Department of Natural Resources
Squirrel pox, or squirrel fibroma, is a viral disease which produces multiple tumors on the skin of gray and fox squirrels. It is a leporipox belonging to the pox group of viruses which produce rabbit fibromatosis, rabbit myxomatosis, hare fibromatosis and some deer fibromas. Multiple skin tumors on gray squirrels from Maryland were reported in 1953. Since then, reports of squirrels with skin tumors have come from Florida, New York, Virginia, North Carolina, and Ontario. In Michigan, they have been found on both gray and fox squirrels in the Lower Peninsula.
This tumor is caused by a virus. Transmission of the virus can occur via direct contact between squirrels and by the bites of mosquitoes and fleas. Aedes aegypti and Anopheles quadrimaculatus mosquitoes have transmitted the virus from squirrel to squirrel in the laboratory.
The tumors develop at the site where direct contact occurred or where the insects fed on the animal.
Naturally, infected squirrels are mostly juveniles. The tumors may be scattered over all the body and range in size from a few mm to 25mm in diameter. Metastasis to the lungs, liver, kidney, and lymph nodes has been reported. In general, there are no obvious signs of illness in naturally infected squirrels except for the presence of the tumors on the skin. In severe cases, when vision is obstructed or the skin becomes secondarily infected, the animal may
be less active, weak, and eventually die. The microscopic lesions of squirrel pox virus in the gray squirrel are similar to those reported from Shopes rabbit fibroma virus in the cottontail rabbit.
The disease is diagnosed by finding characteristic gross and microscopic lesions. Confirmation of the disease is either by histological examination of tissues for intracytoplasmic viral inclusion bodies or by virus isolation.
There is no known treatment; presumably, a vaccine could be developed. However, it would not be logistically or economically feasible to treat free-living wild animals. In cases where the disease is not severe, the lesions probably regress and the animal recovers completely.
As far as we know, the virus only infects squirrels in nature. However, in the laboratory it has been successfully transmitted to woodchucks and rabbits.
In Michigan, where squirrel pox is common, there apparently have been no noticeable effects on squirrel populations. The disease is of no public health significance. The carcasses of affected animals are safe for human consumption, since current knowledge indicates the virus is not transmissible to humans. Furthermore, the tumors are confined to the skin and are removed when the animal is skinned.