One of the most common skin tumors in adult horses is a melanoma, second only in occurrence to a sarcoid. In fact, it is estimated that up to 80% of gray horses older than 15 years old have melanomas. These melanomas are typically black and are usually found around the lips, eyes, ears, salivary glands, anus, penis, and vulva. So generally a black hairless mass found at the junction of a mucous membrane on a gray horse is probably a melanoma. Fortunately, most melanomas in gray horses are benign slow-growing tumors that rarely metastasize or spread to other organs. Because most melanoma lesions occur in regions not exposed to the sun, it seems unlikely that the sun plays as significant a role as it does in human melanoma.
Diagnosis is mainly done by visual inspection, but they can be confirmed through a fine needle aspirate of cells via needle stick followed by placing of contents within the needle onto a microscope slide or by biopsy.
Treatment of melanomas is usually performed by surgical excision. Unfortunately, surgical removal can be difficult because of the number and location of some of these tumors. One medical treatment used to shrink the tumors is cimetidine, which is ironically the same drug used for stomach ulcers in people, although results have been questionable. Intralesional treatment with cisplatin, an anti-neoplastic drug, meaning it inhibits the growth and spread of malignant cells in oil has also had some variable effects in shrinking tumor size.
Melanomas are by no means a death sentence for your horse, and can hang around for many years without problems. However, they can convert to malignancy, which may spread both across the skin and to the internal organs. Suspicious lumps should always be checked by your vet, and monitored at least as frequently as your horse's annual vaccinations. It is very common to find numerous BB sized melanomas in gray horse's sheaths or on the under-side of their proximal tail that are usually impractical to remove. Most horses will outlive any deleterious effects from these. The best recommendation I can give in terms of treatment is to keep your expectations realistic and explore all the available options with your veterinarian.
Dr. Darin Peterson, DVM, was born and raised on a horse and cattle ranch in Rosholt, S.D., and received his B.S. in Animal Science from SDSU. He concentrates most of his work time with large animals. He can be reached at 701-347-5496 or email@example.com