Cutaneous Melanoma in the Grey Horse

Equine cutaneous melanomas are far more commonly observed in grey horses than horses of solid colors. The melanomas have different features, both macroscopically and microscopically, which can be predictive of prognosis and malignancy. The objective of this study of literature regarding cutaneous melanomas in grey horses is to describe the nature and etiology of these lesions. Not everyone doing research in the field of cutaneous melanomas believe that these are in fact true neoplasms. I have focused on why grey horses are more likely to be affected than horses of other colors. A classification of different tumors, as well as common sites for primary lesions and metastases is also addressed.

When working as a veterinarian, these things are important to have insight in so that correct diagnosis can be made. The diagnosis is essential for the expected prognosis of a tumor, and will therefore have a great impact on any decision (e.g. concerning treatment) an owner of a melanoma-bearing horse might have to make. Also, I believe that gaining more knowledge about neoplasms in one species will only help us to understand more about cancer in other species, such as humans.

MATERIALS AND METHODS

When searching for articles I used multiple on-line databases, but mostly PubMed (20110201), Google Scholar (20110201) and Web of Science (20110215). With search terms such as (melanom* OR neoplasm* OR pigmentation disorder*) AND (horse* or equine) AND (grey OR gray) the search results were reviewable. Many articles seemed very well grounded and interesting when reading the abstracts, but unfortunately I found that they were often written in German or other languages. I could therefore not include them in my literature study, since I only understand English and Swedish.

RESULTS

Description, definition and classification

The grey horse

A grey horse is born with a dark fur coat and dark skin. With each shedding, the fur becomes lighter and lighter until finally turning white. The skin however stays dark throughout the horse’s life. Dermal melanomas in grey horses are very common, with 70-80% of all grey horses affected by the age of 15 years (M’Faydean, 1933).

Cutaneous melanomas

Melanomas derive from the mature, pigment producing cells found in the skin and in other areas of the body. These cells are called melanocytes and originate from the neuroectoderm (Moulton, 2002). In a study of 53 horses with cutaneous melanoma four different tumor patterns were recognized (Valentine, 1995):