Avian cholera causes mass mortality at penguin mega-colonies amid H5N1 HPAI outbreaks in Antarctica.

The characterization of pathogens affecting Antarctic wildlife has been historically limited and sporadic. With the introduction of H5N1 high pathogenicity avian influenza virus (HPAIV) in Antarctica in the austral summer 2023/2024, concerns about the health of Antarctic seabirds and mammals prompted immediate surveillance efforts. In March 2024, we visited the Danger Islands archipelago, that hosts two mega-colonies of Adélie penguins, and observed an unusual mortality estimated in thousands of Adélie penguins and other species. Swabs and tissues were collected for molecular detection of infectious agents from 49 carcasses, and additional tissues for histology from a selection of 9 carcasses. We detected Pasteurella multocida DNA in 46 of 49 individuals, and diagnosed avian cholera as the cause of death of most of these animals. By shotgun sequencing, we retrieved the first genomic sequences of the P. multocida strain from this region, and the phylogenetic analysis showed a close relation with strains from other locations retrieved during the same expedition and with strains previously reported in the Southern Ocean area. This study confirms avian cholera as a relevant cause of mortality in the Antarctic, and overall highlights the importance of considering this disease in the differential diagnoses during mortality events in Antarctica.