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Squirrel Fibroma Virus
Background of Squirrel Fibroma Virus
Squirrel fibroma virus (SFV) is classified as a member of the genus Leporipoxvirus, based on the similarity of the gross and histologic lesions in squirrels to those produced by rabbit fibroma virus in rabbits, immunological cross-reaction between squirrel fibroma virus and rabbit fibroma virus, and ultrastructure of the virus. Squirrel fibroma virus, is a species of dsDNA virus in the family Poxviridae. The virus has a unique morphological structure, with oblong particles under the electron microscope. The viral particles are 220~450nm long and 140~260nm wide. The center is a core with a concavity on both sides, and the two lateral bodies are respectively located in the concavity. The core is a nucleoprotein complex composed of DNA and proteins. Immediately around the core is a fence-like core membrane. The core and lateral bodies - together, are surrounded by a lipoproteinaceous surface membrane filled with soluble proteins. There are regularly wound helical filaments (10-20 nm in diameter) on the surface film. SFV proliferate in the cytoplasm of host cells.
Squirrel Fibroma Virus Epidemiology
Squirrel fibromatosis (SF) has been reported from many states in the USA, most commonly along the eastern coast and infrequently in southeastern USA and in Ontario, Canada. Squirrel fibroma virus has been reported in a western gray squirrel, as well as woodchucks, and, presumptively, in a fox squirre. Squirrel fibromatosis generally occurs as a sporadic disease; epizootics are rare. In a few fatal cases, death has been attributed to concomitant immunosuppression or disseminated poxviral disease. Infection is thought to be transmitted by biting arthropods, including mosquitoes and the squirrel flea. In addition, the virus can be transmitted from mother squirrels to nursing young. Occasionally, outbreaks may cause high rates of mortality among affected squirrel populations.
Characterization of squirrel fibromatosis
Squirrel fibromatosis is characterized grossly by single or multiple, firm, alopecic dermal nodules and microscopically by typical poxviral lesions of epidermal hyperplasia, ballooning degeneration, and intracytoplasmic inclusion bodies. The most characteristic feature of SF is the proliferation of atypical fibroblasts in the superficial dermis. In addition to the more common cutaneous form of the disease, a systemic form presenting as nodules in multiple viscera has also been reported. Multiple firm, raised, dry to moist, crusted nodules with a diameter of 0.5-2.5cm were observed in the skin around the eyes, ears, lips, trunk, feet, and genitalia on dead squirrels.
Fig.1 Photographs of the inguinal region and dorsum of an approximately 4-week-old female eastern gray squirrel. (Rivas, 2014)
Squirrel Fibroma Virus Histopathologic Findings
Histologic examination revealed that the skin lesions of diseased squirrels were characterized by moderate to marked epidermal hyperplasia with ballooning degeneration, small to moderate numbers of intraepidermal neutrophils, a few intracorneal pustules, and many circular (5- to 10-m-diameter), intraepithelial eosinophilic intracytoplasmic inclusion bodies. In these regions, the dermis was mildly to markedly expanded by streams of atypical spindle-shaped mesenchymal cells containing oval to elongate nuclei and many circular (5- to 10-µm-diameter), eosinophilic, intracytoplasmic inclusion bodies. Intervening dermal mesenchymal cells were small numbers of viable and necrotic neutrophils, fewer lymphocytes, and plasma cells. The renal interstitium of both squirrels was multifocally mildly expanded by small numbers of similar atypical mesenchymal cells with rare eosinophilic intracytoplasmic inclusion bodies as for the dermis.
Fig.2 Photomicrograph of a section of kidney tissue from the male squirrel. (Rivas, 2014)
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Rivas, A. E.; et al. Pathology in Practice. Journal of the American Veterinary Medical Association. 2014, 245(4): 389-391.