Wildlife Haven Waterloo, an animal rehabilitation facility in Waterloo, Ont., has recently noted an increase in squirrelpox—a highly contagious disease known for causing skin lesions in squirrels. These new cases of the infection are the first the centre has seen in five years.

Caused by squirrel fibroma virus, squirrelpox is a type of leporipoxvirus classified within the pox group of viruses. The disease typically manifests in growths or tumours on infected squirrels.

In conversation with CTV News, Brian Stevens, wildlife pathologist with the Canadian Wildlife Health Cooperative, explained, “Most squirrels affected by this disease will have hairless or poorly haired, raised masses over the face and limbs.”

Squirrelpox predominately infects grey squirrels. Although it has been transmitted to rabbits under controlled laboratory conditions, it is thought to exclusively impact squirrels, and possibly woodchucks, in nature. Species related to grey squirrels, such as red squirrels and fox squirrels, can also be infected. However, Stevens noted, “There is no concern for humans or other animals with this virus.”

In Canada, the disease is primarily focused in Ontario. A 2020 study found while antibodies for the virus were largely absent in introduced B.C. squirrel populations, it was prevalent in 79 per cent of the 19 tested Toronto-area grey squirrels. Squirrelpox has also been reported in nearby American states, such as New York, Michigan, Indiana, and Wisconsin.

As per Michigan’s Department of Natural Resources, there is no known treatment for squirrelpox. While a vaccine could be developed, the logistical and economical challenges of treating free-living wild animals has proved a deterrent. In less severe cases, the lesions regress, allowing for the infected squirrels’ full recovery. However, in severe cases when vision is obstructed, or the skin becomes secondarily infected, animals may face reduced activity, weakness, and eventual death.

A similar squirrelpox virus has devastated red squirrel populations in Europe, where North American grey squirrels have been introduced as invasive species. A 2022 study revealed the virus, carried by non-native grey squirrels, caused a significant number of infected red squirrels to die, contributing to local declines and the replacement of red squirrels by the invasive grey squirrels.

The virus can be transmitted through direct contact between squirrels and through mosquito and flea bites.

Mason Stothart, a recent PhD graduate in veterinary medical sciences from the University of Calgary, told CTV News, “It likely circulates amongst the grey squirrel populations, only to present symptoms in young or physiologically stressed individuals.”

When discussing steps to prevent the spread of disease, Stothart added, “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.”