History

A dull and unresponsive adult 1.18-kg female eastern cottontail rabbit (Sylvilagus floridanus) was discovered outside the Zoo Atlanta Conservation Action Resources Center.

Clinical and Gross Findings

Physical examination of the rabbit revealed blood staining on the ventrum and yellow fluid dripping from the rectum. The hind limbs had firm, fleshy nodules; there was 1 nodule on the lateral aspect of the left hind limb, and there were 2 nodules on the medial aspect of the right hind limb (Figure 1). The rabbit was anesthetized with isoflurane and euthanized by IV injection of pentobarbital sodium and phenytoin sodium solution, and a postmortem examination was performed. At necropsy, the medullas of the kidneys were reddened bilaterally, and the stomach contained 3 or 4 adult tapeworms. The nodules on the hind limbs were alopecic and firm and were white on cut section. All nodules were confined to the dermal and subcuticular tissues. Samples of all major organs and the hind limb nodules were fixed in neutral-buffered 10% formalin and submitted for histologic evaluation.

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Photograph of the medial aspect of the right hind limb of an adult female eastern cottontail rabbit (Sylvilagus floridanus). Notice the 2 raised, firm, cutaneous nodules on the limb. There was another nodule on the lateral aspect of the left hind limb.

Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.19.04.0189

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Histopathologic and Ultrastructural Findings

Microscopically, the cutaneous nodules were unencapsulated, expansile masses that had effaced and replaced the dermis, separating hair follicles. The masses were composed of polygonal to spindle-shaped cells within a scant, lightly basophilic stroma (Figure 2). The cells each had indistinct cell borders and abundant eosinophilic, granular, or finely vacuolated cytoplasm with lighter eosinophilic, oval cytoplasmic inclusion bodies; typically, the cells had 1 round, vesiculated nucleus, although a few cells contained as many as 3 nuclei. One mitotic figure was observed in 10 hpf (X400). The neoplastic spindle-shaped cells formed interconnecting whirls of collagenous connective tissue matrix within and around the periphery of each nodule. Moderate numbers of heterophils were intermixed with the neoplastic cells. Along the periphery of the masses, vessels were surrounded by moderate numbers of lymphocytes, plasma cells, and macrophages. The overlying epidermal and follicular epithelium was hyperplastic with many vacuolated cells (ballooning degeneration) and multiple keratinocytes containing round, brightly eosinophilic, intracytoplasmic inclusions (Figure 3). Hair follicles were frequently dilated with accumulated keratin. Macerated skin nodule tissue underwent negative-stain transmission electron microscopy, which revealed large (271 X 315 nm) rectangular virions within the cytoplasm, consistent with poxvirus¹ (Figure 4). Additional histopathologic changes included microhemorrhages and congestion within the liver, lungs, and kidneys.

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Photomicrographs of a section of a nodule in haired skin. Notice the marked proliferation of polygonal to spindle-shaped cells expanding the dermis. The epidermal and follicular epithelium is hyperplastic, and the follicles are dilated by large amounts of keratin. H&E stain; bar = 100 µm. Inset—Higher-magnification image of the neoplastic, plump spindle-shaped cells. H&E stain; bar = 50 µm.

Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.19.04.0189

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Photomicrograph of a section of a nodule in haired skin. Within the hyperplastic epithelium, notice the ballooning degeneration (asterisk) and eosinophilic intracytoplasmic inclusion bodies (arrow). H&E stain; bar = 20 µm.

Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.19.04.0189

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Transmission electron micrograph of a 271 X 315-nm brick-shaped mature virus particle with rounded corners detected in macerated skin nodule tissue. There are irregular white tubular structures on the surface membrane covering the core, which is a feature typical of all genera of poxviruses, except the genus Parapoxvirus.¹¹ Shope fibroma virus particles have been previously characterized as rectangles that are approximately 268 X 310 nm with rounded corners.¹ Negative stain, 3% phosphotungstic acid, transmission electron microscopy; bar = 100 nm.

Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.19.04.0189

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Morphologic Diagnosis and Case Summary

Morphologic diagnosis: nodular, dermal fibroma with eosinophilic intracytoplasmic viral inclusion bodies in fibroblasts and keratinocytes, epidermal and follicular epithelial hyperplasia with ballooning degeneration, and moderate lymphoplasmacytic and heterophilic dermatitis; and severe, acute, multifocal, hemorrhage with moderate congestion in the liver, lungs, and kidneys.

Case summary: cutaneous Shope fibroma virus fibromatosis (Shope fibroma) in an eastern cottontail rabbit.

