently without showing any outward signs of illness.
In March 2024, an international expedition arrived at the Danger Islands. There, the researchers discovered thousands of dead Adélie penguins, along with the remains of skuas, snowy sheathbills, a giant petrel, and one fur seal. Since avian influenza had reached the region shortly before, researchers initially suspected it to be the cause.
To identify the true culprit, biologists collected samples from 49 dead animals. They performed necropsies, conducted molecular tests, and examined tissues under a microscope. The findings were published in the journal Proceedings of the Royal Society B: Biological Sciences.
The analysis provided a clear answer: DNA of the bacterium responsible for avian cholera was present in 46 of the 49 carcasses. Genetic material from the influenza virus was found in only a few birds, and in very small quantities. Microscopic examination confirmed the presence of bacterial clusters within the penguins’ blood vessels and livers, along with characteristic areas of tissue necrosis.
Genetic analysis shed light on the history of this local pathogen. The bacterial strain found on the Danger Islands proved nearly identical to the one that caused an outbreak of avian cholera among penguins in Hope Bay back in 2001. This indicates that the infection has been circulating in the region for at least 20 years.
The study’s authors estimate that approximately 1% of the adult penguin population on Heroina Island perished. However, the researchers cautioned that this figure might represent only a small fraction of the actual mortality, as scavengers and snowfall quickly conceal the carcasses. While a precise timeline for the outbreak could not be established, observations of nesting sites suggest that the epidemic occurred between January and early February 2024.
Ultimately, the biologists demonstrated that behind the fears surrounding avian influenza lies an old, lethal pathogen—a bacterium that has persisted in the Southern Ocean for decades. The Danger Islands are a sanctuary that hosts 55% of the entire Adélie penguin population of the Antarctic Peninsula. Now, this sole stable refuge for Adélie penguins faces a triple threat: ancient bacteria, a new virus, and melting ice.
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