Melanoma in horses is quite different from melanoma in humans. Equine melanoma is abnormal cell growth resulting in tumors that are most often benign. The tumors consist of an overabundance of melanocytes—melanin-producing cells—and gradually grow larger over time. 

About 80% of gray horses of any breed will develop melanomas by age 15, though they can be found in horses as young as 2.  

The melanomas appear as black growths on the skin. They are most often found on the muzzle, ears, sheath, along the top of the neck at the base of the mane, and around the anus and dock of the tail. The benign tumors don’t metastasize but can grow large and develop separately all over the body. Many horses live full lives with these tumors, but if euthanasia is required, it’s often because large tumors at the tail and anus are preventing the horse from defecating.

Treatment

If you discover a melanoma tumor, have your veterinarian perform an exam right away to determine a course of action. These tumors can grow slowly or quickly. And the larger they become and the more tissue they affect, the harder they are to remove or treat. 

In the early stages, surgical lasers may remove or debulk a tumor, and freezing through cryotherapy can cause the tumor to die. Chemotherapy is also an option, with Cisplatin and other drugs injected into tumors to shrink them. 

A relatively new therapeutic vaccine called Oncept has become available to treat equine melanomas with promising results. Administered as a series of injections by a veterinary oncologist or internal medicine specialist (general practitioners aren’t permitted to administer it), this vaccine tells the body’s immune system to look for cell markers and destroy those cells. Though the vaccine cannot get rid of tumors already in place, results are showing that it can stop their continued growth.