Study: Melanomas More Serious to Solid-Color Horses than Gray Horses

A recent Austrian study has shown that melanomas (malignant tumors of pigmented skin cells) are not as serious in gray horses as melanomas found in solid-colored horses characterized by early spread. Researchers believe this might be because gray horses havespecial genetic factors that inhibit the spreading process.

Additionally, the researchers found that the heritability of melanomas in a population of gray horses is about 30%.

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.

"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. There was spotted depigmentation around the anal and peri-anal areas in 50% of the cases and in six cases, depigmentation was extensive.

Although advanced-stage melanoma-bearing gray horses were found, none of the affected horses suffered any severe problems 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 (spread)."

Melanomas have been scientifically linked to gray horses before; similar results were found in a study using gray horses of a particular French breed (Camargue). Interestingly, the Lipizzaners were more likely to have melanomas 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. 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 spread, and finding out a horse's melanoma status with a simple blood sample. Researchers also plan to develop therapies for treatment of melanomas involving the immune system.