Highly Pathogenic Avian Influenza – Devastating impact on wildlife in southern Ontario

Highly Pathogenic Avian Influenza (HPAI), particularly the H5N1 subtype, is having a devastating impact on wildlife in southern Ontario. While public focus has largely been on the virus’s impact on poultry farms and the potential for human infection, the effects on wild bird populations are already catastrophic. Mass mortalities have been reported in areas like Rouge National Urban Park, Caledon, and along Toronto’s waterfront, where Canada geese, snowy owls, cormorants, gulls, and birds of prey have been found dead in alarming numbers. The virus spreads most readily during cold months and migration seasons, when birds gather in close quarters, creating ideal conditions for transmission.

The CWHC provides essential diagnostic surveillance capacity in Ontario and Dr. Brian Stevens, wildlife pathologist with CWHC Ontario/Nunavut, was recently interviewed by The Local, in Toronto. Dr. Stevens and his team receive carcasses of wild birds suspected to have avian influenza from across the province, submitted by municipalities, rehabbers, and members of the public. Each bird is examined and tested to determine the strain of influenza and its potential threat. “Geese seem to be particularly susceptible to avian flu,” Stevens noted, explaining the disproportionately high number of Canada geese and crows that test positive in their lab.

So far this year, CWHC Ontario/Nunavut has tested 458 wild birds, with nearly 40 percent returning positive for H5—a dramatic increase from the 13 percent positivity rate recorded in 2024. According to Stevens, harsh winter conditions likely played a role, forcing birds to gather in tight groups where the virus could more easily spread. Not all strains of avian flu are lethal, nor are all bird species equally vulnerable. However, the growing number of confirmed HPAI cases suggests a troubling trend, particularly for species already facing conservation pressures. Stevens pointed to environmental factors exacerbating the situation: extreme cold freezing over lakes and ponds, combined with heavy snow, forced birds to cluster closely together—ideal conditions for viral spread. “Viruses, in general, also typically survive better in the cold,” he said.

The story appears in The Local, a Toronto-based nonprofit magazine recognized for its deep reporting on health, equity, and social issues. Known for shining a light on stories that intersect science, public policy, and the lived experiences of Torontonians, The Local provides a thoughtful and community-oriented lens on pressing urban challenges. You can read the full article by Wency Leung here: Bird Flu Comes For Toronto’s Wildlife

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