A growing body of research is revealing that the H5N1 avian influenza virus, once considered primarily a threat to poultry and wild birds, is now spreading through an unexpectedly wide range of wild mammal species — from sea lions on South American coastlines to red foxes in European forests. Wildlife biologists warn that the virus’s adaptation to mammalian hosts represents one of the most significant ecological and public health developments of the past two years, with implications that stretch from biodiversity conservation to human pandemic preparedness.
The latest surveillance data indicates that H5N1 has now been detected in more than 48 mammalian species globally, including grizzly bears, polar bears, mountain lions, harbour seals, and dairy cattle. The unprecedented host range has prompted researchers from multiple institutions to ramp up monitoring efforts and reassess long-held assumptions about how influenza viruses move between ecological compartments.
From Birds to Mammals: A Worrying Crossover
Highly pathogenic avian influenza (HPAI) of the H5N1 lineage first emerged in domestic geese in southern China in 1996. For decades, its spillovers into mammals were considered rare and largely dead-end events. That changed dramatically in 2021, when a new clade — designated 2.3.4.4b — began circulating widely among wild birds across Europe, Africa, and the Americas. Unlike earlier strains, this variant has shown a striking ability to infect and, in some cases, transmit between mammals.
According to information published by the [World Organisation for Animal Health](https://www.woah.org/en/disease/avian-influenza/), tens of thousands of wild and farmed mammals have died or been culled as a result of H5N1 infection since 2022. The mass die-offs of South American sea lions in 2023, which killed an estimated 24,000 animals along the coasts of Peru, Chile, and Argentina, were among the first events to suggest that the virus may have acquired mutations enabling mammal-to-mammal transmission. Genetic analyses published in peer-reviewed journals indexed on [PubMed](https://pubmed.ncbi.nlm.nih.gov/) have since identified mutations in the PB2 gene — a known marker of mammalian adaptation — in viruses recovered from these outbreaks.
The Dairy Cattle Surprise
Perhaps the most unexpected development came in early 2024, when H5N1 was detected in dairy cattle in the United States — a host species in which influenza A had never been documented at scale. The U.S. Department of Agriculture has since confirmed infections across multiple states, with the virus appearing to spread between cows through milking equipment and close contact in herds. Several farm workers have tested positive for the virus, though human cases have so far been mild and largely confined to conjunctivitis. Detailed updates and case counts are maintained by the [Centers for Disease Control and Prevention](https://www.cdc.gov/bird-flu/), which continues to classify the public health risk as low but warns that the situation requires vigilant monitoring.
Dr. Richard Webby, a leading influenza researcher at St. Jude Children’s Research Hospital, has publicly cautioned that each new mammalian host represents a fresh opportunity for the virus to acquire mutations that could enhance human transmissibility. Wildlife biologists echo that concern from a conservation angle: endangered species such as the California condor and certain pinniped populations have suffered devastating losses, with recovery programs decades in the making now threatened.
What’s Next for Surveillance and Conservation
Researchers are calling for expanded genomic surveillance of wild mammal populations, better integration between veterinary and public health systems, and accelerated development of vaccines tailored to non-poultry species. Some governments have begun trial vaccination programs for endangered birds, while others are exploring whether dairy cattle vaccination could be feasible. The One Health framework — which links human, animal, and environmental health — has gained renewed urgency in policy discussions.
For wildlife biologists, the coming year will be critical. As migratory birds redistribute the virus across continents during seasonal movements, scientists will be watching closely for further mammalian spillovers, novel reassortment events, and any signs of sustained mammal-to-mammal or human-to-human transmission. The story of H5N1’s expansion is a reminder that the boundaries between ecosystems are far more permeable than once assumed — and that careful biological monitoring is no longer a niche concern, but a global imperative.
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