Question:Will all gray horses develop melanomas? I’m considering buying an 8- year-old mare. She is perfect in just about every way, but I’m very concerned that she will develop melanomas eventually. I’ve seen a few extensive cases, and two where the horses had to be put down. If melanoma is likely in this mare’s future, is there a way to control the spread of the tumors?
Answer: Melanomas are an extremely common skin tumor in gray horses. Approximately 80 percent of gray horses develop melanomas by their late teen years. Fortunately, most of these growths are benign. It is important to keep in mind that most gray horses with melanoma lead normal lives with no or relatively simple treatments. However, melanomas are still a significant concern because they can become malignant, often occur in multiple locations and can form large tumors that are locally invasive, meaning they can grow into deeper tissues and invade important structures.
Where melanomas commonly develop
Melanoma tumors most commonly develop under the tail and in the perineal region. They can also occur in other locations such as the head, prepuce or elsewhere on the body. Some melanomas grow very slowly, and others increase in size rapidly. In a small number of horses melanomas affect internal organs, however, most melanomas remain in the skin. (Melanomas in non-gray horses are rare but tend to be malignant.)
Although most gray horses develop melanomas, many require no treatment. Owners can be proactive by frequently inspecting horses for melanomas, particularly in the most common anatomic locations. If a few small tumors are detected, they can simply be monitored. It is a good idea to take measurements with calipers or a ruler so that changes in size can be readily detected.
Your veterinarian can provide guidance as to whether a tumor requires treatment. Rapidly growing or larger melanomas are typically treated. Simple surgical removal can be curative for small, solitary melanomas. Larger and locally invasive tumors require more invasive surgery and potentially complicated aftercare. Other common methods of treatment include laser removal, local chemotherapy or oral medications (e.g., cimetidine).
A melanoma vaccine—given once a tumor is diagnosed to prevent further growth—is available. The vaccine can improve success in some horses, usually in combination with other local treatment methods. Other experimental treatments are available from equine medical and surgical specialists. These include gene therapy and electroporation treatment, which is a method of disrupting melanoma cells with electric pulses.
While treatment of small tumors can be curative, other melanomas may develop over time so continued monitoring is necessary. In a small percentage of cases melanomas can become large and aggressively invasive to the point that treatment is not feasible and quality of life declines. In these cases, euthanasia is the best option.
Overall, gray horses with melanoma have a good prognosis for normal quality of life. The most important factor in maintaining a good prognosis is diligent monitoring and, when necessary, early treatment before melanomas grow in size or become malignant.
Chris Byron, DVM, MS, DACVS
Virginia-Maryland College of
Chris Byron, DVM, MS, DACVS, received his Bachelor of Science degree from Cornell University in 1994 and earned his veterinary degree at the same school in 1998. He later earned a master’s degree in Large Animal Clinical Sciences from Michigan State University, and he is Board certified by American College of Veterinary Surgeons.