Equine melanomas are a common occurrence for older grey horses and ponies . “Whiskey”, a 25 year old grey pony had numerous dermal melanomas which had been present for a number of years. Dermal (or skin) melanoma’s, present as black masses and commonly occur under the tail, around the anus, in the scrotum and on the face. Dermal melanomas in older grey horses are unlikely to be malignant, however melanomas in non grey horses are much more likely to be malignant and should be thoroughly investigated. Malignant melanomas can spread to other parts of the body by metastasis.
Treatment Options for Melanomas
Treatment options for melanomas have varied success. Localised single benign masses present on the body and can be surgically removed with complete success being likely. Larger, coalescing masses can be treated with oral anti-histamines though it is not always effective and only reduces the size of the melanomas present. Also treatment is very labour intensive, requiring treatment orally every eight hours for 4-12 months. Larger lesions can be injected with cisplatin but once again, it will only reduce them in size. Vaccination against melanomas can reduce tumour size but is very expensive and requires six monthly boosters.
All dermal melanomas have the potential to spread locally, involving large amounts of tissue and affecting important structures, so surgical removal should be considered if this is likely. Unfortunately treatment once metastatic melanomas have developed does not affect this cancerous growth.
Coming back to Whiskey, he had a few melonomas under his tail, one on his neck and one on his face. None of these masses, where interfering with his tack, so no treatment had been undertaken. He had however started to develop swelling of his sheath and a foul smelling discharge was present. On investigation he had a few melonomas present up high in his sheath and an ulcer like lesion just inside his sheath. He was started on antibiotics for the infection and took a smear of the ulcerated lesion. Unfortunately the smear was undiagnostic, so it was elected to surgically remove the lesion with wide margins. The entire lesion was sent to the lab and was diagnosed as a squamous cell carcinoma. Unfortunately squamous cell carcinomas, spread locally and can metastasis to the lymph nodes. Luckily, the tumour had been completely excised and the margins of the incision where clean (i.e. no tumour cells were detected.) Ongoing for Whiskey his melanomas in the sheath are regularly monitored, as these could grow and cause problems with urination.
Katherine Hansen BVSc