These lumps and bumps can mean trouble, but researchers are making headway in defining tumor treatment strategies

The luster and shine of a horse’s coat speaks volumes about his inner health. But problems might lurk beneath the skin in the form of tumors or other lumps. As you groom your horse, take extra care to notice any suspicious changes, as some skin lesions can pose troubling health consequences. The most common equine skin tumors to watch for are melanoma, squamous cell carcinoma (SCC), and sarcoids.


Horses’ tumor susceptibility, especially for melanoma or SCC, correlates with the degree or lack of skin pigmentation. Researchers claim in studies that 75-80% of gray horses over the age of 15 eventually develop melanoma. However, the equine form of this skin tumor is not usually malignant or as rapidly progressing as that which occurs in humans or dogs. In 2008 Leif Andersson, PhD, and his colleagues in the Department of Animal Breeding and Genetics at the Swedish University of Agricultural Sciences examined the association between melanoma and gray coat color in horses. Andersson reports that gray horses carry "a unique (genetic) mutation, inherited from generation to generation, that predisposes to melanoma development."

While most melanomas in gray horses are benign, histologic changes during tumor development (e.g., expanding into surrounding muscle) can cause them to become malignant. In humans, for example, a genetic predisposition such as fair skin coupled with exposure to sunlight’s ultraviolet (UV) rays can be a melanoma-causing combination.