Metastatic Melanoma in Horses

The clinical and pathologic findings are reviewed for 14 horses with metastatic melanoma. All were older gray horses, with an average age of 16 years. The most common sites of primary tumors were the ventral tail, perineum, and parotid salivary gland. Metastases were found in multiple locations and caused a variety of clinical syndromes. The most common sites for metastases were the lymph nodes, liver, spleen, skeletal muscle, lungs, and surrounding or within blood vessels throughout the body. Many of the horses had dermal melanomas for years (range 1–6 years) before succumbing to metastatic disease. Histologic characteristic of dermal masses was not predictive of malignancy in the majority of cases. Treatment consisting of surgical debulking and administration of cimetidine, an autogenous vaccine, or both was attempted in 4 horses with no effect on outcome.