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Melanoma Incidence Lower in Quarter HorsesBy Kentucky Equine Research Staff · October 24, 2014

Melanomas are common equine skin tumors, particularly in older gray horses. Some estimates put the incidence of melanomas as high as 80% in gray horses over the age of 15. They can occur below or above the skin surface, and are often found around the anus and on the underside of the tail. Unlike human melanomas, these tumors in horses are generally slow-growing and may be present for many years without causing significant problems.

A recent study conducted at the University of Minnesota indicated that older gray Quarter Horses may develop melanomas less frequently than horses of other breeds, possibly because of genetic mutations responsible for variations in coat color. In addition, Quarter Horses that do develop melanomas are often not affected as severely as horses of other breeds.

The researchers collected samples of DNA from more than 330 gray Quarter Horses ranging in age from 1 to 33. Slightly over half of the horses older than 15 years had visible melanomas, a percentage that is considerably below the average for older gray horses of all breeds. Melanomas were scored from 0 (smaller, less developed) to 4 (larger, more developed) based on size and appearance. The tumors on Quarter Horses scored an average of 0.35, far below the average grade of 1.19 for tumors on a comparable group of Lipizanner horses.

Coat color in horses is genetically complex, with many variations in dilution and expression. Horses can show one coat color when they are young and gradually develop more roaning (white hairs mixed with another coat color), turning grayer as the years pass. In Lipizzaner horses, most of which are born black or very dark-coated and become lighter as they age, those with two copies of the gene that distributes black pigment are at greater risk for melanoma development than horses with one copy of the gene. The researchers looked for evidence that the genetic pattern for chestnut coat color in Quarter Horses might be linked to a protective mechanism that minimizes the risk of tumor formation. Though this hypothesis was not borne out in the current study, further research could reveal a genetic component that would allow horses to be tested for melanoma risk before the tumors appear.