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As with humans, melanoma in a horse is a cancer of the skin that develops in the melanin cells. These melanomas usually appear as round, black nodules found near the base of the ears, around the eyes, around the neck, under the tail, and around the vulva or rectum.
The nodules are usually smooth and are not painful. All equine melanomas are malignancies; however studies show that about two-thirds of all melanomas in horses grow slowly at their original sites and may cause no problems. Diagnosis is done by taking a section of the mass and examining it under a microscope.
- Smooth, round, dark or black nodules on the skin around the eyes, ears, neck, vulva or rectum, and under the tail
- Dark, black granules when seen through a microscope
Causes of melanoma are not known, but it is likely that a number of mutations linked to coat color may be the basis. Gray horses are most susceptible, but horses of any color can develop melanoma cancer. These tumors can develop at several sites in the body at once and some will have a malignant pattern of growth very early in the course of the disease.
Some scientists believe that solar exposure may play a role in the development of equine melanoma, but not all sites where the tumors develop, such as under the tail and on the belly, are exposed to much direct sunlight. .
Because the exact cause of equine melanoma has not been established, the best prevention of devastating results is to contact a veterinarian as soon as the characteristic nodules are seen.
A biopsy of all tumors by a specialist familiar with melanoma should be done, since actively malignant melanomas are likely to kill the horse in a relatively short time if not properly diagnosed.
Although some equine surgeons argue that operating on melanomas can activate the cells and increase the chances of tumor growth or metastasizing, being proactive with biopsies and laser surgery, where necessary, to remove large masses that interfere with tack or make defecating difficult, for example, are in the best interests of the horse.
Since melanomas are thought to be the result of mutations, scientists are working on identifying the cause of these mutations with hopes of using gene therapy to prevent them from developing.
If, after a biopsy, it is determined that the lump is not causing any problems, one option is to leave it alone. On the other hand, because a melanoma can kill a horse in a relatively short time if it metastasizes, laser surgery to remove it is often the recommended treatment. The laser cuts and seals blood vessels and enables the site of the surgery to heal quickly.
Medical management of melanomas is a traditional approach. Tagamet, a drug commonly used to treat stomach ulcers in humans and horses, has a potent effect on some melanomas. Although it may not cure the cancer, it is known to drastically reduce the size of some melanomas throughout the body.
Good responses have also been obtained with a tissue-based vaccine made from the horse's tumor cells. Radiation and other cancer drugs and treatments are also recommended by veterinarians.
A number of scientists are working on gene therapy, with the hope that manipulation of genetic signals will help prevent cells from becoming cancerous.
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