Commentary: Equine Melanoma and the Nature of Malignancy

One veterinarian shares his views on equine melanoma and malignancy.
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One veterinarian shares his views on equine melanoma and malignancy.


Whether equine melanoma is a benign or malignant neoplasm has been discussed and debated for at least 100 years. Many consider it a common benign growth of the skin, particularly of gray horses, which is precisely as stated in a manual on recognition and treatment of equine diseases published by the USDA in 1916. However, equine melanoma was considered at that time a serious malignancy–"melanosarkoma"–in German literature (1909).

During pathology training, many veterinarians are taught that most equine melanomas are focal aggregates of locally-growing, variably pigmented cells–akin to human moles (pigmented nevi), which is a somewhat classic view of benign tumors. Armed with this knowledge, they counsel owners of horses with tumors that melanoma is slow-growing and of little consequence. This theme is common in pre-purchase examinations.

Like all neoplasms, equine melanoma must begin with the transformation by mutation of a single stable cell faithfully reproducing its genotype and phenotype to that of a neoplastic cell displaying a propensity for unregulated growth and variations in differentiated phenotype. This process is not in dispute. The small melanoma under the tail of the gray horse already has formed a cluster of cells that infiltrate and compress surrounding skin and connective tissue–two things that normal, differentiated melanocytes do not do. These small melanomas could be quiescent for many years

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