Wart-like growths found on the skin of white-tailed deer and other members of the deer family are fibromas. They are popularly referred to as skin tumors, or simply warts. Histopathologists identify skin tumors from deer as papillomas, fibromas, or papillofibromas depending upon the predominate type of tissue making up the tumor. Since there is evidence that skin tumors common to deer are caused by one kind of virus, and the differences that have been described for them is due to their age, all will be treated as one, the fibroma, in this discussion. An infection with fibromas is called fibromatosis.
Fibromas are conspicuous as firm, nodular masses fastened only to the skin and varying in diameter from 10 to more than 100mm. All are fleshy. Some are covered with gray or dark skin which often is scratched and bleeding. Others have a black, dry, hard surface that may be fissured much like the head of a cauliflower. The larger ones tend to be pendulous because of their weight and stem-like attachment to the skin.
Fibromas are randomly distributed on deer but occur most frequently about the eyes, neck, face, and forelegs. They may be single or multiple. Light infections are a common occurrence in white-tailed deer. A heavily infected deer may have 25 or more. Occasionally, in multiple infections, they are so numerous and close together that they join into a coalescent mass. However, the incidence of severe involvement is extremely low, judging from the fact that not more than 2 or 3 cases in the whole state are reported to us a year.
The disease occurs statewide in Michigan, and has been reported in white-tailed deer, black-tailed deer, and mule deer over much of their respective North American ranges. Similar skin tumors have been reported from moose on Isle Royale and in British Columbia.
In a survey made in New York state it was found that the incidence of fibromatosis is highest in deer 2.5 years of age and younger, and 5 times higher among bucks than does. Records obtained for Michigan through necropsies, field reports, and check station operations also indicate this is primarily a disease of young male deer.
Transmission and Development
Fibromas can be experimentally transmitted by rubbing fibroma tissue onto the scratched skin of a susceptible deer. How transmission is accomplished in the wild is not known. Possibly, it is through contact of broken skin with infectious material, either from an infected deer or vegetation which has been rubbed or brushed by an infected deer. The fact the incidence is highest among bucks suggests that fighting is a means for the spread of the disease. Biting insects may possibly be responsible since many viruses are transmitted by insect vectors.
Clinical Signs and Pathology
The fleshy or wart-like growths attached to the skin are the only sign of fibromatosis. Infected deer behave normally unless the location of the fibromas blocks vision or results in other physical impediment to normal activities.
On cross section, fibromas show a white, tough core of uniform texture covered with a rind of varying thickness and color. Those with a thin layer of skin show a thin and lightly pigmented rind, while those with a hard fissure external surface have a thick dark rind.
Fibromas involve only the skin and have no direct effect on the general health of deer. There may be a secondary effect if the growths are particularly large and located where they cause physical difficulties as in seeing, eating, and running.
Diagnosis is based on the gross appearance and structure of the growths, and the fact they are attached only to the skin, not the underlying muscles and bones. Confirmation is based on microscopic examination of the tumor tissue.
Treatment and Control
It has been learned from experiments that infections stimulate an immune response in host deer, and thus become self-limiting. In most deer, the fibromas develop to only a few mm in diameter; they abruptly stop growing, dry up, and disappear. Only in an occasional deer do they develop into conspicuous skin tumors. Results of the New York survey indicate that wild deer are exposed and develop an immunity to the fibroma virus early in life.
Treatment is not feasible for most wild populations. The disease has not been reported as a problem in captive herds. Presumably, the growths could be removed surgically if it became important. Since exposure to the virus leads to immunity, it should be possible to develop a vaccine if prevention becomes necessary. To date, clinical disease has been too rare to justify such actions.
Fibromatosis is not an important cause of deer mortality. The disease is not known to infect humans. It's main significance lies in the consternation and concern experienced by the hunter who shoots a deer covered with ugly-looking lumps. Though they don't harm the meat, fibromas are repulsive to most persons and therefore render a fine trophy aesthetically undesirable.
Some domesticated animals (cattle, dogs, etc.) are subject to "warts" common to their kind. There is no reason to believe that fibromatosis of deer is infectious to domestic animals.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.