Melanomas: Gray Horses vs. Solid-Colored Horses

A recent Austrian study has shown that melanomas in gray horses are less malignant than those found in solid-colored horses characterized by early metastases (cancer that started from cancer cells from another part of the body). Researchers believe this might be because gray horses have specific genetic factors that inhibit the metastatic processes. Additionally, the researchers found that the heritability of melanomas in a population of gray horses is about 30%.

Idaho
KIMBERLY S. HERBERT PHOTO 

Melanomas in gray horses (like this one on the horse's dock) are less malignant than those found in solid-colored horses, according to a recent Austrian study.

The study, published in the Equine Veterinary Journal, was completed by researchers from the University of Veterinary Medicine in Vienna, Austria, collaborating with individuals from the University of Agriculture and University of Vienna Medical School in Vienna and the University of Zagreb in Croatia.

The scientists performed a clinical study on 296 gray Lipizzaners with pedigrees traced back 32 generations. Each was classified according to his stage of disease using a 0-5 scale (0 being free of melanoma).

"Of the 296 horses, dermal melanomas were present in 148 horses (50%), 68 of which were more than age 15 years; 51 (75%) of these were melanoma-bearing," said the scientists in the study. "A vitiligo-like, spotted depigmentation of the peri-anal and anal regions was observed in 50% of the cases, particularly in older horses," said the researchers. "In six cases, depigmentation was extensive."

The scientists also analyzed the effects of stud farm, age, and sex on melanoma incidence. While the animal's sex didn't seem to be a factor for incidence, the effects of stud farm and age were highly significant. Horses from particular stud farms had significantly lower melanoma gradation than horses from other farms.

Although melanoma-bearing gray horses were found up to stage 4, none of the affected horses suffered any severe clinical effects or was handicapped in performance. This contrasts with solid-colored horses with melanomas. Monika Seltenhammer, DVM, of the Clinic for Surgery and Ophthamology of Vienna's University of Veterinary Medicine, said, "Although melanomas are more seldom in dark-colored horses, they show human melanoma-like features. And melanomas in humans are one of the most malignant tumors due to the early development of metastases."

Melanomas have been scientifically linked to gray horses before; similar results were found in a study using Camargue-type (originating in Southern France) gray horses three years ago, thus giving the current study results more significance. Interestingly, the Lipizzaners had more incidence of melanoma than the Camargue horses, and the Lipizzaner is the product of 400 years of inbreeding, whereas the Camargue horse population is more heterogenous. The authors suggest that, "A hereditary predisposition (due to the inbreeding) seems probable," although another study in 2000 reported that Lipizzaner inbreeding has no effect on melanoma incidence.

Further analysis of melanoma development is needed to indicate whether the tendency to develop melanomas is due to parentage or merely an influence of genes with vigorous effects that are unique to gray horses.

"On the one hand, the gene which is responsible for graying seems to be connected with the development of melanomas in such horses," she said. "Whereas on the other hand, there are existing grays, old and white ones, without any melanomas. So there are obviously more genes responsible that are affecting and influencing each other, and it depends on which genes are more expressed."

Regardless of their derivation, "A histological (tissue) investigation should be done in any case of a lesion that arouses the suspicion of melanoma," added Seltenhammer.

Future Austrian genetic studies focus on the genes responsible for melanoma development and metastasis, and prognostic and diagnostic marker systems for melanoma. "A simple blood sample should deliver significant answers," said Seltenhammer. "Moreover, therapy solutions on an immunological basis are going to be developed."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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