Grey horse melanoma - Is it a true neoplasm? (abstract)

Authors: Sutton RH, Coleman GT
Publication: New Zealand Veterinary Journal, Volume 41, Issue 1, pp 43-44, Mar 1993
Publisher: Taylor and Francis

Animal type: Horse, Livestock
Subject Terms: Integument/skin/wool/hair/fur/feather, Neoplasia, Oncology, Pathology
Article class: Abstract

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Abstract: Grey horse melanoma or equine melanotic disease occurs concurrently with the greying of coat colour. The tumours are frequent in greying horses more than 6 years of age and it has been stated that 90% of grey horses older than 20 years are affected.
The earliest, most consistent change seen is the accumulation of melanin-containing cells in the deep dermis on the ventral surface of the tail. Dermal nodules may subsequently develop in this area and also in the perineum, in the commissures of the lips and, less commonly, the udder or penile sheath. It is not known if internal tumours develop concurrently or if they are true metastases. Tumour formation may extend from the perineal area to the pelvic canal nodes and they may be found in a wide range of tissues but most commonly in the parotid gland area and in the facial planes of skeletal muscle. The epidermis is not involved, junctional activity is not seen and ulceration is rare and due to trauma. The course is long and the horse suffers little discomfort unless the size and/or location of the tumours interferes with normal function.
Histologically the tumour cells appear to be a mixture of spindle cells and large polyhedral cells with peripheral nuclei crammed with melanin granules, which represent melanocytes and melanophages. Preliminary differential immunostaining, using antigen markers such as S100 and CD68, tend to support this.
Possibilities for this condition include a true neoplastic disorder with malignant features, a pigmentary storage disease with focal accumulations of melanophages or the presence of ectopic melanocytic nests stimulated to produce excess melanin. It is distinct from the typical malignant melanoma of humans, dogs and goats which appears rarely in the horse with no apparent association with coat colour. It has been frequently compared to the human blue naevus which is a benign melanocytic dermal tumour. In each condition, inert appearing deposits may be found in regional nodes. Rarely malignant changes may supervene. Both human blue naevus and equine melanocytic disease may represent disorders of the dermal melanocyte.

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