Congenital and acquired melanocytomas (benign melanomas) in eighteen young horses

In a retrospective study, cutaneous melanocytic tumors from 18 horses, less than 2 years old, were examined histopathologically and clinical follow-up requested. Melanocytomas (benign melanomas) occurred in a variety of breeds and in horses of varied coat color. The age of the horses at the time of biopsy ranged from 3 weeks old to 2 years old. Four melanocytomas were congenital, 11 melanocytomas were acquired by 1 year of age, and three were acquired prior to 2 years of age. Of the 18 horses, five were male, and 13 were female. All tumors were solitary and located on the legs or trunk; none were in the perineal region. Ulceration of the overlying epidermis was common. Tumors were generally localized and were not encapsulated. The tumors had a variety of cell patterns ranging from sheets, to streams, or nests of melanocytes. Cellular morphologic findings also ranged from epithelioid, to a mixture of epithelioid and spindle cells or to a spindle pattern. The nuclei were large and euchromatic, especially in the epithelioid cells. Several tumors had moderate cellular pleomorphism and binucleate cells. Mitotic activity was generally low (less than 1/high-powered field), but was readily detected (1-2/high-powered field) in bleached sections of four cases. Melanin pigmentation varied from mild to heavy. Melanophages were admixed with the tumor cells or in the adjacent tissue. Follow-up information was obtained on 15/18 horses and revealed that 14/15 horses were free of recurrence following excision. One neoplasm, that was poorly demarcated and had a spindle cell pattern, was not completely resected and continued to grow. These melanocytic tumors in young horses are distinct from melanomas in aged horses in their location, epithelial involvement, and age of horses affected. The majority of these tumors appear to be benign and share features of melanocytic nevi of human beings.