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H5N1 and H5N5 Were Detected in the Same Great Black-backed Gull Population; H5N5 Also in 17 Grey Seals

Adult great black-backed gull standing on a rocky shore
Research · Disease, parasites & microbiology

During a seabird mortality event on the Norfolk coast, two highly pathogenic avian influenza subtypes were detected within the same local great black-backed gull (Larus marinus) population. In a study first published on 24 January 2026, H5N1 was detected in five great black-backed gulls and H5N5 in 17. These were detections in the same local population during the same event; the study did not report individual gulls co-infected with both subtypes. Site staff had recorded 46 dead great black-backed gulls, together with one European herring gull (Larus argentatus) and one common gull, around Blakeney Point during the winter 2024–2025 event.

H5N5 was also detected in one European herring gull, one atlantic puffin and one northern fulmar, while H5N1 was found in one northern fulmar. The authors describe this as the first recorded instance in Great Britain of H5N1 and H5N5 being detected within the same avian population at the same location.

H5N5 was also detected in grey seals

The investigation also tested grey seal (Halichoerus grypus) carcasses from the same site. H5N5 was detected in 17 seals, whereas none tested positive for H5N1. The study reports this as the first mass detection of highly pathogenic H5N5 avian influenza in mammals in Great Britain. Blakeney Point holds England’s largest grey seal breeding colony, bringing large numbers of seals and scavenging gulls into close contact during the breeding season.

Map of bird, environmental and grey seal samples at Blakeney Point showing H5N1 and H5N5 results
Bird, environmental and grey seal samples from Blakeney Point and their H5N1/H5N5 test results. Figure 1 from Falchieri et al. (2026), Transboundary and Emerging Diseases, CC BY 4.0.

The carcass counts could not provide a precise total for the bird mortality. Many remains were decomposed or incomplete, and the length of the beach and tidal conditions prevented a complete search. The researchers therefore considered the number of dead birds to be greater than the number sampled.

Genome data revealed markers linked to mammalian adaptation

Influenza A subtypes are named from two surface proteins: hemagglutinin (H) and neuraminidase (N). H5N1 and H5N5 therefore share the H5 subtype but differ in their neuraminidase, N1 versus N5. Genome sequencing showed that the H5N1 virus was closely related to viruses circulating in British wild birds and poultry at the time. The H5N5 viruses from birds and seals shared a 22-amino-acid deletion in the neuraminidase stalk and the PB2 E627K change. PB2 is part of the viral polymerase complex, and E627K is associated with mammalian adaptation. However, the same change has also been found in avian H5N5 viruses without obvious mammalian involvement, so it does not by itself show that the virus had adapted to seals.

The H5N1 sequence did not contain mutations that the researchers considered suggestive of increased mammalian adaptation or zoonotic potential. Despite H5N1 and H5N5 occurring in the same bird population and place, the genomic analyses found no evidence of reassortment between them — the exchange of gene segments that can occur when different influenza viruses infect the same host.

The direction of transmission remained unresolved

The study could not establish the direction of transmission between birds and seals. Great black-backed gulls and European herring gulls feed around seal haul-outs, including on carcasses and afterbirths, creating opportunities for exposure in either direction. The observed seal mortality remained within the expected range for the colony and did not support sustained mammal-to-mammal transmission, although the authors could not rule it out.

Sources and image credit

  1. Falchieri, M., Bentley, E., Coombes, H. A., Mollett, B. C., Terrey, J., Holland, S., Stubbings, E., Mcginn, N., Cooper, J., Ahmad, S., Lewis, J., Clifton, B., Collinson, N., Aegerter, J., Venkatesh, D., Russell, D. J. F., James, J., Reid, S. M., & Banyard, A. C. (2026). Detection of two high pathogenicity avian influenza virus (HPAIV) subtypes, H5N1 and H5N5, in a mass mortality event in wild seabirds and co-location with dead seals. Transboundary and Emerging Diseases, 2026, 1–9. Article 4680980. https://doi.org/10.1155/tbed/4680980Primary open-access research article first published 24 January 2026. H5N1 was detected in five Great Black-backed Gulls and H5N5 in 17 at Blakeney Point, Norfolk; H5N5 was also detected in 17 Grey Seal carcasses. The authors report the first recorded co-detection of H5N1 and H5N5 within the same avian population at one British location and the first mass detection of H5N5 in mammals in Great Britain.

Image: Ryan Hodnett — great black-backed gull (Larus marinus) photographed in Øksnes, Norway, 14 July 2023; representative species image, not a bird from the Norfolk mortality event. Source: Wikimedia Commons. · CC BY-SA 4.0 · Source

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