Equine Melanoma and the Nature of Malignancy

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Whether equine melanoma is a benign or malignant neoplasm has been discussed and debated for at least 100 years. Many equine practitioners, by virtue of their training and experience, 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 United States Department of Agriculture in 1916. However, equine melanoma was considered at that time a serious malignancy—“melanosarkoma”—in thought and 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. The statement “your horse will die of something else” is arguably correct considering that the truly common causes of death of horses include musculoskeletal injuries (and euthanasia), gastrointestinal catastrophes (and euthanasia), and a variety of cardiorespiratory lesions resulting in poor performance (and euthanasia).

Little justification exists for prolonging discussion of the nature of equine melanomas. Equine melanoma is a progressive malignancy.

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 may be quiescent for many years.

In 30-40% of affected horses, these cell clusters continue to evolve at one or more sites. We know from the study of many animal and human tumors (malignant gliomas, for example) that the progressive growth of, and invasion by, tumors is driven by further mutation of the unstable neoplastic genome. Progression involves the selection of neoplastic cells capable of exploiting their environment—acquiring and processing nutrients, soliciting vascular growth, avoiding detection and destruction by immune and inflammatory systems—and increasing their numbers. As they progress, these neoplastic cells become more and more difficult to control and eradicate, fully expressing their malignant character.

It is common to see severely affected horses. A small cutaneous lesion that has been present for months to years begins to grow rapidly and may ulcerate. In some cases, multiple masses form at different sites, possibly representing simultaneous growth of several unique tumors (something being investigated with genotyping), rather than metastatic spread. Virtually all of these tumors are infiltrating tissues around them, a well-recognized problem in Stage III-IV human melanoma (in which the survival rate is less than 10% five years after diagnosis). These malignancies in horses are difficult to excise and are rarely removed completely. When they interfere with defecation, reproduction, or eating, they cause suffering and result in death (euthanasia).

Equine melanomas are not benign and should not be ignored. The current proven method for control of these tumors is removal, through surgery, by laser, or with cautery. Veterinarians should recognize the need for this approach and deal realistically with horses with equine melanomas until we have a better understanding of the mutations that drive their formation and progression.

Reprinted from Equine Disease Quarterly