Equine melanoma

Sooner or later, your gray horse will very possibly develop melanomas. That's the bad news. The good news is that they are rarely fatal.

A melanoma is a cancer that develops in the melanin cells of the skin. Melanin, the pigment that makes some skin darker than others, is abundant in the skin of gray horses. While horses of other colors can develop melanomas, grays are especially vulnerable.

Melanomas are solid, hard, usually round black tumors. Melanomas in humans are very serious because this cancer tends to spread internally. The rapid internal spread (metastasis) is usually consistent with a poor prognosis, and many human melanoma
patients die within six months to a year of onset.

Melanomas in horses act very differently. They are usually only locally invasive and are slow growing. These round, black nodules are commonly found near the base of the ears, around the eyes, around the neck and jugular groove (the indentation on the side of the neck where the jugular lies between muscle groups), under the tail and around the vulva or rectum. These lumps are generally smooth, hard and non-painful. Occasionally, equine melanomas can metastasize and cause devastating internal problems, but most tumors remain in the skin and grow slowly during the horse's lifespan.

Diagnosis is made by taking a piece of the mass and examining it under a microscope. The characteristic dark, black granules of melanin within the tumor cells confirm the disease.

If the lump is small and not causing any other problems, it is usually best left alone. Some equine surgeons argue that operating on melanomas "activates" the cells and increases the chances of tumor growth. They recommend a cautious approach to the problem.

Large masses that interfere with tack or prohibit horses from defecating, for example, have to be treated.

Laser surgery has really helped in the surgical approach to melanoma management because it has enabled veterinarians to remove melanomas located in areas that would be difficult to operate on using traditional techniques. The laser cuts and seals blood vessels and makes dissection easier in areas where there is not a lot of extra skin, such as under the tail.

Medical management of melanomas is the more traditional approach. Cimetidine (Tagamet) is a drug commonly used to treat stomach ulcers in horses and humans. This drug also has a potent effect on some melanomas and, while not curative, will drastically reduce the size of melanomas throughout the body.

Researchers have developed a tissue-based vaccine made from the horse's own tumor cells. Good re-sponses have been obtained for mel-anomas on the face, body and legs; tumors on the tail/vulva and along the jugular do not respond as well.

Additional anti-cancer drugs and treatments, such as radiation, are

currently being developed. In the future, gene therapy may enable us to manipulate genetic signals to prevent cells from becoming cancerous, or turn off cancer-like functions once they begin.

Ken Marcella, D.V.M., is based at the Chattahoochee Equine Center in Canton, Ga.