Ask the Vet: Equine Skin Tumors

Question: I have a grey 5-year-old Arab/Azteca mare. She is greying pretty slowly and I am wondering if horses that grey more slowly are more or less likely to develop melanomas as they age.

Answer: Melanocytic tumors in gray horses are thought to arise because of a disturbance in melanin (or pigment) transfer from dermal melanocytes to follicular cells. This accumulation of pigment and eventual transformation to a melanoma has been documented in melanoma-prone locations.  Interestingly, common sites of occurrence are typically the areas that first begin to show loss of pigment as the horse ages.

The majority of melanomas are located around the perineum and base of the tail, with lesions around the head (lips, eyes, parotid region) and other sites less frequently reported. Melanomas have also been reported in the foot, meninges, thorax, ocular structures, and abdominal cavity. These unusual locations appear to be associated with a poor prognosis.

Logically you might argue that horses that lose pigment or “gray” more slowly would also be slower to develop tumors but data is lacking.  In reality the vast majority of gray horses will ultimately develop melanomas.  In some reports it appears that “flea bitten” grays are less likely to develop melanomas compared to other gray coated horses.

There is good news to be had despite this information!  The majority of melanomas in horses are benign meaning they rarely metastasize or cause systemic problems. The biggest issues with melanomas relate their location and size.  Melanomas around the lips can interfere with training, melanomas around the vulva can impact a mare’s ability to carry a foal.  When melanomas get large (called dermal melanomatosis) they can ulcerate, attract flies and cause more discomfort.  Malignant melanomas are very rare and usually detected in aged horses.

In a study looking at gray horses at a Lipizzaner stud farm they reported that melanomas were more common in horses >15 15 years old (75%) verses all horses (50%).  The vast majority of horses were clinically unaffected by the melanomas.

Should your horse develop melanomas there are many options for treatment. Depending on the size, growth rate and location your veterinarian can discuss the best approach to ensure your horse remains comfortable and functional.

Answered by, Elizabeth Carr, DVM, Ph.D., DACVIM, DACVECC, CVA, Michigan State University