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 belongs 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.
These tumors are presumably all caused by a virus. Although the natural history of the virus is not known, both Aedes aegypti and Anopheles quadrimaculatus mosquitoes have transmitted it from squirrel to squirrel in the laboratory. It is probably transmitted by insects in nature as well.
The tumors develop at the site where the insects feed 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, but rarely. In general, there are no obvious signs of illness in naturally infected squirrels except for the presence of the tumors over 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 Shope's rabbit fibroma virus in the cottontail.
The disease is diagnosed by finding characteristic gross and microscopic lesions. Confirmation of the disease is either by histologic 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 man. Furthermore, the tumors are usually confined to the skin and are removed when the animal is skinned. If metastasized areas are found in the viscera, even though the lesions are not of public health significance, for aesthetic reasons the carcass should be discarded.