If you read the veterinary literature, you can find reports that 80 percent of gray horses over the age of 15 will develop melanoma. While this may not be the actual number affected, many older gray horses do develop melanoma lesions in the skin. Unlike the aggressive and malignant nature of human or canine melanoma, equine melanoma tends to affect the skin in most cases; typically horses can live with this form of skin cancer for a very long time with little repercussion. In an article that originally appeared in an issue of Horse Illustrated magazine, Nancy S. Loving, DVM (a performance horse veterinarian based in Boulder, Colo., and the author of All Horse Systems Go), discusses melanoma.
Q: I have a gray horse with multiple melanomas under his tail. Can these be treated or cured? Will they affect his daily health?
A: Common areas for melanoma to occur are on the underside of the tailhead, around the anus and/or vulva, within the prepuce, along the penis, in the mouth, and around the salivary gland in the throat latch. However, it’s possible for melanoma nodules to develop just about anywhere on the body.
Chemotherapeutic agents, like cisplatin, have achieved success in control, and on occasion, resolution. Medications like oral cimetidine and other oral or injectable formulations that target the immune system have also been used in attempts to stimulate the horse’s immune system to slow the growth of cancer cells. Another treatment uses a “vaccine” made from melanoma cells that are harvested directly from the affected horse. Such efforts to boost a horse’s natural immunity have worked in some cases but not in others. Ongoing research into immunotherapy treatments may eventually present viable options that thwart the growth of equine melanoma. Although unusual, on rare occasions a horse’s immune system will eliminate them without any intervention.
If the lesions are small (pea-sized) and don’t ooze or bleed, and if they seem fairly slow-growing, it is often best to leave them alone. Surgical excision or biopsy tends to “anger” melanoma cancer cells so that they are more likely to become active and expand; this has the potential to increase their malignancy and to elicit internal metastases.
On the other hand, if the lesions are obviously invasive and/or are ulcerated and draining, then they need to be managed, often with surgical intervention using scalpel and/or laser excision or cryotherapy (freezing). It gets tricky when there are small, benign lesions directly beneath critical areas of tack. You’ll want to discuss your options with your veterinarian, since those lesions may interfere with riding but intervention may worsen the problem.
It helps to take photos of each melanoma when you discover it; use a size reference in the photo so you can track any changes over time. As long as they aren’t bothering your horse and aren’t obviously active, then the best course to take is to monitor them.