Commentary: Equine Melanoma and the Nature of Malignancy

Commentary: Equine Melanoma and the Nature of Malignancy

Whether equine melanoma is a benign or malignant neoplasm has been discussed and debated for at least 100 years.

Photo: The Horse Staff

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.

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 could 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.

CONTACT: John L. Robertson, VMD, MS, PhD, 540/231-4643,, Department of Biomedical Sciences & Pathology, Virginia-Maryland Regional College of Veterinary Medicine, Blacksburg, Va.

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

About the Author

Equine Disease Quarterly

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More