H5 Avian Influenza Exposure Appears in East Greenland Polar Bears
Serology from East Greenland polar bears has added a new mammalian sentinel to the map of highly pathogenic avian influenza in the Arctic. Plasma collected during subsistence hunts between 2016 and 2025 showed no influenza A virus antibodies before 2024, then rose to 3 of 14 bears in 2024 and 6 of 14 in 2025. Four bears harvested in 2025 also carried antibodies to H5 clade 2.3.4.4b, the globally expanding lineage responsible for widespread outbreaks in birds and spillover into mammals.
The finding matters because polar bears do not simply drift through the Arctic food web. They sit at the top of it, moving between marine and terrestrial prey sources as sea ice retreats and feeding options narrow. In that setting, even a brief encounter with infected birds, carcasses, or contaminated environmental material can become biologically consequential. The new serologic signal suggests that East Greenland bears were exposed recently enough to mount a measurable antibody response and then survive long enough for that response to be detected during later hunts.
Serologic Signals Rose Sharply After 2023
Investigators screened 109 plasma samples from polar bears harvested near Scoresby Sound in East Greenland. The sample set covered hunting seasons from January through May and spanned nearly a decade, but the key pattern emerged only in the most recent years. A competitive ELISA targeting influenza A nucleoprotein found influenza A virus antibodies in 21% of bears sampled in 2024 and 43% in 2025. Before 2024, no samples were positive.
The rise was not subtle. Three male bears were positive in 2024, and six bears in 2025 were positive, including five males and one female. The jump in prevalence over such a short interval fits the ecology of a recent introduction into the local food web rather than a long-standing background signal. It also fits the seasonal biology of Arctic influenza, in which migratory birds serve as amplifiers during the months when they occupy northern breeding grounds and then depart before the hunting season begins.
A second round of testing narrowed the interpretation. Positive samples, along with a small subset of negative samples, underwent hemagglutination inhibition testing using avian influenza viruses representative of H5 and H7 subtypes. Four bears harvested in 2025 inhibited hemagglutination of H5N1 viruses within clade 2.3.4.4b at a titer of 1:40. None of the tested sera reacted against clade 1 viruses. In practical terms, the pattern supports infection with a virus from an exclusively HPAI lineage rather than a nonspecific avian influenza exposure.
The Arctic Food Web Is Now a Viral Interface
The serology aligns with recent detections of H5 viruses in Arctic birds. H5N1 and H5N5 had already been confirmed in seabirds along the west coast of Greenland, including Uria aalge, Larus hyperboreus, Fulmarus glacialis, and Larus marinus. East Greenland polar bears likely encountered the virus during the summer–fall migration period before the animals were harvested, because migratory birds are absent during the hunting season itself. That timeline fits a plausible sequence: spillover into scavenging bears during a period of intense bird congregation, followed by survival and seroconversion detected months later.
Hunters reported no clinical signs in the bears, which is important. Deaths in Alaska and Svalbard have shown that HPAI can kill polar bears, but seropositivity in East Greenland indicates a different endpoint for at least some exposures. The animals may have recovered after a sublethal infection, or they may have cleared the virus after transient replication without obvious disease. Either way, the antibody pattern argues that mammalian infection is occurring without necessarily producing carcasses that would be easy to detect in remote Arctic terrain.
This is the core epidemiologic concern. When a pathogen can circulate through birds, spill into mammals, and leave only a serologic trace, passive surveillance will miss part of the picture. Polar bears may now function as a living readout of environmental contamination and prey-level viral activity in a region where field observation is sparse and weather limits routine sampling.
What the Antibody Pattern Says About Transmission
The exact route of exposure in polar bears remains unresolved, but the biology points toward ingestion. HPAI spillover into mammals often occurs after eating infected birds or coming into contact with contaminated carcasses and environmental material. In polar bears, the most plausible pathway is scavenging dead or infected birds and mammals, especially given their opportunistic feeding behavior and the nutritional stress that accompanies sea ice loss. Direct contact through mucosal or nasal secretions is possible, and mother-to-offspring transmission through lactation is biologically plausible in mammals more broadly, but the hunting ecology in East Greenland makes scavenging the strongest candidate.
One detail strengthens the inference. All four seropositive bears tested by hemagglutination inhibition showed activity against clade 2.3.4.4b, the clade associated with current global HPAI outbreaks. Cross-reactivity with low pathogenic H5 viruses can never be ruled out completely in serologic work, yet the absence of reactivity against clade 1 viruses narrows the interpretation and supports true exposure to a recent HPAI lineage.
Surveillance in Greenland May Offer an Early Warning System
Greenland links the East Atlantic and North American Atlantic flyways, making the region a natural junction for avian virus movement. In that geography, an Arctic carnivore can serve as more than a curiosity from wildlife virology. It can act as a sentinel. Monitoring polar bears may help identify where HPAI is moving across migratory superhighways before the signal becomes obvious in birds, mammals, or remote coastal communities.
The study also shows the value of collaboration with Indigenous hunters. Without subsistence harvests and long-term local partnership, the blood samples that made this serologic pattern visible would not exist. The practical gains extend beyond virology. Better surveillance in Arctic ecosystems depends on cultural knowledge, field access, and sustained trust, especially where environmental change is redrawing animal movement patterns and increasing contact between species that rarely overlapped in the past.
A final caution remains embedded in the data. Serology can demonstrate exposure, but it does not prove the precise timing, source species, infectious dose, or clinical severity of infection. The cohort was also limited to harvested animals near one region of East Greenland, so the broader distribution of H5 exposure across the Arctic is still unknown. What the blood does show is more concrete: HPAI H5 clade 2.3.4.4b has reached East Greenland polar bears, and at least some of those bears lived long enough to leave antibodies behind.