Grey Horse Melanomas

DNA Vaccines

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Cancer isn't diagnosed nearly as frequently in horses as it is in humans, but approximately 80% of all white or gray horses will develop melanomas by the time they are 15 years old. Partly because of melanomas' preferred location (near the tail, anus, groin, or salivary glands) and partly because they often aren't diagnosed early enough, there are currently few effective treatment options, according to a veterinary internist presenting at the 2011 American Association of Equine Practitioners convention.

"One novel strategy for treating cancer in human medicine is the use of DNA vaccines that 'target' cancer cells," relayed Jeffrey Phillips, DVM, MS, PhD, Dipl. ACVIM, of Lincoln Memorial University's College of Veterinary and Comparative Medicine, during the meeting, which was held Nov. 18-22 in San Antonio, Texas.

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Melanocytic tumors (tumors arising from melanin-containing cells), including those diagnosed in horses, have a high level of "tyrosinase" molecules within their cells, which typically are not present in noncancerous cells. The DNA vaccine helps the immune system learn that these high levels of tyrosinase molecules are foreign, stimulating the immune system to identify and kill any cell containing elevated tyrosinase levels. In this respect the process is similar to other vaccines used in horses, such as those directed against bacteria (e.g., Streptococcus equi [strangles]) and viruses (e.g., West Nile virus), in which the immune system is stimulated to attack molecules deemed "foreign."

In a small, yet innovative, pilot study Phillips and colleagues used an FDA-approved (commercially available) canine DNA vaccine against tyrosinase in five apparently cancer-free horses (one gray and four bay in color). None of the horses experienced any adverse reactions such as painful swelling or hives, etc. All horses developed an "appropriate," detectable, and long-lasting (likely more than six months) immune response--both humoral (in the blood) and cell-mediated ([the protective function of immunization associated with cells]; thus activating both "sides" of the immune cascade)--to the vaccine.

"The DNA vaccine used in this study appears promising (based on safety and significant immune response against tyrosinase which is a component of the melanomas) in the treatment of equine melanomas," Phillips concluded. "Our previous work published in American Journal of Veterinary Research documented that melanomas in horses express high levels of tyrosinase while normal tissue does not. By generating an immune response against tyrosinase we would thus expect to have clinical activity. This same approach and rationale has been used successfully in multiple species including dogs. Our initial results in horses with melanomas confirmed these findings and demonstrated tumor shrinkage, as was presented (previously) at AAEP. We are currently conducting field trials in horses with tumors."