Equine melanoma is a common type of skin cancer seen primarily in older, grey horses (over 8 to 10 years of age). In one study, up to 80% of grey horses over 15 years of age had developed melanoma. Melanomas are especially common in the Lipizzaner, Percheron, and Arabian horse breeds.

Melanomas can occur anywhere on the horse's body, however they are most frequently located at the perineum, the base of the tail, neck, mouth, eyelids, and mane. Other common locations include the parotid lymph nodes and salivary glands. Tumors often involve multiple pedunculated nodules. Growths can also develop in internal organs. The majority of equine melanomas grow slowly causing few problems however in cases where their size begins to interfere with vital body functions.

Clinical Signs of Melanoma

Clinical signs depend on the location and size of the tumor. Melanomas may exist for many years without growing in size or causing any problems for the horse. However, once they begin to grow in size is when they can present a significant problem for the horse. If melanomas develop and enlarge in the throat latch area, they can cause horses to be unable to flex at the poll, turn their heads from side to side, or eat or drink comfortably. Geldings or stallions that develop melanomas in the sheath can cause discomfort for the horse to urinate or breed. Horses with large melanoma growth on their anus or tail base can develop extreme intermittent discomfort when defecating. Since over 80% of melanoma lesions will ultimately develop some degree of malignancy, early recognition and treatment are key for managing the disease.

Management and Treatment of Equine Melanoma

Treatment options include surgical removal, cimetidine, acupuncture, Chinese herbs, and a recently introduced vaccine. Large tumors can obstruct the rectum or prepuce, inhibit food intake, and impinge upon the upper airway. Surgical excision of the mass is usually locally curative but is controversial for large or confluent tumors. In one study, 50% of horses who had tumors larger then 4 cm surgically removed, had continued or new growth of distant melanocytic tumors.