Equine melanoma is a skin tumor that arises from melanin producing cells and can be benign or malignant. The skin is the most common site for the tumor in horses with the typical locations being the ventral tail, perineum, external genitalia, eyelids, periocular skin, parotid salivary glands and the commissures of the lips. It is most common in gray horses especially Arabians and Percherons and it is estimated that up to 80% of gray horses more than 15 years old have melanomas.
Tumor development is possibly due to disturbances in melanin metabolism. In horses as compared to humans, UV radiation does not seem to be a risk factor. Tumors are usually firm and nodular, may be solitary or multiple, and may be hairless and ulcerated. They are usually black. A diagnosis can usually be made based on clinical signs and visual inspection. Further diagnostics may be warranted if the mass is atypical in appearance.
Most small early lesions are benign but with time may become more malignant by invading local tissues or possibly metastasizing to remote sites such as spinal cord, lungs, or intestine. It is thought that metastasis may occur via blood or lymphatic vessels. But there is debate over whether or not metastasis truly occurs or if additional internal melanomas occur. Although some lesions are large in size, clinical signs are usually minimal and are typically associated with the space-occupying nature of the mass. Depending on the location, a horse may have difficulty chewing or eating leading to weight loss or an impaired ability to defecate or urinate.
There is no uniformly useful treatment for equine melanoma. Spontaneous resolution of small masses may occur. Surgical excision of small lesions or those that appear to be fast growing is possible but regrowth and/or new lesion development is also possible. Prolonged administration of cimetidine has been effective is some cases. One study showed that the number and size of melanomas was reduced by 50-90% in horses treated for 3 months. When effective, daily maintenance therapy is recommended.Newer therapy options include immunomodulation with vaccines. The Oncept canine melanoma vaccine is a DNA vaccine which uses human tyrosinase, a protein which has an elevated expression in melanoma tumor tissue. The vaccine generates a tyrosinase specific anti-tumor immune response. Canine and equine tyrosinase share 92% and 90% similarity respectively to human tyrosinase. Horses are vaccinated at 2 week intervals for a total of 4 doses and then boostered at 6 months and every 6 months thereafter if effective. The goal is a reduction in the size of smaller melanomas, possible resolution, slowing of tumor growth of larger masses, and possible prevention. Researchers are still collecting data on its use in horses but it appears to be safe. Some report regression of the tumors 50% of the time, stabilization of the disease/prevention of further growth 40% of the time, and 10% of the time there seems to be no effect meaning the disease continues to progress and melanomas grow. It does not seem to be effective in very advanced cases including horses with large melanomas that are affecting the horse’s ability to defecate. In addition, autologous vaccines provide other options to create an immune response. They either combine surgical excision with stall-side sterilization of tumor tissue and subsequent implantation of that excised tissue under the skin of the horse’s neck or the production of an individualized vaccine at a commercial laboratory.
These services are available at Atlantic Equine Services. Please contact us if you have any questions regarding equine melanoma or would like to schedule an appointment.