In the literature, it is reported that as many as 80% of gray horses over the age of 15 will develop melanoma. Common locations for melanoma lesions are around the eyes, lips, muzzle, throatlatch, sheath, perineum and under the tail, but they can appear just about anywhere on a gray horse’s body.
Melanomas are not confined to occurrence on gray horses, but they are more prevalent on grays—gray horses carry a unique mutation inherited from generation to generation that predisposes them to melanoma development.
Historically, malignant melanoma skin tumors of the horse have been difficult to manage, particularly when they are aggressively invasive and/or become open sores. Treatments with laser excision, direct injection with cisplatin and oral cimetidine have produced variable and often unsatisfying results.
Now it looks like several equine melanoma vaccines that are in the making show promise for future management and control of equine melanoma. While most vaccines are used to prevent disease, the objective with these products is to give horses already affected with these skin tumors a vaccine that stimulates their immune response to either rid the body of the cancer or at the very least to retard growth of the tumors.
One product, Oncept (by Merial), has been used to manage canine melanoma and is now in clinical trials for horses at Lincoln Memorial University in Tennessee. By targeting tyrosine, which is present in abundance on melanoma cells, Oncept ramps up the horse’s native immune response. Results of the clinical trials are encouraging: Horses with melanoma tumors that were given a series of the transdermal vaccine demonstrated a measurable immune response. A six-month booster followed the initial four biweekly doses.
Another vaccine, ImmuneFX (by Morphogenesis), is being tested as well. It uses a bacterial gene to evoke a strong immune response by the horse to attack tumor cells without harm to normal tissue. Results from this product have yielded 40-48% reduction depending on whether the vaccine was injected directly into the tumors or elsewhere in the same horse.
Yet another melanoma vaccine is in investigative stages and clinical trials at the University of Florida’s College of Veterinary Medicine.