Penn Vet | Fact Sheet Detail




By: Wildlife Futures Team Date: Jun 23, 2020

Other names: Rabbit fibromatosis, Shope fibroma, squirrel pox virus, Deer fibroma


Fibromas are wart-like growths most commonly confined to the skin. These types of lesions, found in both domesticated animals and wildlife, result from viral infections which are largely species-specific. A group of poxviruses cause fibromas in wild rabbits (Shope fibromas), squirrels (squirrel fibroma virus) and deer. Papilloma viruses also cause fibromas in deer.


Shope fibromas, though relatively common, have not been implicated as a mortality factor in rabbit populations of North America. The disease can however cause local outbreaks.

Similarly, outbreaks of the Squirrel Fibroma Virus causing high rates of mortality can occasionally occur, but overall populations do not seem to be negatively affected by this disease.

Interestingly, in the United Kingdom the invasive grey squirrels are displacing the native European red squirrels, in part due to the squirrel pox virus which is exotic to the UK and fatal to the red squirrels.

Though they can be dramatic when they occur, fibromas are of no significance to the survival of the deer population.

Species Affected

The Shope fibroma virus was first isolated in 1932. This virus is widespread in wild cottontail rabbits in both North and South America. Jackrabbits, Snowshoe hares and European rabbits have all been experimentally infected.

The Squirrel Fibromas virus is known to be infectious to grey and fox squirrels, as well as woodchucks. In a laboratory setting it has also been successfully transmitted to rabbits.

Papilloma and pox viruses affect white-tailed deer, mule deer, black-tailed deer and other members of the deer family across the US. 

None of these viruses are known to infect humans.


Rabbit fibromatosis infects cottontail rabbits in the Eastern and Midwestern United States.

Squirrel pox has been reported in grey squirrels on the Eastern coast (Delaware, Pennsylvania, Maryland, North Carolina, New York, Connecticut, Virginia) Florida, Indiana and Michigan.

Deer fibromatosis occurs throughout the white-tailed and mule deer range in North America.


Rabbit fibroma virus is transmitted by biting insects such as fleas and at least four different species of mosquitoes. Mosquitoes serve both as a biological and a mechanical vector for virus transmission as it has been shown that they can transmit the virus for up to 5 weeks after feeding on infected tissue.

In squirrels, biting insects such as mosquitoes and squirrel fleas are also the primary mode of transmission. The virus can be transmitted through direct contact of infected individuals, fecal-oral routes and lesion contact, as well as through inanimate objects or materials.

The exact mechanism in which deer fibromatosis is transmitted is not entirely clear but it has been suggested that both direct contact and biting insets play a role. The incidence of fibromas is highest amongst bucks which suggests that fighting or rutting may play a role in disease spread through contact of broken skin with infectious material.

Because it brings susceptible animals together artificial feeding can play a role in starting and maintaining local outbreaks.

Clinical Signs


Generally, most rabbits remaining unaffected by the growths. In the rare case where they interfere with vision, eating, or movement, individuals may become weak, lose body condition, and become more vulnerable to predation or fatal trauma.

Fibromas are often single, but occasionally multiple or numerous, raised nodules of the skin. They can affect any part of the body but are mainly observed on the legs, feet, face and ears. The growths are sparsely haired and colored gray, black or tan. The size can vary but most are less than 1 inch in diameter.

In adult rabbits, most fibromas are localized, persist for 10-14 months but often regress spontaneously following development of antibodies. It is thought that animals are immune to reinfection following resolution.  

All age groups are affected, but the young and the immunocompromised are more susceptible. In these cases, fibromas become extensive and, can become invasive affecting the internal organs. These individuals can develop malignant and fatal tumors.


Fibromas in squirrels do not cause overt signs apart from the presence of raised, flattened nodules in the skin. The growths can range in size from 1/16 to 1 inch in diameter and are often located on the head and limbs of squirrels.  Most infections are self-limiting and spontaneously regress. As with rabbits, the young are typically affected. In suckling squirrels, fibromas often form around the mouth and those individuals infected with numerous growths can become debilitated, emaciated or die.

Though lesions commonly involve only the skin, squirrel fibroma virus has also been reported to cause visceral nodules, affecting the lungs, liver, kidney and lymph nodes.


Cutaneous fibromas are hairless tumors that can be found on any part of the skin but are more commonly confined to the face, eyes, neck and forelegs. Fibromas can be gray or black in color. They may be smooth, but may also have a roughened or “warty” appearance. They can be single or multiple, appearing in clumps and can range in size from ¼ to 8+ inches in diameter.

In most cases the overall health of the animal is not affected. Rarely, due to the location and size of fibromas, they can interfere with vision, eating, breathing or movement.  Large fibromas are also more prone to secondary infections.

Individuals of all age groups can be affected but the highest incidence of fibromas is in bucks. Deer that are exposed to the virus at a young age and develop fibromas develop immunity. If they are then re-exposed again as an adult, they are less likely to develop clinical signs due to acquired immunity. 


A tentative diagnosis of fibromas (in rabbits, squirrels and deer) can be made on the basis of visible lesions. Confirmation can be attained by microscopic examination of the affected tissue or virus isolation from fresh tissue.


Fibromas in healthy infected squirrels, rabbits and deer are self-limiting and naturally regress. However, in immune-comprised individuals disease can progress and become fatal particularly in rabbits and squirrels. There is no effective treatment for fibromas, but wildlife rehabilitators can provide supportive care for moderately affected animals. Humane euthanasia should be considered for the severely infected animals.


Fibroma viruses appear to have little impact on populations of wild rabbits, squirrels and deer, so management is not necessary.

The disease is of no public health interest as it is not transmissible to humans. The carcasses of infected animals are safe for human consumption because the viruses are confined to the skin. However, if lesions are present in the internal organs or there is evidence of a secondary bacterial infection the carcass should be discarded for aesthetic reasons.

Suggested Reading


Rabbit Fibromatosis

WF rabbit fibroma 

Rabbit fibromatosis; photo courtesy of the Michigan Department of Natural Resource.

Squirrel Pox

WF squirrel pox 

Squirrel pox; photo courtesy of the Michigan Department of Natural Resources.

Deer with Severe Fibromas

WF Deer with fibromas 

Deer with severe fibromas; photo courtesy of the Pennsylvania Game Commission.