Melanoma—cancer of the skin’s pigment cells—behaves quite differently in horses than in other species, including dogs and humans, and is most commonly, though not necessarily always, associated with gray coat color. In fact, it’s thought that something along the lines of 80 percent of gray horses are going to develop melanomas at some point during their lives.
Usually, melanomas in horses present as black lumps near hairless areas, such as under the tail, around the anus or in the sheath of geldings. However, enlargements can develop under the skin just about anywhere.
Most commonly, the tumors are benign, although malignant melanomas have been reported. Even benign tumors tend to grow and can almost completely disfigure the areas where they are growing. For example, it can be difficult to see or even find the anus in a horse with advanced melanoma.
Sometimes, the tumors can grow large enough to make passing manure difficult. Larger tumors burst and ooze, which can lead to local inflammation and infection.
Four Tumor Types
Melanocytic nevi, usually benign, are small, single discrete masses (less than 2 to 5 centimeters) seen in younger horses of any color, and they can appear just about anywhere on the body.
Dermal melanomas also usually are benign; however, if they are large or develop in atypical locations, they can become malignant. They vary in size, usually are discrete masses and can appear in clusters.
They are most often seen in more mature gray horses and most commonly appear under the tail, in the perineum and on external genitalia, but also can invade the parotid salivary gland, lips, eyelids and the neck.
Dermal melanomatosis tumors are more likely to be malignant and metastatic. Usually seen in horses older than 15 years of age, dermal melanomatosis presents most commonly as multiple, large, poorly circumscribed masses of various sizes.
Anaplastic melanoma also tends to be malignant and metastatic but is relatively uncommon. The tumors typically are seen in horses older than 20 years of age, and they can occur in horses of any color.
Diagnostic testing, such as fine needle aspiration or biopsy, can help determine tumor types. Metastasis—which most commonly occurs in the lymph nodes, the liver, the peritoneal lining, in blood vessels or in the spleen—sometimes can be detected by rectal examination or ultrasound.
Surgical removal can be curative for some cases, especially when tumors are small. Success in removing larger tumors with a CO2 laser has been reported. Intralesional injection with cisplatin has been reported to be effective in some cases.
In the 1980s. the antihistamine, cimetidine, was reported as causing tumor remission in a small group of horses; however, that promising report has not been accompanied by clinical success in most horses. An autogenous vaccine made from submitted tumor tissue has been available, but there is little evidence as to its efficacy.
In 2007, a vaccine—something of a Holy Grail in cancer therapy—was approved for the treatment of canine melanomas. The problem with trying to develop a vaccine against cancer is that, to the immune system, cancer cells appear just like all of the other cells in the horse’s body. Because cancer cells are so normal (mostly), the horse’s immune system doesn’t recognize them as a problem, unlike virus particles, bacteria or tetanus toxin. The body typically doesn’t mount an immune defense against cancer cells.
However, there is something different about melanoma cells. They contain a large amount of tyrosinase, which is important in controlling the body’s rate of melanin production. Even though tyrosinase is a normal protein, researchers have been able to “trick” the dog’s body (and, apparently, the horse’s body, too) into mounting an immune response.
The vaccine is made with a human tyrosinase gene, which is placed into a small ring of DNA. Human tyrosinase is different enough from canine tyrosinase to elicit an immune response from the dog, but it’s close enough to canine tyrosinase that the immune response being stimulated also works specifically against the dog protein. While tyrosinase is in almost all of the dog’s skin cells, melanomas produce much more of it, so the immune response is able to target the cancer cells specifically.
It wasn’t a great leap to wonder whether similarities existed between canine and equine melanomas. Research conducted at the University of Tennessee concluded that the canine vaccine was efficacious in controlling—or occasionally causing complete regression of—equine melanoma, and research currently is being conducted on the vaccine at the University of Florida.
The canine vaccine currently must be obtained from either an oncologist or a small animal internal medicine specialist, and research is ongoing to develop a vaccine specifically for horses. However, for a horse with melanoma, the vaccine does appear to be a reasonable treatment option. For those that respond, the vaccination will be a lifelong therapy.
Dr. David W. Ramey is a Southern California equine practitioner who specializes in the care and treatment of pleasure horses. His website is doctorramey.com.
Originally published in the May 2017 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!