Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose


Synonyms: Melanosis, Melanocytosis, Gray Horse Melanoma


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.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

Very Common
Less Common
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The prognosis depends on the type of melanoma, its presentation, and your horse's age.

Prognosis in gray horses is good in most cases as these tumors tend to be more of a cosmetic blemish than cause severe disease. Most gray horses have one or more on their body after about 6 years of age.

The prognosis for amelanotic melanoma in non-gray horses is poor. It tends to progress quickly and ultimately be fatal.

Local recurrence after complete removal of a melanoma is uncommon. However, recurrence elsewhere on the body is common.

Removal should be considered for rapidly growing, single masses in atypical locations.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • Based on the appearance of this mass today, will it be a problem in the future?
  • In purchasing this horse with melanoma, what should I expect?
  • Are you aware of any new treatments that can rid my horse of melanoma?
  • What is the cost/benefit of available melanoma vaccines?
  • Should I buy this gray horse knowing that melanoma may be a problem?

There is likely a genetic basis for the development of this type of tumor in horses. Selection for horses without this problem reduces the prevalence of this problem within the population.

Recognize how common this condition is in true gray horses, and consider this when you buy a gray horse. Regularly monitor masses for number, size and growth. Track their progression by taking a digital photo on a regular basis.

further reading & resources

Related References:

B. A. Valentine M. B. Calderwood Mays H. S. Cheramie Anaplastic malignant melanoma of the tail in non-grey horses Equine Veterinary Education Volume 26, Issue3
March 2014 Pages 156-158


Higgins AJ, Snyder JR eds. The Equine Manual. 2nd Ed. Edinburgh: Elsevier Saunders 2006.

Author: Doug Thal DVM Dipl. ABVP