Pedal osteosarcoma in a donkey.

An 18 1/2-year-old castrated male donkey with progressively worsening right forelimb lameness presented with a mass on the distal dorsal aspect of its P3 bone. Grossly, the firm, gritty mass was infiltrative, disrupted the contours of the overlying hoof wall, and had mottled and cavitated areas on cut surface. Histologically, the growth was composed of densely cellular sheets of mildly pleomorphic mesenchymal cells forming irregularly shaped islands of poorly mineralized osteoid. The neoplastic mass had patchy areas of necrosis. The diagnostic possibilities considered for this donkey's mass include osteosarcoma, osteoma, ossifying fibroma, and fibrous dysplasia. Careful consideration of the gross and histological characteristics of this donkey's mass support a diagnosis of osteosarcoma.