The latest research indicates that equine practitioners and the general public need to understand that a melanoma is not just a benign bump.

The statistics are a little daunting:


  1. Up to 80% of gray horses older than 15 years will develop at least one melanoma, a type of cancerous tumor.

  2. Approximately 30% of equine melanoma cases seen by at least one referral hospital had developed into extremely large, advanced, infiltrative, multinodular, metastasizing (spreading) or multicentric (having multiple centers of origin) lesions.

  3. All melanomas are malignant (even the tiny ones) and possess the propensity to have further and unpredictable patterns of growth.

Yet many veterinarians and horse owners are fairly dismissive of melanomas, considering them to be benign lesions that merit neither biopsy nor treatment–unless or until the tumor becomes threatening.

Some veterinarians think that’s a potentially fatal attitude. “It’s important to realize that melanomas grow into serious problems in many horses,” warns melanoma researcher John L. Robertson, VMD, MS, PhD, Director of the Center for Comparative Oncology (CeCO) at the Virginia Maryland Regional College of Veterinary Medicine in Blacksburg, Va. “I routinely see horses that have the advanced stage of the disease. Most of those advanced cases will die from the disease, as we have very few treatments for it.”

Various studies have found melanoma incidence ranging from 3-15%, with 30% of melanomas occurring in horses that aren’t white or gray.

Melanomas 101

Most melanomas occur externally. “The common sites for primary melanomas include the undersurface of the tail, perineum (the area of skin under the tail and around the anus), and the external genitalia,” says Robertson, who is also a professor of pathology in t