What is squirrelpox and should Canadians be concerned?

A squirrel eats a peanut in the snow Wednesday, February 13, 2019 in Deux-Montagnes, Que. (THE CANADIAN PRESS/Ryan Remiorz)

A viral disease that can cause squirrels to break out in unsightly skin lesions has been reported by an animal rehabilitation centre in Ontario. Known as squirrelpox, experts say the highly-contagious disease does not affect humans, pets or most other animals.

"Most squirrels affected by this disease will have hairless or poorly haired, raised masses over the face and limbs," Brian Stevens, a wildlife pathologist with the Canadian Wildlife Health Cooperative, told CTVNews.ca. "In Ontario, it is rare for squirrels to carry any diseases that are a major concern for the health of humans."

Caused by the squirrel fibroma virus, the disease predominately infects grey squirrels, which are commonly found in forests and cities throughout the eastern parts of North America. The tree squirrel species, which can also have brownish or black fur, typically survives infection with growths or tumours disappearing over time.

"There is no concern for humans or other animals with this virus," Stevens, who studies wildlife diseases and mortality, said. "It is specific to squirrels and cannot be transmitted to other species."

After a five-year absence in cases, Joy Huggins from Wildlife Haven Waterloo says the Ontario animal rehabilitation facility has experienced a recent increase in calls about squirrels with probable infections. Wildlife Haven is currently caring for a symptomatic juvenile squirrel that arrived in early December with siblings that have since died.

"It's hard for everybody in the winter, let alone a young one, because she is a baby," Huggins told CTV News Kitchener. "So if they can come in here and get reprieve and rest, they’ve got more chance of living."

The disease has also been reported in nearby states like New York, Michigan, Indiana and Wisconsin. A 2020 study noted that antibodies for the virus were largely absent in introduced populations in B.C., but found in 79 per cent of 19 grey squirrels tested in Toronto. Related species like red and fox squirrels can also be infected.

In Canada, Stevens says sick animals can be reported to local wildlife rehabilitation centres while dead animals can be reported to the Canadian Wildlife Health Cooperative.

"It’s likely relatively common out there as we get a handful of reports every year," Stevens said. "[M]ost of the reports describe Ontario and the northeast U.S. as the typical regions affected by this disease; however, it is likely more widespread in other adjacent provinces and into the Midwest."

While the virus has been transmitted to rabbits in a laboratory setting, it is believed to only affect squirrels in nature, and possibly woodchucks.

Michigan's Department of Natural Resources also states the "disease is of no public health significance."

"Transmission of the virus can occur via direct contact between squirrels and by the bites of mosquitoes and fleas," the department explains. "In severe cases, when vision is obstructed or the skin becomes secondarily infected, the animal may be less active, weak, and eventually die."

Mason Stothart recently completed a PhD in veterinary medical sciences at the University of Calgary, and previously studied eastern grey squirrels at the University of Guelph, where he saw the effects of squirrelpox first-hand in southern Ontario.

"It likely circulates amongst the grey squirrel populations, only to present symptoms in young or physiologically stressed individuals," Stothart told CTVNews.ca. "If folks want to help limit the spread and protect squirrels in their backyards, they could consider not putting out bird seed, or otherwise cleaning their feeders on a regular basis."

Indiana's Department of Natural Resources says infected animals can even be safely eaten.

"There have been no reports of infected humans, including from consumption, if the tumours do not reach internal organs," the department states. "If internal tumours are found, do not consume the animal and discard the carcass properly."

In Europe, where previously introduced North American grey squirrels are now considered an invasive species, a similar squirrelpox virus has devastated some European red squirrel populations, which typically lack natural immunity.

"In Europe, squirrelpox virus is carried by non-native grey squirrels and spread into native red squirrel populations," a 2022 study explained. "The virus causes a large proportion of infected red squirrels to die and contributes to local declines and the replacement by grey squirrels."

With files from CTV News Kitchener