Metastasizing Melanoma

If a melanoma spreads to a horse's central nervous system, treatment isn't likely to be effective.

Photo: The Horse Staff

Q. There’s a gray lesson horse in his early 20s at my barn who has had melanoma growths under his tail for years. They don’t seem to cause him discomfort, and his riders always keep that area very clean. In the past week, however, he’s started losing his balance occasionally and is showing neurologic signs. The vet is coming out to see him, but is it possible that the melanoma has spread to his central nervous system? If so, is there anything that can be done?

Ashley, via e-mail

A. Your question is a good one, given this horse’s age and his long history of melanomas. The short answers to your questions are:

  1. Yes, metastasis (i.e., spread) of the melanomas to his central nervous system is certainly possible, and
  2. If metastasis has occurred, no treatment is likely to be effective.

A thorough veterinary physical exam can confirm that the horse is truly ataxic (incoordinated), rather than lame from a non-neurologic cause. If actual ataxia exists, a focused neurologic exam can define the precise location of a central nervous system lesion. This information can help generate a realistic list of possible causes, or differential diagnoses.

It is also important to rule out other possible causes that may have treatment options, such as equine protozoal myeloencephalitis, or EPM. Currently, there are injectable vaccines available that are designed to shrink melanoma tumors. However, outcome data from actual cases is minimal. At present, this technology cannot be expected to offer a satisfactory outcome in a case such as this. Finally, it’s critical that no one rides this horse as long as any ataxia is present; ataxic horses pose a serious risk of human injury.

About the Author

Harry Werner, VMD

Harry W. Werner, VMD, is a Connecticut equine practitioner with special interests in lameness, purchase examinations, wellness care, and owner education. Dedicated staff, continuing education and technological advances enable his practice to offer high-quality patient care and client service in a smaller, general equine practice environment. A committed AAEP member since 1979, Dr. Werner is has served as AAEP Vice President and, in 2009, as AAEP President, and he is a past president of the Connecticut Veterinary Medical Association.

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