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Treatment Approaches for Equine Melanomas
Melanomas are tumors that tend to be found in the skin of grey horses. Typically they are dark brown or black firm raised areas. They are the third most common skin tumors in horses following Sarcoids and Squamous Cell Carcinomas. The most common locations include along the bottom of the tail, just around the rectum, and around the genitals. Less commonly they occur just behind the jaw, along the neck and ears. They may occur individually or group together in bunches that can give a cobblestone appearance. In grey horses they are most frequently slow growing and localized in nature but can become big enough that they interfere with functions such as defecation or cause irritation.
When they occur in non-grey there is greater tendency for the melanomas to be aggressive and become widespread shortly after diagnosis. In these cases they have a high risk of becoming metastatic.
At this time, among available treatments, there are three approaches that I believe are the most reasonable.
The first is surgical resection followed by injections of anti-cancer drugs such as cis-platin or carboplatin. This has proven to be a very effective treatment. One study followed cases for two years following treatment. 13 of 14 melanomas horses treated with surgical removal followed by implantation of cis-platin beads did not have the tumor re-occur. Some cases required multiple treatments. More than half did not require additional treatments. New tumors particularly in areas such the area around the rectum can develop even if the original tumors are treated and do not regrow.
A second approach is the treatment with a vaccine to induce your horse’s own immune system to attack the melanoma. The vaccine is actually licensed for use for melanomas in dogs, which can be very aggressive and quickly fatal. In horses, melanomas are much slower growing.
The vaccine contains a gene encoding human tyrosinase, which is an enzyme associated with skin pigmentation. Research showed that high tyrosinase expression is found in melanoma tumors. This suggested that a vaccine that induces a host to attack this enzyme may well reduce the tumor.
The tyrosinase produced from the human DNA in the vaccine is similar to equine tyrosinase and stimulates an immune response against melanoma cells producing tyrosinase. Using DNA from a different species produces tyrosinase that is considered foreign by a horse’s immune system results in a potent immune response.
Because the human tyrosinase is similar enough to a horses tyrosinase, the immune response will also target a horse’s melanoma cells.
In talking with one of the lead researchers on the use of this vaccine in horses, he related the following experience: Of about 70 horses treated 25% had a noticeable decrease in tumor size; 50% a modest decrease in tumor size and/or arrested tumor growth; 25% no response. These are not spectacular results, but they are significant. Ongoing research is being done to determine if a better dose or protocol can be created to increase the response. Because of the nature of this treatment an initial series of injections is given with follow-up boosters given every six months.
A third approach is quite novel and comes from research out of Wake Forest University that peaked in the interest of a researcher at the Virginia-Maryland veterinary school. It was found that oil of Frankincense had cell killing properties in the lab. When injected into equine melanomas it appears to have the same effect. It can be used in cases such as large masses of melanomas around the rectum. Tumors are injected on a weekly basis. The tumors react with inflammation and burst creating an open wound. Frequently, injections of oil of Frankincense are repeated weekly over two or more months until tumor size has been reduced sufficiently.
A final word about Cimetidine treatment. This is widely known treatment and is to the best of my knowledge, based on one publication. In 1990 a report was published on the use of Cimetidine, an anti-ulcer drug, for the treatment of equine melanomas in 3 horses. Beyond the obvious, that this was a report of treatment in just 3 horses, there have been no follow up larger studies that I am aware of to verify the effectiveness of this treatment.
What makes this treatment attractive is that Cimetidine is a a relatively inexpensive medication to use. However, in my discussions with researchers in the area of equine melanoma treatment, they cautioned that they had seen nothing in the way of response to support this treatment and would not favor it over the ones I outlined above.
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