Beginning in the fall of 1998 and extending into the spring and early summer of 1999 there was a large epizootic of squirrel fibromatosis in squirrels in seven counties in peninsular Florida. Hundreds of gray squirrels (Sciurus carolinensis) with multiple cutaneous tumors were submitted or reported to biologists, veterinary hospitals, and private wildlife rehabilitators. Most squirrels died or were euthanized soon after submission. Twenty squirrels were submitted for necropsy. The majority of the squirrels examined were adults (12/20) and male (15/20). The number and location of tumors varied widely among the affected squirrels; however, a consistent finding was involvement of the eyelids (20/20). Histopathology revealed a proliferative population of mesenchymal cells within the dermis and marked ballooning degeneration of keratinocytes in the overlying epidermis. Intracytoplasmic viral inclusions were present in the neoplastic mesenchymal cell population and the degenerating keratinocytes. Ulceration and necrosis of the surface of the tumors or associated tissues was present in 14 of the 20 squirrels. Virions consistent with poxvirus were observed via electron microscopy in samples collected from a representative tumor. Death of the squirrels was attributed to emaciation, tissue damage, and severe negative energy balance associated with poxvirus infection and massive tumor growth. The underlying cause of this unusual epizootic of fibromatosis in gray squirrels remains unknown.

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