Melanoma in Horses

What Is Melanoma in Horses?

Melanomas are one of the most common tumors in aging gray horses, with about an 80% incidence rate occurring in gray horses over the age of 15.

Melanomas are a tumor of the melanocyte, the pigment producing cell that gives skin and hair its color. Breeds that may be predisposed to melanoma include Lipizzaners, Arabians, and Percherons.

The most common locations for melanomas to occur include:

• Under the tail

• Vulva

• Penis

• Parotid gland region (behind the jaw)

• Lips

• Eyelids

• Internal organ systems (such as abdominal or nervous system) can also be affected

Symptoms of Melanoma in Horses

Melanomas typically appear as firm, black, raised nodules on the skin. They generally start as a solitary mass and progress to multiple nodules. With time, melanomas will enlarge and invade local tissue and metastasize (spread) and affect distant sites. Clinical signs that can be associated with melanomas will depend on the tissue and location they are located. They most commonly affect a horse's ability to pass feces, urinate, or eat.

If a horse has an internal melanoma, these can lead to significant health concerns such as ataxia and colic. In advanced cases, horses may exhibit significant weight loss.

Causes of Melanoma in Horses

Recent studies have shown that there is a genetic mutation that causes horses to gray with age. It is known that gray horses are predisposed to melanoma development but the underlying genetic mutation that causes the tumor growth in these horses is still unknown.

How Veterinarians Diagnose Melanoma in Horses

Veterinarians are generally able to diagnose melanomas based on history and physical exam. A sample from the mass can also be sent off to pathology for evaluation and diagnosis if needed.

Treatment of Melanoma in Horses

Treatment for melanomas should be addressed and discussed with your primary vet when they are small and first noticed. Once the melanomas become bigger and more of a concern, common treatments are often applied too late and hold relatively low success rates. It is also important to note that while these tumors are often benign and slow growing, they are frequently in locations that do not make treatment successful.

Topical therapies are often ineffective or difficult to use especially when they are applied in the late stages of the disease. Surgical removal of tumors can be performed successfully but often the location of the tumor makes this difficult or another mass grows in the same area. Chemotherapy drugs such as cisplatin have been used but the use of a toxic drug can make quality of life difficult.

Currently there is one available USDA approved plasmid DNA vaccine for melanomas. This vaccine is still new but has shown positive results with stopping growth of existing melanomas. There is also evidence for significantly reducing the size of any melanomas. If you suspect your horse has a melanoma, reach out to your primary vet to discuss the treatment options and develop an individualized plan for you and your horse.

Recovery and Management of Melanoma in Horses

While most melanomas are benign, they can lead to significant health concerns. If you have any concerns that your horse has melanoma, it is recommended to have your veterinarian perform an exam and discuss available treatment options and medical conditions that could occur based on the location and severity of the tumor. Some medical conditions that could arise due to a melanoma include:

• Colic

• Ataxia

• Unable to urinate

• Unable to pass feces

• Weight loss

• Decreased appetite or trouble eating