Avian cholera bacteria were found in 46 of 49 Adélie penguins sampled after a 2024 die-off
Researchers detected avian cholera bacteria in 46 of 49 Adélie penguins sampled after a 2024 die-off in Antarctica's Danger Islands. H5N1, the first suspect, turned up in only a few, so a test for flu alone would have named the wrong cause.
The Scientist · Science desk
Key figures: genetic analysis suggests avian cholera has circulated in the region for at least 20 years; close genetic links were found to strains more than 8,000 kilometers away in the Southern Ocean; the Danger Islands are home to more than 1.2 million Adélie penguin breeding pairs.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Years avian cholera has likely circulated, at least | 20 years | 7 | |
| Distance to related strains, more than | 8,000 km | in the Southern Ocean; 5,000 miles | 8 |
| Adélie penguin breeding pairs, more than | 1,200,000 breeding pairs | on the Danger Islands | 3 |
What happened
- An international expedition to the Danger Islands in March 2024 found thousands of dead Adélie penguins across two islands, plus carcasses of several other seabird species.
- By the authors' reading, avian cholera has been present in the region for at least 20 years: the outbreak strain closely matched one identified in Antarctic seabirds more than two decades before.
- The strain also has close genetic relatives among those detected more than 8,000 kilometers away in the Southern Ocean.
Why it matters
- decision A die-off survey that tests only for H5N1 would have returned a few positives and the wrong cause, so bacterial testing and pathology belong in the same protocol.
- constraint With avian cholera circulating for at least 20 years, the arrival of H5N1 on the Peninsula no longer dates or explains a mortality event on its own.
- exposure Remote colonies that nobody samples cannot say which pathogen is at work, and Iervolino says outbreaks there can go undetected or uninvestigated.
Pasteurella multocida was detected in 46 of the 49 animals sampled [6], or 93.9% [14]. That leaves three animals without a detection [15]. Set against thousands of carcasses across two islands [2], 49 is a small sample, and the phys.org account of the study does not say how the animals were chosen or whether any carried both the bacterium and the virus.
The paper's claim is that most of the investigated deaths were caused by avian cholera [5]. Finding a bacterium in a carcass does not by itself show it killed the bird, and the team paired laboratory testing with pathological examinations [5]. In our view that pairing is what lets the authors speak of cause of death at all.
The genetics argue against a recent arrival. The outbreak strain was closely related to one identified in Antarctic seabirds more than two decades earlier, and the authors suggest avian cholera has circulated in the region for at least 20 years [7]. They also found close links to strains detected more than 8,000 kilometers away in the Southern Ocean [8]. H5N1 had only recently reached the Antarctic Peninsula [4]. At the moment of discovery, timing was the only thing connecting bird flu to the carcasses.
Ralph Vanstreels, a wildlife veterinarian at the UC Davis One Health Institute's Latin America Program and a senior author [11], put the caution plainly. "However, this case is an important reminder that other pathogens continue to circulate and cause mortality in these populations," he said [9].
One expedition in March 2024 [2], two islands and 49 animals show that avian cholera can drive a mass die-off in a colony of more than 1.2 million breeding pairs [3]. The study gives no rate across Antarctica. Lead author Matteo Iervolino of Erasmus University hedged accordingly. "The findings suggest avian cholera may be a more widespread and underestimated cause of seabird mortality in Antarctica than previously recognized," he said [10]. He added that sustained surveillance matters in remote ecosystems, where outbreaks can go undetected or uninvestigated [13].
The study is by Iervolino and colleagues in Proceedings of the Royal Society B, titled "Avian cholera causes mass mortality at penguin mega-colonies amid H5N1 HPAI outbreaks in Antarctica" [12].
What to watch
- Whether follow-up work reports how the 49 animals were chosen and whether any carried both the bacterium and H5N1.
- Whether surveys of other Antarctic colonies test for Pasteurella multocida alongside H5N1 after a mass death.
- Whether further sequencing ties the Danger Islands strain to the Southern Ocean strains more than 8,000 kilometers away.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence64
- Adoption
- Insufficient
- Hype gap0
- Incentives
- Insufficient
- Confidence62
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
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 in Proceedings of the Royal Society B.
- [2]
In March 2024 an international research expedition to the Danger Islands found thousands of dead penguins across two islands, along with carcasses of several other seabird species.
- [3]
The Danger Islands are home to more than 1.2 million breeding pairs of Adélie penguins.
- [4]
H5N1 had recently reached the Antarctic Peninsula, so bird flu was the initial suspected cause of the deaths.
- [5]
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.
- [6]
The bacterium was detected in 46 of 49 animals sampled, while H5N1 was present in only a few.
- [7]
Genetic analysis showed the bacterial strain responsible was closely related to one identified in Antarctic seabirds more than two decades earlier, suggesting avian cholera has circulated in the region for at least 20 years.
- [8]
Researchers also found close genetic links to strains detected more than 8,000 kilometers (5,000 miles) away in the Southern Ocean.
- [9]
"However, this case is an important reminder that other pathogens continue to circulate and cause mortality in these populations," said Ralph Vanstreels.
- [10]
"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.
- [11]
Ralph Vanstreels is a wildlife veterinarian with the UC Davis One Health Institute's Latin America Program and one of the senior authors of the study.
- [12]
The paper is Matteo Iervolino et al., 'Avian cholera causes mass mortality at penguin mega-colonies amid H5N1 HPAI outbreaks in Antarctica', Proceedings of the Royal Society B: Biological Sciences (2026), DOI 10.1098/rspb.2026.1271.
- [13]
Iervolino said the findings underscore the importance of sustained wildlife health surveillance in remote ecosystems, where outbreaks can go undetected or uninvestigated.
- [14]
46 of 49 sampled animals is about 93.9%.
- [15]
Three of the 49 sampled animals had no bacterial detection.
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Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgScientists identify avian cholera as major cause of penguin deaths in Antarctica
1 article · October 14, 2026
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Entities
- Adelie penguinFollow
- H5N1 avian influenza virusFollow
- Pasteurella multocidaFollow
- Proceedings of the Royal Society BFollow
- One Health Institute, University of California, DavisFollow
- Ralph VanstreelsFollow
- Matteo IervolinoFollow