Comments

The gross and histologic appearance of the skin nodules was consistent with Shope fibromas. First described by Richard E. Shope in 1932, Shope fibromas are benign, self-limiting proliferations of fibroblasts within subcutaneous tissues.² Typically, the overlying epidermal and follicular epithelium is hyperplastic with prominent rete ridges extending into the neoplastic tissue and an inflammatory cell infiltrate present throughout the mass.^2,3 Neoplastic cells, as well as hyperplastic epithelial cells, contain characteristic eosinophilic intracytoplasmic inclusion bodies.^2,3 These lesions are caused by the Shope fibroma virus (SFV), also known as the rabbit fibroma virus.⁴ The SFV belongs to the genus Leporipoxvirus within the subfamily Chordopoxvirinae of the family Poxviridae.⁴ The genus Leporipoxvirus is composed of SFV, rabbit myxoma virus (MYXV), hare fibroma virus, and squirrel fibroma virus.⁴ The SFV and MYXV infect wild and domestic rabbits (genera Sylvilagus and Oryctolagus).⁴ In both rabbit genera, SFV infection causes benign cutaneous fibromas, whereas MYXV infection causes cutaneous fibromas in Sylvilagus spp and fatal myxomatosis in Oryctolagus spp.⁴ Interestingly, SFV can be used in a heterologous live virus vaccine against MYXV.⁵ Hare fibroma virus infects hares (Lepus spp), and squirrel fibroma virus infects squirrels (Sciurus spp and Tamiasciurus hudsonicus), both causing cutaneous fibromas similar to those associated with SFV infection in rabbits.⁴

In healthy adult rabbits, SFV infection results in local cutaneous tumor formation that reaches its maximum size 7 to 12 days after infection.³ Typically, the tumor then completely regresses by 25 to 35 days after infection; however, a tumor may persist for several months.^3,6 Following initial infection, healthy adult rabbits develop protective immunity.⁷ In newborn rabbits, SFV infection can be fatal.^3,7,8 Depending on the immune status of the animal and the virus load, infected newborns develop either a large local tumor with systemic metastasis or disseminated inflammatory and degenerative lesions consistent with systemic MYSV infection.⁸ The differences in clinical progression of SFV infection in healthy adults and newborn rabbits are thought to be attributable to an inadequate development of cytotoxic T-cell responses and delayed-type hypersensitivity reactions in newborns.⁷

Rabbits transmit SFV to other rabbits via vectors.⁹ Shope fibroma virus can be transmitted by 3 species of fleas (Cediopsylla simplex, Odontopsyllus multispinosus, and Hoplopsyllus affinis); 5 species of mosquitos (Aedes aeqypti, Aedes triseriatus, Anopheles quadrimaculatus, Culex quinquefasciatus, and Culex pipiens); 3 species of reduviid bugs, which are also known as kissing bugs (Triatoma infestans, Triatoma phyllosoma pallidipennis, and Rhodnius prolixus); and the bedbug Cimex lecturlarius.^6,9 It has been suggested that the fibromas may be preferred feeding sites for blood-sucking arthropods because of their superficial and highly vascular nature.^6,9 Shope fibromas in rabbits are most frequently reported in the fall.^3,5,10 This seasonality may reflect the concurrence of a large population of mosquitos and infected cottontail rabbits in the fall.^5,10 Shope fibromas in rabbits have been reported throughout the eastern and Midwestern US, Texas, and Ontario, Canada.^3,5 Occasionally, outbreaks in domestic rabbitries may occur.^3,5 Of all cutaneous tumors in domestic rabbits, 10% are Shope fibromas.¹⁰

Shope fibromas are typically diagnosed on the basis of the gross appearance of a freely moveable skin nodule that is 0.5 to 6.0 cm in diameter and located on the forepaws, ears, or head.⁵ On examination of sections of such nodules, histopathologic findings of well-demarcated proliferations of polygonal to spindle-shaped cells, hyperplastic epithelium, eosinophilic intracytoplasmic inclusion bodies, and other changes (as described in the present report) can confirm the presumptive diagnosis.^2,3,5 Diagnosis of Shope fibromas can also be made on the basis of PCR assay results, virus isolation, or detection of poxvirus particles by electron microscopy.^1,3,5 Because these tumors typically spontaneously regress, treatment is rarely needed.⁵ Protection from blood-sucking arthropods is a preventative measure.⁵

For the rabbit of the present report, the blood-stained ventrum and dull, unresponsive mentation were likely results of systemic vascular changes, as evidenced by microhemorrhages and congestion in multiple organs. Exposure of the rabbit to anticoagulant rodenticides was a possible cause, given that a bromadiolone-containing rodenticide was used on the zoo grounds for rodent control. Other potential causes included trauma or sepsis. Additional fresh or frozen tissues were not available for further ancillary diagnostics.