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Malignant Melanoma in Horses

Many common types of tumors that occur in domesticated animals and in people (lymphoma and mammary tumors, for example) are not seen frequently in horses. The reasons that horses, as a species, seem relatively resistant to some of these common types of tumors are not known, but will very likely be the subject of comparative genomics research, now that genomic probes for horses are now available.

Several types of skin tumors (melanomas and squamous cell carcinomas), however, are very common in both horses and people. Melanomas are seen primarily in horses with dilute coat color (white or grey), but may occur in horses of any color. Although these tumors can occur in horses of any age, melanomas are more common in horses as they age. Published literature indicates that up to 80% of white or grey horses will develop melanomas by the age of 15 years (1). Thoroughbreds, Quarter Horses and Arabian horses are all breeds of horses that commonly develop melanomas, especially grey horses of these breeds. The most common sites for melanomas to grow are under the tail and around the anus, in the groin, and in the neck at the site of the parotid salivary glands.

The underlying cause of these tumors is not known, although it is likely that a number of key mutations, linked to coat color, may be the basis. There are few good treatments for invasive or multicentric tumors. A focused research program that will determine the underlying cause(s) of melanomas, how to detect and prevent them, and how to more effectively treat them is underway, and is necessary if we are to control and eliminate them from all breeds of horses.

Preliminary research studies of equine melanoma

We now believe that all equine melanomas are malignancies. From our research studies, we know that about two-thirds of all melanomas will initially behave as tumors that will grow slowly at their site of origin. These small, local tumors may cause no problems, or they may interfere with work, bodily functions such as defecation, or create cosmetic problems for affected horses and their owners. We know from evaluation of biopsies that even the small, locally growing tumors will grow into surrounding tissue, making early treatment of small lesions desirable (before they grow extensively).

Other equine melanomas will display a malignant pattern of growth very early in the course of disease. These tumors can occur at several sites in the body at once, or they may spread throughout the body from a primary tumor. Malignant behavior melanomas are likely to kill the horse outright or may necessitate the humane destruction of the affected animal. Since some veterinarians consider melanomas to be benign (based on dated concepts about these tumors), they do not usually biopsy the tumors or remove them in time to control the serious problems associated with the growth the tumor. We feel it is important to biopsy tumors for accurate diagnosis and treatment planning.

We think that we can eventually prevent the development of melanomas if we can understand what causes them. We know that these tumors are more likely to develop in light-colored horses but we do not know why. The common sites of melanoma on the body (around the tail and on the belly) are not exposed to significant amounts of sunlight, but we cannot discount the possibility that solar exposure may play a role. We think that there may be families of horses (that is, horses closely related to each other) that have a higher incidence of these tumors, primarily due to inherited mutations in critical genes, but this, too, is unproven. However, we know that all tumors are due to mutations and so it is likely these tumors have common mutations that we could identify and then control.

Current research directions and projects

Several equine melanoma research projects are underway at the Center for Comparative Oncology. These include:

  • Analysis of breed and familial genetics and comparison of the genome of grey horses compared to other coat colors
  • Computational analysis of serum samples from healthy horses and those with melanoma, to see if a marker molecule for the presence and extent of disease can be found
  • Evaluation of surgical methods to reduce tumor volume in horses with large melanomas
  • Development of several new therapies, including injectable and topical treatments (see "Chili's Story"), and photoactive molecules

For more information on research, participation in clinical therapy trials, or to support ongoing research, please contact:

Dr. John Robertson
Director of the Center for Comparative Oncology
e-mail: drbob@vt.edu
Phone: 540-231-7666

Contributions to support research are tax-exempt, under the 503(C) designation of the Virginia Tech Foundation.