What is Melanoma?
Melanomas are tumors that are caused by an abnormal growth of melanocytes and are the most common form of skin cancer in horses. Although humans get melanomas from ultraviolet light, this is not the case with horses. Melanomas are most common in gray horses. In fact, close to 80% of gray horses over 15 will develop a melanoma. When these cancers do spread, it is most commonly found in the lymph nodes, abdomen, blood, lungs, liver, and spleen.
Melanoma in horses is a type of abnormal cell growth (tumor) that are usually benign (not cancerous), but may be malignant (cancerous) rarely. Gray horses are affected more often than any other color horse. These melanomas are solid black growths that are most common on the base of the tail, lips, ear, jaw, sheath, and anus. Most often, these growths do not cause any pain and they grow slowly unless they are malignant. They rarely cause any problem although, in rare cases, the tumor can spread inward into a vital organ, which is fatal.
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Symptoms of Melanoma in Horses
The main symptom is a hard black nodule on any part of the body, but most commonly found on the:
- Anal area
- Base of the tail
- Lips and gums
- Benign dermal melanomas are not cancerous and may be any size and can be a singular nodule or a group of nodules. They are found most often in gray horses
- Malignant dermal melanomas are the same as benign dermal melanoma except there are more nodules and they are cancerous
- Melanocytic nevi are usually found in young horses between 4 and 5 years of age; the nodules are small and can be any color
- Anaplastic malignant melanomas are found in older horses and they are cancerous and fast moving
Causes of Melanoma in Horses
Certain types of horses are more susceptible such as:
- Gray horses
- Over 15 years old
Diagnosis of Melanoma in Horses
The veterinarian will first need to get your horse’s history, including medical and shot records. After that, a detailed physical examination will be done, which starts out as your veterinarian watching from a distance, watching for any lameness and studying your horse’s attitude, behavior, stature, and conformation. Moving on, the veterinarian will record your pet’s breathing with palpation, heart rate, blood pressure, weight, height, body temperature, and body condition score. The veterinarian will also have you walk and trot your horse in a circle while watching how the muscles and joints perform in motion. Following this, a thorough assessment of the skin from head to tail, looking for lesions and any other abnormalities. The veterinarian will use a fine needle aspiration to get a tissue sample from several of the lesions found and, in some cases, the veterinarian will go ahead and remove the entire lesion. Also, imaging is important in determining if the melanoma has spread (metastasized) so digital radiographs (x-rays) and ultrasound are important. If a more detailed view is necessary, an MRI and CT scans are used, and possibly a bone scan. The veterinarian will need to do a thorough examination of the lymph nodes and take samples with a fine needle aspiration if necessary. In addition, the lungs, abdominal wall, liver, and spleen should be examined.
Next, laboratory tests will be performed such as a CBC (complete blood count), BUN (blood urea nitrogen), chemistry panel, fungal and bacterial cultures, PCV (packed cell volume), and glucose level.
Treatment of Melanoma in Horses
Treatment varies depending on the size and type of the tumor and whether it is spreading or not. The smaller nodules can be removed, but this is not usually successful with the larger masses. There are other ways to treat melanomas such as:
For a small tumor, it will usually be left alone if it is not malignant. However, if the veterinarian thinks it will spread or become cancerous, surgical removal is preferred.
For malignant tumors, the chemotherapy drug, cisplatin, will be used intralesionally. This procedure is done by using a long needle to puncture the lesions and inject the drug into the tumors. It may be done alone or coupled with removing the melanoma.
Cimetidine is a histamine blocking drug that slows the tumor’s growth that sustains the immune system to help kill tumor cells. Although this drug has some success in slowing the tumor, it has not been successful in completely getting rid of the tumor.
If your horse has already had a melanoma or if you have a true gray horse, the veterinarian will suggest that you give your horse a melanoma vaccine. The vaccine is similar to the melanoma vaccine for dogs. This is still in the experimental stages, so the success rate is not well known yet.
Recovery of Melanoma in Horses
The prognosis for melanoma varies, depending on whether the tumor is malignant or benign. It also depends on if the tumor is able to be removed and if it has spread to any lymph nodes or vital organs. Many horses can live for several years with melanomas without having any problems, but it is best to have the melanomas removed when they are small because they are easier to remove.