Melanomas in gray horses are most frequently found on the undersurface of the tail (about 95%). The next most likely location is around the anus and vulva of mares. They can also be found on the lips, eyelids, penis and sheath.
Melanomas are not limited to gray horses. Melanomas in other horses are found on the body or legs. Metastasis (spread) apparently occurs via the blood stream, carrying the tumor cells virtually anywhere. Metastases have been found in the skeletal muscles, gutteral pouches, salivary glands and anywhere in the chest or abdomen. There was even a recent report of spread to the brain.
However, while any horse with melanomas might eventually experience spread, metastasis to distant sites is much more common when melanomas appear on non-gray horses. On microscopic examination, melanomas in non-gray horses are much more similar to the aggressive and deadly melanomas found in people. (See table for types of melanomas.) Gray horses may live several years after the appearance of melanomas before the tumors show any spread beyond a local increase in the number of tumors.
Why gray horses are so prone to melanomas is currently unknown. In humans, sunlight is implicated as a factor, especially in the transformation of benign lesions into malignant melanomas, but melanomas in gray horses typically occur ”where the sun don’t shine.” One theory is that as gray horses age and their coats typically get whiter, the melanin that would otherwise be deposited in the hair is stored in increasing amounts in the skin.
There’s still considerable disagreement regarding how melanomas in horses can/should be classified microscopically, and what microscopic findings mean in terms of how aggressive the tumors are. However, the bottom line for an individual horse is still that it’s often impossible to predict how these tumors are going to behave. In horses with the slow-growing tumors, they’re often more a cosmetic problem than anything else.
Problems related to internal tumors depend heavily on where they are located. In rare cases of spread to the spinal cord or heart, consequences can be fatal, while melanomas in the salivary glands are disfiguring but rarely interfere with eating. Large tumors in the anal or vulvar region can cause problems with defecation, urination or breeding. Many, if not most, gray horses with skin melanomas also have melanoma growths in their gutteral pouches. Those can change frequently in their size and number but usually don’t cause serious problems such as invasion of blood vessels and serious hemorrhage, although this can happen.
One common problem with all types of melanomas is irritation of their surface resulting in ulceration and bleeding. Cryotherapy (freezing) is often the treatment of choice for this. It won’t cure the tumor, but does significantly reduce the size and stop the bleeding problem.
The prevailing veterinary opinion is that melanomas should be considered incurable and that benign neglect is the best treatment approach, until and unless problems arise. To the extent that melanomas in aged gray horses likely represent a body-wide change/abnormality in production and/or handling of melanin, that is probably correct. However, some of the prevailing wisdoms regarding melanoma should be re-examined.
You may have heard that surgical removal of melanomas is dangerous and triggers spread. However, a search of the literature does not support that idea. While it’s true that melanomas with particularly malignant features may exhibit explosive spread if disturbed by a surgery, this is apparently a rare occurrence with melanomas. As a safeguard, needle aspiration of the tumor for cytology can be done prior to surgery.
Surgery is usually only considered if a tumor becomes a problem in its location, e.g. is interfering with function or is ulcerating. However, it’s also an option for horses who have only one or two tumors and is usually curative. In horses with multiple tumors, surgery can be considered for only the ones that are causing problems. Tumors that have ulcerated or are causing problems because of their location can also be effectively reduced by cryotherapy (freezing), although this is not curative.
Otherwise, there are few treatment options. Melanomas respond poorly, if at all, to radiation and chemotherapy. Based on work with human melanomas, vaccines have been suggested but have not produced exciting results to date. The antiulcer drug cimetidine (Tagamet) apparently causes tumor shrinkage for variable time periods in some horses, and in others does not work at all. Immunotherapy showed promising results in one trial, but we’re a long way away from having this be an option.