Case 170421-1

Three-year-old, female spayed German shepherd dog

The patient received a tibial-plateau-leveling osteotomy. When examined for a routine recheck, ten weeks later, an approximately 1.5-cm-diameter, ulcerative cutaneous mass was noted on the medial aspect of the third digit of the left pelvic limb. The patient was very reactive to palpation of the mass. It was removed and submitted for histopathology.

The entire sample was 1 cm in diameter with an exophytic, 0.6-cm-diameter, verrucous mass that protruded 0.5 cm above the surface of the skin. It contained a central cavity and on cut surface was mottled light tan to gray.

Four sections of haired skin and paw pad are examined, in which there is a mass-like, papillary proliferation of the epidermis overlain by a dense layer of lamellar keratin. The stratum basale and stratum spinosum are markedly thickened by epithelial cells having moderate anisocytosis and anisokaryosis. The epithelium contains an average of 11 mitotic figures per high-powered (400x) field. Occasionally cells within the stratum spinosum have amorphous eosinophilic intranuclear inclusions that are 10 to 15 micrometers in diameter. Hypereosinophlic, individualized, epithelial cells are scattered multifocally throughout the stratum spinosum (single-cell necrosis), along with large numbers of large ballooned keratinocytes and irregular keratohyalin granules in the granular layer. Nests of basal-type cells have variably distinct cell borders, a moderate amount of eosinophilic cytoplasm, a lightly basophilic nucleus with finely stippled, dispersed chromatin, and one to three nucleoli. These aggregates of cells are most common in proximity to the stratum basal, but they are occasionally nested within the stratum spinosum. Moderate numbers of lymphocytes and plasma cells and lesser numbers of neutrophils are scattered throughout the dermal connective tissue.

Immunohistochemistry for papillomavirus demonstrated specific cytoplasmic staining of cells in the epidermis.

Haired skin: Severe, chronic, focal epidermal hyperplasia and dysplasia with keratinocyte ballooning, apoptosis, hypergranulosis, and hyperkeratosis with multifocal intranuclear inclusion bodies (papilloma virus)

The mass examined was consistent with a viral papilloma. The basal cell layer was very hyperplastic, which most likely represents an early, proliferative phase of the lesion. Recurrence of these lesions in the same or other areas is common due to viral persistence in the skin.

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