H5N1 in Canadian wildlife: waterfowl, raptors and reassortant viruses dominate four years of surveillance

A national analysis of highly pathogenic avian influenza A(H5N1) in Canadian wildlife has identified 2,657 detections between January 2022 and March 2026, with wild birds accounting for more than 93% of cases. Waterfowl and raptors were the principal host groups, while mammalian infections represented a smaller but epidemiologically important component. The study also shows that reassortant Eurasian–North American viruses became dominant and that autumn and spring remain the main periods of wildlife detection.

Published in Comparative Immunology, Microbiology and Infectious Diseases, the study analysed national surveillance data from all 13 Canadian provinces and territories, combining temporal trends, host species, viral lineage and spatial clustering. The dataset covered 112 wildlife species and 2,657 laboratory-confirmed H5N1 detections recorded from 7 January 2022 to 25 March 2026.

Birds accounted for 93% of detections

Of the 2,657 detections, 2,478 involved birds, or 93.3%, while 179 involved mammals, or 6.7%. Nearly three quarters of all records (1,980 detections, 74.5%) involved animals found dead or euthanised. Waterfowl and raptors were particularly prominent. During the large 2022 wave alone, waterfowl represented 36.3% of detections and raptors 26.7%, together accounting for almost two thirds of wildlife cases. Corvids contributed 11.4%, seabirds 9.3% and gulls and terns 8.9%.

The importance of raptors probably reflects both direct infection and exposure through predation or scavenging of infected birds.

2022 remains the largest wildlife wave

The first major Canadian wave was also the largest in the study period. There were 1,199 wildlife detections in 2022, representing 45.1% of the complete dataset. Numbers then fell to 423 in 2023 and 363 in 2024 before rising again to 605 in 2025. The smaller 2026 total reflects surveillance only through late March.

The 2022 episode had major conservation consequences. Previous assessments cited by the authors estimated more than 40,000 H5N1-attributable wild-bird deaths in eastern Canada during spring and summer 2022, with seabirds and sea ducks particularly affected.

Autumn and spring are the main risk periods

Across the full 2022–2026 dataset, autumn produced 906 detections and spring 880, representing 34.1% and 33.1% of all detections respectively. Winter accounted for 581 detections and summer only 290. These peaks closely correspond to major bird migration periods, but the authors caution against restricting surveillance to migration alone. Almost one third of detections occurred outside the defined spring and autumn migration windows.

That pattern supports year-round monitoring, with intensified surveillance during periods of major bird movement.

Ontario, Alberta and British Columbia were the main hotspots

The geographical distribution was highly uneven. Ontario recorded 618 wildlife detections, followed by Alberta with 429 and British Columbia with 391. These three provinces were classified as very-high detection hotspots. Quebec recorded 298 detections, Saskatchewan 249 and Nova Scotia 195, placing them in the high-detection category.

During the 2022 wave specifically, Quebec recorded the largest share, followed by British Columbia, Ontario, Saskatchewan and Alberta. Together these five provinces accounted for 70.9% of detections that year.

The study also identified local clustering within parts of the Prairie region, although there was no evidence of uniform spatial clustering across Canada as a whole.

Reassortant viruses became dominant

One of the strongest virological findings concerns reassortment. Across the study period, 80.8% of detections involved reassortant Eurasian–North American H5N1 lineages. These viruses combine genetic material from the Eurasian H5N1 lineage introduced into North America with segments derived from North American avian influenza viruses. During the 2022 epidemic wave, reassortant viruses already accounted for 76.5% of wildlife detections, compared with 22.4% for fully Eurasian viruses.

The modelling showed that viral lineage, year and season were among the principal factors associated with detection counts, and reassortant lineages were strongly associated with higher numbers of detections.

Mammals remain a smaller but important surveillance target

Mammals accounted for only 6.7% of detections, but the authors consider these infections important because they document continued spillover beyond avian hosts.

The expansion of clade 2.3.4.4b H5N1 has already demonstrated an ability to infect a growing range of mammalian wildlife. For surveillance programmes, mammalian cases therefore provide information not only about disease burden but also about repeated opportunities for viral adaptation to non-avian hosts.

Toward more targeted wildlife surveillance

The authors use the Canadian data to propose a more risk-based surveillance strategy. Rather than distributing sampling effort evenly, surveillance could prioritise Ontario, Alberta and British Columbia, migration-associated periods, waterfowl and raptors, reassortant lineages and mammalian wildlife. Retrospective testing of this framework suggested that such targeting could capture a large proportion of historical detections while concentrating effort on the most informative hosts, places and seasons.

The study therefore portrays Canadian H5N1 surveillance as an evolving One Health system rather than simply a count of infected birds. The dominant signal since 2022 has been the interaction between migratory waterfowl, predatory birds, geographically concentrated hotspots and increasingly prevalent reassortant viruses, with mammalian spillovers adding another layer to the surveillance picture.