A colder than average winter has meant an increase in highly pathogenic avian influenza (H5N1) among birds, according to officials.
Heather Reid, the Toronto Wildlife Centre's head veterinarian, said the organization began to notice signs of the virus among the city’s bird population at the start of 2025.
"In January, after we started having very cold weather, we started seeing increased reporting of suspicious birds that were showing symptoms that seemed consistent with avian influenza,” she said.
"The virus can survive cold weather, and as the water freezes, the birds are forced to be together in the open waters,” Reid explained. “Probably the stress of the cold also might weaken the birds a little bit.”
The seasonal uptick in cases of H5N1 among birds is not unheard of. Following a cold snap in 2024’s winter season, Reid said there were also increased reports of avian flu among birds in Toronto.
It is possible, but very rare, for a human to contract bird flu. Spread through animal feces and secretions, including mucus and saliva, human transmission can occur when the virus gets into a person's mouth, nose, eyes or is inhaled.
There is no evidence to suggest eating cooked poultry or eggs could transmit H5N1 to humans.
Canada has so far seen one confirmed human case of avian flu.
In November 2024, a 13-year-old in British Columbia contracted the country’s first known human case of avian flu after testing positive for the virus. The teen spent about two months in hospital, some of it on life support in an ICU.
There is no evidence of sustained person-to-person spread of the virus — though that doesn’t mean Canada’s government is simply standing by.
On Wednesday, the Public Health Agency of Canada announced it has secured an initial supply of 500,000 doses of a vaccine against the H5N1 strain of avian influenza that is currently circulating.
The highest priority for eventual vaccine distribution will be given to people who may be at risk of contact with infected animals.
Will avian flu continue in the spring?
Avian flu, similarly to human influenza, is seasonal and there are several strains of the virus, with some worse than others.
According to Reid at the Toronto Wildlife Centre, it's still an open question what will happen with the rate of bird flu during warmer weather.
Reid said there could be an increase of avian flu among birds when migratory flocks return to Toronto in the spring.
"Birds are coming back and they're also grouping together again, and maybe some of them are bringing the virus,” she said.
But even if there is an uptick in the spring months, Reid expects rates to drop again come June.
"Usually, the virus doesn't survive well in hot temperatures, so we do see it resolving in the summer months," she said.
In the meantime, there are still risks associated with the presence of bird flu in Toronto.
Other animals that feed on the carcasses of birds that have died of H5N1 can become infected.
Though waterfowl are the most common natural carriers of avian influenza, Reid said other birds of prey like owls, hawks, falcons and crows can die after feeding on infected birds.
In the spring of 2023, a dog died in Oshawa after it was found chewing on a dead goose that likely carried avian flu pathogens.
The risk of infection is a good reason to keep dogs on leash and cats indoors, Reid said.