In the past, melanoma has been used to describe both the benign and malignant neoplasms. In general, melanocytoma is used to describe the benign neoplasm arising from melanocytes and melanoma is used synonymously with malignant melanoma.
Melanocytes are neuroectodermal cells and proliferations may arise from the epidermis, the dermis or the adnexa.
The link between UV-light radiation and the development of melanocytic tumours, which is seen in humans, has not been seen in domestic animals.
These tumours are most common in dogs, gray horses and miniature pigs. They are uncommon in goats and cattle and rare in cats and sheep.
Melanomas in Dogs
Melanocytomas are more frequent than malignant melanomas.
Melanocytomas are frequently found on the head and forelimbs of middle-aged and older dogs. There may be a predilection for males.
They appear as macules or patches, papules or plaques, or as elevated, occasionally pedunculated masses. Most have a pigmented surface.
Malignant melanomas are found in older dogs. The mucocutaneous junction of the lips, the oral cavity and the nail bed are the most common sites affected. They appear as raised, ulcerated nodules that are variably pigmented.
When present on the lip, the tumour may be pedunculated. When present in the nail bed, they appear as swelling of the digit, often with loss of the nail and destruction of bone.
Cytology of the mass will help identify cells containing melanin granules or blue-green material consistent with melanin. It may be difficult to identify whether the cells are of epithelial or mesenchymal origin, and depending on the grade, the cells may appear poorly-differentiated.
Digit lesions may require radiography and a deep punch biopsy for diagnosis.
Staging of the tumour should be performed, and the regional lymph node sampled, as well as further abdominal and thoracic imaging to determine any metastatic spread, which is very common with malignant melanomas.
Treatment of melanocytomas involves complete excision. The tumours are benign and this is curative.
Malignant melanomas are aggressive and have considerable metastatic potential. Complete excision is generally indicated, however this may be difficult if the tumour is very infiltrated into the tissue. Digit tumours require amputation and oral tumours may be treated by hemimandibulectomy.
Radiotherapy and chemotherapy have not been shown to be effective.
A vaccine has been developed which encourages the development of antigen and a T-cell response and has shown potential in treating the disease in some cases.
Lesions at the rostral end of the oral cavity show the best survival times.
Prognosis is generally poor, with an overall survival time of less than a year.
Melanomas in Horses
They are mostly found in gray horses, especially in Lipizzaners, Arabs and Percherons.
They begin their development when horses are 3-4 years old, but are usually recognised in older horses.
Most are benign but some invasive variants with metastatic potential exist.
Predilection sites include: the perineum and the base of the tail, but they can also occur in other areas including the parotid gland.
The tumours are often multiple and appear as coalescent, pedunculated nodules. They increase in size and number over time.
Most tumours are black on cross-section.
Cytology is helpful in identifying the tumour cells.
Regional lymph node involvement should be checked for, but there is a debate as to whether involvement represents metastasis or whether the intranodal melanocytes and melanophages represent a stimulation of extracutaneous melanocytes that are normally present in the lymph node.
This consists of surgical or cryosurgical removal. Animals are predisposed to developing additional tumours over time.
Intra-lesional chemotherapy with cisplatin or carboplatin has been used after surgical debulking if the mass cannot be entirely removed.
Melanomas in Pigs
Melanocytic neoplasms of pigs are seen as congenital lesions and sporadically in adults of the Sinclair (Hormel) miniature pigs and Duroc and Duroc crosses.
These tumours can be found anywhere on the body. Generally multiple, they can appear as pigmented macules or patches with smooth borders, ulcerated pigmented lesions, or as deeper, slightly raised, blue masses. Deeply invasive melanomas are often associated with metastatic disease.
The lymph nodes and lungs are the most common sites of metastasis.
Treatment and Prevention
Not all of these tumors become invasive, and many undergo spontaneous regression associated with an intense lymphocytic infiltrate.
Melanocytic lesions in pigs are not treated. Prevention by selective breeding is recommended if lesions are frequently recognised in a herd.
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Merck and co (2008) Merck veterinary manual Merial
Meuten, D. (2002) Tumors in domestic animals Wiley-Blackwell
Withrow, S. (2007) Withrow and MacEwen's small animal clinical oncology Elsevier Health Sciences
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