A mass mortality event of unprecedented scale is unfolding among grey seal pups in the North Sea, with UK scientists reporting that avian influenza H5N1 has killed three-quarters of pups born this season at key breeding colonies. The data, collected by the Sea Mammal Research Unit at the University of St Andrews, paints a stark picture of cross-species transmission and viral persistence in marine mammals.
The outbreak shows no signs of abating. Since December 2024, seal pups have been washing ashore along the coasts of Scotland and northern England, their bodies showing the characteristic neurological symptoms of highly pathogenic avian influenza. The mortality rate among pups has reached 75% in the worst-affected haul-out sites, such as the Isle of May and the Farne Islands. Adult seals appear to be less susceptible, but the loss of an entire year class could have long-term implications for population recovery.
The virus has been present in UK bird populations since 2021, but its jump to seals marks a concerning evolutionary step. H5N1 has now adapted to replicate efficiently in mammalian hosts. The implications extend beyond conservation. Each spillover event represents a potential pathway for the virus to acquire mutations that could threaten human health. The World Health Organisation has classified the current risk to humans as low, but this assessment is contingent on the virus not gaining further adaptations.
The UK Animal and Plant Health Agency has deployed rapid response teams to conduct necropsies and swab samples from both live and dead seals. The data are being shared through the Global Influenza Surveillance and Response System, a network of 159 laboratories across 129 countries. UK scientists are leading the effort to sequence the viral genomes from these marine mammals, tracking the mutations in real time. So far, no human cases have been linked to this outbreak, but the proximity of seal haul-outs to fishing communities and tourist beaches is a vector of concern.
The biosphere is a complex network. When we see a 75% mortality rate in a top predator, it is not an isolated event. It is a signal that the ecological pressures we are placing on the planet are accelerating viral evolution. Climate change is a compounding factor: warming waters and altered prey distributions are already stressing seal populations, making them more vulnerable to disease.
The UK government has allocated 10 million pounds for a new National Wildlife Disease Surveillance Centre, to be based at the Pirbright Institute. This facility will coordinate responses across species and agencies, closing the gap between veterinary and human health. But the broader challenge remains. We are in an era of what epidemiologists call 'spillover emergence', where pathogens move from wildlife to livestock to humans with increasing frequency. The seals are a sentinel.
As spring arrives and the remaining pups wean, the mortality rate will likely decline. However, the virus is now established in the seal population. It will persist in the marine environment, potentially circulating indefinitely. The question is not whether we can eradicate it, but whether we can build the surveillance capacity to detect the next mutation before it becomes the next pandemic. The seals have given us a warning. The question is whether we are prepared to act on it.
