Four years ago the Norwegian Veterinary Institute reported both H5N1 and H5N5 for the first time in wild birds on Svalbard, which lies above the Arctic circle (see More HPAI (H5N5 & H5N1) Detected In Arctic (Svalbard).
Since then we've been tracking a small - but growing - number of spillovers of H5N5 to mammals in both Europe and Canada, including seals in the UK, domestic cats in Iceland, and raccoons (and other small mammals) in Canada.
Two summers ago (2024) in Cell Reports: Multiple Transatlantic Incursions of HPAI clade 2.3.4.4b A(H5N5) Virus into North America and Spillover to Mammals, researchers reported finding the mammalian adaptive E627K mutation in a number of samples.
Note: Many of the H5N5 viruses that have spread across Canada and the United States have reassorted with local LPAI viruses, and are now genetically distinct from the Svalbard & European lineages, making direct comparisons difficult.
Two months ago (May 20th) the Norway Veterinary Institute, announced the detection of HPAI H5N5 virus in both a dead walrus and a polar bear on Svalbard Island.
Although no other animals were confirmed infected during this investigation, there were eye witness accounts of two other polar bears exhibiting potential neurological symptoms.
Today we've an unusually rapid follow-up report; a preprint published yesterday on the bioRxiv server that reports that H5N5 exposure is far more common among polar bears on Svalbard island than previously thought.
Somewhat surprisingly, most polar bears appear to survive H5N5 infection, although severe (and likely neurotropic) illness can occur.
First, the Abstract and a few excerpts from the preprint, but you'll want to read it in its entirety. I'll return after the break with a bit more.
Highly Pathogenic Avian Influenza H5N5 in a Polar Bear and Atlantic Walrus, Svalbard, 2026, with Widespread Seroconversion in Polar Bears
Knut Madslien, Johanna Hol Fosse, Jon Aars, Cathrine Arnason Bøe, Magnus Andersen, Kayla Buhler, Ingvild Fjeldheim, Britt Gjerset, Torhild Jørgensen, Ida Kristin Myhrvold, Andreas Rohringer, Kjersti Sturød, Morten Tryland, Bjørnar Ytrehus, Ragnhild Tønnessen, Ingebjørg Helena Nymo
doi: https://doi.org/10.64898/2026.07.20.739480
This article is a preprint and has not been certified by peer review [what does this mean?].
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Abstract
Highly pathogenic avian influenza virus (HPAIV) subtype H5N5 was detected in a one-year-old polar bear (Ursus maritimus) and an adjacent adult Atlantic walrus (Odobenus rosmarus rosmarus), both found deceased in Raudfjorden, Svalbard. This represents the first confirmed case of HPAI in a European polar bear and the second in an Atlantic walrus.Viral genomes were nearly identical and harbored PB2-E627V, a marker associated with mammalian adaptation. Several polar bears, including the deceased individual, had previously been observed feeding on the walrus carcass.Antibodies against H5 were detected in 75% of polar bears in 2023 (n=36) and 97% in 2024-2025 (n=65), suggesting extensive circulation of HPAIV in the population following the first detections in birds in Svalbard in 2022, whereas no antibodies were detected in samples from 2014-2022 (n=243).
(SNIP)
Although no human cases have been linked to the HPAIV H5N5 PB2-E627V cluster, its detection across diverse mammalian species indicates a broad host range. Occurrence in both terrestrial and marine mammals suggests that ecological interactions, including predation and scavenging, may facilitate cross-species transmission.
These findings are consistent with incremental, multigenic adaptation enabling infection of multiple hosts without evidence of full mammalian adaptation. Although HPAIV H5N5 can be zoonotic (43), the risk of zoonotic spillover to humans is considered very low for H5N5 detected in wildlife species with little or no human contact.
Nevertheless, the transmission to novel host species underscores the importance of timely risk assessment, surveillance, and infection preparedness, as well as awareness and adherence to appropriate biosafety measures among wildlife professionals.
Continued circulation within an avian-associated lineage as well as uncertainties on disease reservoir and adaptation underscores the need for disease surveillance at the wildlife–human interface in Arctic and North Atlantic regions.
The current report highlights that H5 HPAIV circulates in Arctic ecosystems and infects apex predators. Although no population-level effects are currently evident, detection in brain tissue and field observations suggest that disease can occur at the individual level. The identification of PB2-E627V and additional polymerase-associated changes is consistent with adaptation to mammals within an avian-associated lineage. These findings highlight the need for continued surveillance to clarify transmission pathways, evolutionary dynamics, and potential health effects.
Perhaps the bigger question is how much `silent' transmission of HPAI H5Nx is occurring unnoticed in mammalian wildlife?
The above preprint lists only one confirmed infection of a polar bear with H5N5 on Svalbard island, yet seroprevalence studies suggest 97% exposure.
How common this kind of disparity might be in other species is unknown.
Specifically, antibodies to avian influenza virus nucleoprotein were detected in 54.9% (n = 61) of samples using enzyme-linked immunosorbent assay; antibodies to Eurasian-lineage highly pathogenic avian influenza H5 clade 2.3.4.4b and North American low pathogenic avian influenza H5 were detected in 43.2% (n = 48) and 22.5% (n = 25) of samples, respectively, using virus neutralization assays; and antibodies to avian influenza virus neuraminidase were detected in 44.1% (n = 49) of samples using enzyme-linked lectin assay.
While all of this is admittedly based on just two surveys - involving different subtypes and different mammalian species - this reminds us that much of what H5Nx is doing in the wild occurs outside of our view.
