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Melanoma in Gray Horses

Date Published: 04/02/2006
Date Reviewed/Revised: 08/04/2014

If you have a gray horse, it is likely the horse may have one or more tumors on his body called a melanoma, especially if the horse is older. In fact, one report indicated 80% of gray or gray dapple horses over 15 years of age have at least one melanoma. The tumors most commonly occur on the perineum or ventral surface of the tail, and are usually firm, nodular, hairless, and may be ulcerated. They are almost always black and can grow slowly without metastasis, grow slowly with sudden metastasis, or grow rapidly and metastasize from the onset. Metastasis indicates a tumor is malignant and can spread to other organs, and usually this involves spread to internal organs that cannot be readily examined by your veterinarian.

The majority of these tumors that occur in gray horses are benign and metastasis is unusual, so many vets are not concerned with them. However, Dr. Harry Werner indicates in this year’s AAEP proceedings that there is still the potential for a malignancy to occur. If you are looking at buying a horse that has a melanoma, it is important to realize that the future biological activity of this tumor is not predictable. Also, the potential exists for local multiplication and invasion, cosmetic deformity, and life-threatening metastasis. Currently available therapy may not be successful. Medical therapy has been effective at reducing the size of tumors in some cases with Tagamet ― the same Tagamet people take to help prevent stomach ulcers ― but it is not effective in all cases. Surgical excision or cryotherapy can be performed but it is possible the tumors can recur. The most important fact to remember if you are considering purchasing a horse with a melanoma is that although most are benign, no one can predict the future metabolic activity of these tumors.

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