Melanoma is a kind of tumor (abnormal cell growth), that is usually benign (slow growing or stagnant), but occasionally malignant and cancerous (fast growing, or invading other tissues or organs).

A very high percentage of gray horses develop melanoma masses somewhere on their body as they age. It is a problem unique to the true gray horse. It may be more common in gray horses of specific breeds. The genetics of melanoma are being studied but are not well understood.

The most common locations for these hard, black growths are around the anus and tail base, on the tail itself, behind the jaw and under the ear, on the sheath, and within the lips. In most cases, these nodules are painless and very slow growing.

Although they are unsightly, these growths usually do not interfere with bodily functions or performance. However, some are or become rapidly growing and, in rare cases, the nodules spread into other organs, and the disease is fatal. What determines the course melanoma takes is still not fully understood.

Very occasionally, a form of melanoma can develop on non-gray horses. It is most common on the tail. This usually does not behave as it does on gray horses, and can be rapidly fatal.

Diagnosis is usually made on visual evaluation. Almost any gray horse with a hard nodule has a melanoma. If there is any doubt, a sample of the mass can be removed and sent to a lab (biopsy and histopathology).

It is difficult to completely rid a gray horse of melanoma. In our practice, we surgically remove masses that become large enough to cause problems defecating, affect other critical body systems, or that repeatedly open and bleed.

I have treated some melanoma with cimetidine, but I have not seen real evidence of effectiveness. The science currently suggests that it is not a particularly helpful treatment. The most exciting area treatment for melanoma involves the development of new melanoma vaccines. These too though lack strong evidence for effectiveness as of the time of this writing. So be sure to ask your vet for whatever evidence can be provided.