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Description and Distribution
R. E. Shope, in New Jersey in 1931, first reported tumorous growths from the skin of a cottontail rabbit. These growths were later classified as fibromas and appropriately named Shope fibromas. The Shope fibroma is a small (25 mm diameter), firm pinkish-red, spherical tumor generally found on cottontail rabbits. There may be from 1 to 10 or more fibromas present on the animal's body, with growths usually on the legs and head.
Shope fibromas have been reported in rabbits from a number of states but are usually found on cottontail rabbits living east of the Mississippi River. In Michigan, fibromatosis is a common disease in cottontail rabbits in the southern half of the lower peninsula, but the prevalence of disease is unknown for in the northern half of the lower peninsula and the upper peninsula.
Two studies performed at the Rose Lake Wildlife Research Center reported the prevalence of fibromas in rabbits in southern Michigan. Forty cases of papillomatosis (probably fibromatosis) were found in rabbits examined between September 1 and December 13, 1938. Between 1950 and 1966, 19 of 170 rabbits examined at the pathology laboratory exhibited tumorous growths, with 8 of the 19 having fibromas. Both studies found juvenile rabbits to be more susceptible to infection with Shope fibromas.
Transmission and Development
Shope fibromas are caused by a filterable poxvirus. The virus is transmitted between cottontail rabbits through the bites of fleas, mosquitoes, and possibly other biting insects. Four species of mosquitoes have been shown to transmit the causative organism, with the species being Aedes aegypti, Anopheles quadrimaculatus, Aedes triseriatus and Culex pipiens. Mosquitoes may transmit the virus to susceptible rabbits up to 5 weeks after the mosquitoes feed on the tumorous tissue, thereby serving as both a biological and mechanical vector.
Once the fibroma virus is transmitted to other rabbits by subcutaneous and intramuscular inoculation, the tumor begins development. The loosely attached tumors occur most frequently on the skin of the feet, nose, eyelids, lips, ears and back. The fibroma virus acts directly on the epidermal and connective tissue cells causing the formation of a localized tumor. The tumors are slow growing, often multiple in number, and are described as elevations of the skin by the fibrous mass. The overlying epidermis thickens and sends bulbous proliferating epithelium deep into the tumorous tissue.
Fibromas are usually seen on juvenile or subadult rabbits and occur most frequently during late summer, fall and early winter. The fibromas may remain for as long as 10-14 months, spontaneously regressing following the development of antibodies in the rabbit. Usually the fibroma regresses during late winter or spring. It is believed that the animals are immune to reinfection following the regression of a fibroma.
Viruses similar to the Shope fibroma virus have been known to cause fibromas in brush rabbits in California, in hares in Europe and in gray squirrels in North America. Domestic rabbits can become infected by inoculation of the fibroma virus and the virus has been demonstrated to be closely related to the agent causing infectious myxomatosis, a fatal disease of domestic rabbits.
Clinical Signs and Pathology
Generally, there are no clinical signs of illness and the rabbit is unaffected by the growths. Fibromas may be important to the animal's well-being if locomotion or feeding is hindered due to the infection.
The tumorous growths usually cause no significant pathological changes due to the location of the fibromas.
Treatment and Control
There is no effective treatment for Shope fibromas, and none is needed since the infection is self-limiting and will usually regress within a year, allowing complete recovery of the host.
Gross examination is generally sufficient to identify the tumorous growths as Shope fibromas. The characteristic size, shape and appearance allow for a quick and fairly accurate diagnosis.
To confirm the tumorous growths as fibromas, a histopathological examination should be performed. Shope fibromas have uniform spindle-shaped connective tissue cells and there is no inflammatory reaction present unless the growth has been traumatized.
Shope fibromas have not been implicated as a mortality factor in cottontail rabbit populations in North America, even though the disease may reach epizootic proportions in certain areas. The fibroma virus does cause a localized, relatively insignificant, self-limiting reaction on the skin which usually disappears within one year. Experimentally, nestling cottontail rabbits have been killed by intradermal injections of the Shope virus.
The Shope fibroma virus is of no public health significance as it is not transmissible to humans either by handling or eating affected carcasses. Furthermore, because fibromas are confined to the skin, they are removed when the animal is pelted. Carcasses of affected rabbits are safe for human consumption.