LONDON. A spike in seal mortalities off the British coast has heightened concerns among virologists that the H5N1 avian influenza virus is adapting to mammals, increasing the threat of a human pandemic. Scientists at the Animal and Plant Health Agency (APHA) have documented a significant rise in infections among grey seals in Norfolk and Cornwall, with preliminary genetic sequencing suggesting the virus has acquired mutations associated with mammalian adaptation.
The findings, published in the journal Emerging Infectious Diseases, indicate that the circulating strain now carries a substitution in the PB2 gene known to facilitate replication in mammals at lower temperatures. This change, while not yet conferring efficient human-to-human transmission, represents a step along a well-documented evolutionary pathway that virologists have warned about for decades.
Professor Ian Brown, director of scientific services at APHA, said that the data represent a clear signal of increasing risk. "We are observing changes in the virus that are consistent with adaptation to mammalian hosts. Each spillover event provides an opportunity for the virus to acquire mutations that could make it more transmissible in humans."
The United Kingdom has recorded 37 seal deaths attributable to H5N1 since January, a marked increase from previous years. The virus has now been detected in a broad range of mammals including foxes, otters, and polecats. In the United States, the Centers for Disease Control and Prevention confirmed this week that a dairy farm worker in Michigan tested positive for H5N1, the third human case linked to the ongoing outbreak in cattle. That patient reported conjunctivitis and mild respiratory symptoms and has since recovered.
The World Health Organization maintains that the overall public health risk remains low, but acknowledges that the situation warrants intense surveillance. Dr. Maria Van Kerkhove, acting director of the WHO's epidemic and pandemic preparedness and prevention department, noted that the expanding host range is unprecedented. "The breadth of mammalian species now infected with H5N1 is historically unparalleled. Each new host represents an opportunity for viral reassortment and adaptation."
The UK Health Security Agency has activated its incident management protocols and is working with veterinary authorities to monitor both animal and human populations. Enhanced surveillance of wastewater for influenza A has been implemented at six sites across England. The UK has also secured advance purchase agreements for two H5N1 vaccine candidates, though production capacity remains limited.
For the international community, the situation in the UK mirrors a global trend. Since 2020, the highly pathogenic avian influenza H5N1 clade 2.3.4.4b has spread via migratory birds to every continent except Australia and Antarctica. It has caused mass die-offs of seabirds, seals, and has infected dairy cattle in the United States, where more than 90 herds across 12 states have tested positive.
Dr. Tom Peacock, a virologist at Imperial College London, cautioned against alarm but emphasised the need for preparation. "We are not in a pandemic situation. But we are in a phase where the virus is performing experiments in real-time. Our job is to be ready for the outcome."
The UK government has been urged by opposition MPs to publish its pandemic preparedness plan, last updated in 2023, and to outline investment in surge capacity for testing, antiviral stockpiles, and vaccine manufacturing. The Department of Health and Social Care said it would provide an update in the coming weeks.
As the virus consolidates its presence in new mammalian hosts, the core question for policymakers is not whether a pandemic will occur, but whether the window for containment has already closed. For now, the answer depends on whether the virus gains the final key adaptation: efficient airborne transmission between humans.
