Navigating the mysteries of gray horse melanoma

Horse ownership offers many joys, but as owners navigate the needs of their equine companions, they may find that each breed has its own unique requirements. What owners may not know is specific coat colors can signal distinct health challenges that they should attentively watch for and address.

In the spirit of National Day of the Horse this Dec. 13, Dr. Dustin Major, a clinical assistant professor in large animal surgery at the Texas A&M School of Veterinary Medicine & Biomedical Sciences, explores one of those distinct health challenges — that all gray horses are predisposed to melanoma — and how owners can manage this skin condition through early intervention. 

What is melanoma?

Melanoma, a form of skin cancer, develops when the cells that control skin pigmentation grow uncontrollably, causing a mole or age spot to grow and change size, shape, or color. While it is not fully understood why this happens, Major says that owners can reasonably expect for all gray horses to eventually develop melanomas to varying degrees.

“Melanoma is very common in gray horses that are middle-aged or older, but it is uncommon for horses of any other color,” Major said. “Some gray horses will only have one small melanoma spot, while others will have more expansive melanomas, particularly under the tail, around the anus, and in the corners of the mouth.”

Gray horse melanoma tends to start as small lesions, or parts of the skin that have an abnormal appearance and feel compared to the rest of the skin. They tend to display as round, firm, almost spherical masses that are clearly distinct from the surrounding tissue. While they are not typically fatal, melanomas can disrupt daily activities if left untreated. 

“It is rare for the melanomas to metastasize, or spread to distant organs such as the lungs or brain, but they can grow large enough to become locally invasive, meaning they prevent organs from working in the way that they should,” Major explained. “For example, if a horse has large melanomas around their anus, it can be difficult for them to defecate.”

Exploring treatment options

Because gray horse melanoma prevention is unlikely, early intervention is essential for maintaining the horse’s quality of life.

“Owners should keep an eye on the common areas where melanomas appear and record what they look like,” Major said. “They can then work with their veterinarian to monitor the melanomas and intervene before the lesions grow so big that they are harder to deal with. The earlier lesions are addressed, the better the outcome.”

Surgery is one option for removing melanoma lesions, but depending on a lesion’s size, the surgery can be difficult for both surgeons and animals.

“When melanoma lesions are small, they tend to be like little marbles and are very easy to remove surgically,” Major said. “Once they start to get bigger, like the size of a tennis ball, the surgery to remove them becomes more complicated, expensive, and traumatic for the patient.”

There is also a vaccine that offers a potential treatment for gray horse melanoma, though it’s less likely to fully remove melanomas present.

“A veterinary oncologist has to first prescribe the vaccine before a veterinarian injects the vaccine with a special device, and the vaccine is given every two weeks, for four treatments, which can become very expensive,” Major said. “There is no guarantee that all melanomas will go away even though the vaccine can possibly slow down the growth of melanomas, keep new melanomas from growing, and shrink the ones that are already there.” 

Because of the difficulties that come with surgery and vaccinations, Major strongly emphasizes that owners rely on early intervention as the best method to treat and manage melanomas in gray horses.

“The key thing to remember is to get all melanomas checked out early and then monitor them so that when it is time to intervene, the melanomas can be removed before it’s too late,” Major said.

Without the proper care and attention, gray horses can face challenges that compromise their well-being; by working to understand and meet the specific needs of these horses, owners can ensure that despite any melanoma that may appear, gray horses can lead fulfilling and healthy lives.

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Pet Talk is a service of the School of Veterinary Medicine & Biomedical Sciences, Texas A&M University. Stories can be viewed on the web at vetmed.tamu.edu/news/pet-talk. Suggestions for future topics may be directed to vmbs-editor@tamu.edu.