#18,388
Five weeks ago the UK made a brief announcement on the detection of HPAI H5N5 In 2 Grey Seals in Norfolk. This was the first detection of HPAI H5 in a non-avian species in the UK in nearly 2 years, and the first such detection involving the H5N5 subtype.
Last November, the UK reported an outbreak in poultry of H5N5 in Yorkshire along with > 2 dozen dead birds infected with H5N5 from around the kingdom (see map below).HPAI H5N5 is a reassortant offshoot of the clade 2.3.4.4b H5N1 virus, which has turned up sporadically in European birds, cats in Iceland, birds in the the UK - and since 2023 in Canada - infecting both birds and mammals.
They wrote:
Thus, while A(H5N5) viruses are comparably uncommon, their high virulence and mortality potential demand global surveillance and further studies to untangle the molecular markers influencing virulence, transmission, adaptability, and host susceptibility.
Over the weekend the UK's APHA announced the detection of an additional 13 grey seals with H5N5, and the disclosure that the mammalian E627K mutation was detected as well.
Note 1: findings in grey seals in North Norfolk
Following the initial detection of avian species (great black-backed gulls (Larus marinus)) and two grey seals (Halichoerus grypus) testing positive for high pathogenicity avian influenza virus (HPAIV) H5N5 at Blakeney point, Norfolk in February 2025, further sampling was undertaken by the avian influenza national reference laboratory in collaboration with APHA’s Diseases of Wildlife Scheme to establish the broader impact of the virus on seals and birds in that location.
A total of 40 seal carcasses were sampled from the site and from that 37.5% (n=15) tested positive for HPAIV H5N5.
These animals are part of a well-established pinniped colony in Norfolk. Mortality noted at this seal colony was below the expected baseline of 10% during pupping and sampling efforts in seals were triggered following the observation of a mortality event in gulls that also tested positive for HPAIV.
The sequence generating from positive seal samples included a mammalian adaptive mutation (E627K) in one gene (PB2) but this mutation has been observed in numerous positive samples from mammals detected previously both in Great Britain and globally, and in isolation isn’t considered to represent an increase in zoonotic risk.
It is likely the animals were exposed to the same source of infected wild birds.
We cannot determine with certainty whether influenza of avian origin was the sole cause of death in these wild animals, and it is possible other factors may have contributed. All findings are also reported to the World Organisation for Animal Health (WOAH).
While H5N5 remains a minor player, it is not impossible that we could a shift from H5N1 to H5N5, or H5N6 (or to some other n-type), in the months or years ahead. After all, little more than 5 years ago H5N1 was in serious decline, while HPAI H5N8 was dominant.
The H5Nx virus has an large - and growing - array of diverse avian and mammalian hosts to inhabit, and no lack of genetic diversity on which to draw.
While past performance is no guarantee of future results, its current trajectory is nevertheless concerning.
