East Greenland polar bears now join the list of Arctic H5 sentinels
Serologic evidence from East Greenland polar bears shows recent exposure to influenza A virus and, in a subset, to the globally emerging highly pathogenic avian influenza H5 clade 2.3.4.4b. The finding matters because polar bears sit at the top of the Arctic food web, where a spillover event can signal broader viral movement across migratory flyways and marine-terrestrial interfaces.
Plasma collected during subsistence hunts near Scoresby Sound in Ittoqqortoormiit, East Greenland, revealed a sharp shift in antibody detection over a short time window. No bears sampled before 2024 had influenza A antibodies. In 2024, 3 of 14 bears were positive on a competitive ELISA, and in 2025 the proportion rose to 6 of 14. Four of the 2025 positives also reacted in hemagglutination inhibition testing against H5N1 viruses within clade 2.3.4.4b. In practical terms, that pattern is hard to explain without genuine wildlife exposure to a recent H5 virus.
The bears themselves were harvested as apparently healthy animals. Hunters reported no obvious clinical illness, which suggests these large carnivores can survive infection long enough to mount and retain an antibody response. In a virus family known for high mortality in some mammalian hosts, survival after exposure is a biologically important clue. It implies either a nonlethal encounter, a lower infectious dose, or host factors that blunt disease severity despite infection.
Serology points to recent spillover rather than historical background noise
Researchers screened 109 plasma samples collected between 2016 and 2025 with a competitive ELISA targeting the influenza A nucleoprotein. That assay detected no antibodies before 2024, then identified positive samples in both 2024 and 2025. The temporal pattern is central. It argues against a long-standing background signal in the population and instead points to a recent exposure event in East Greenland.
The confirmatory hemagglutination inhibition assay strengthened the interpretation. Plasma from 4 bears harvested in 2025 inhibited hemagglutination of H5N1 viruses from clade 2.3.4.4b at a 1:40 dilution. The same plasma did not show the same pattern against clade 1 viruses, which helps narrow the result toward the modern HPAI lineage rather than broad, nonspecific cross-reactivity. Because clade 2.3.4.4b is exclusively a highly pathogenic lineage, the serology fits with exposure to a virus of real ecologic consequence.
A single sentence captures the signal: East Greenland polar bears had antibodies consistent with exposure to contemporary H5 HPAI.
The likely exposure window also follows the ecology of the Arctic rather than the hunting season. Migratory birds are absent during the winter–spring hunt period, so the bears were probably infected during the preceding summer or fall, when birds pass through the region and when contamination of coastal environments is more plausible. The timing aligns with prior reports of H5N1 and H5N5 in seabirds from Greenland and adjacent Arctic zones. Those bird infections create the upstream source from which bears can be exposed through scavenging, predation, or contact with contaminated carcasses and nesting sites.
Spillover routes in the Arctic are broad, but scavenging is the leading candidate
The biology of HPAI spillover in wild mammals is straightforward even if the exact route in this setting remains uncertain. Infection can follow ingestion of infected birds, contact with contaminated surfaces, exposure to mucosal or nasal secretions, scavenging of dead birds or eggs, feeding on infected mammals, or, in some species, mother-to-offspring transmission through lactation. For polar bears, the most plausible route is ingestion of infected birds or mammals, because these predators and scavengers exploit carrion when sea ice retreat forces longer periods on land.
Climate-driven sea ice loss increases that contact. As polar bears spend more time on shore, they encounter bird colonies, carcasses, and marine mammal remains that may harbor pathogens moving along the East Atlantic and North American Atlantic flyways. Those flyways converge through Greenland, turning the region into an ecologic crossroads where avian viruses can circulate between continents and spill into mammals.
This is not the first time HPAI has reached the Arctic mammalian ecosystem. Deaths linked to H5N1 in Alaska and H5N5 in Svalbard and Jan Mayen have already shown that these viruses can cross species and geography in high-latitude environments. The East Greenland data extend that pattern by showing exposure in living polar bears, not just fatal infection in stranded or deceased animals.
Why the polar bear matters as a sentinel species
Polar bears may function as an early warning species for HPAI activity in Arctic ecosystems because they combine two risk factors: a carnivorous diet that includes scavenging and a habitat that places them in the path of migratory birds and contaminated coastal food webs. Serologic surveillance in this species does more than document infection. It can reveal when and where viral incursions are happening before they are obvious in humans or domestic animals.
The study also shows the value of collaboration with Indigenous hunters. The samples came from a long-running partnership with local subsistence harvesters, and those relationships made it possible to access animals from remote terrain that would otherwise remain unsampled. In regions where wildlife, people, and pathogens overlap, surveillance depends on that kind of field infrastructure as much as on laboratory testing.
Still, the findings remain limited to serology. Antibody detection cannot identify the exact infecting virus, the infectious dose, or the clinical course in each bear. Cross-reactivity with low pathogenic H5 strains is possible in theory, even though the hemagglutination inhibition pattern in these samples favored clade 2.3.4.4b. The work also cannot determine whether infection occurred from birds, mammals, or contaminated environmental material. Those questions require virologic surveillance, additional sampling, and, ideally, paired ecological data from the same seasonal window.
Even with those limits, the message is direct. East Greenland polar bears have been exposed to HPAI in recent years, and the serologic trail points to the same H5 lineage now moving through birds and mammals across the Arctic. The next step is not speculation about whether the virus reached the region; it is tracking how often it arrives, which species carry it, and how far it can spread through a rapidly changing northern ecosystem.