Melanomas in horses.

In human beings a striking loss of pigment may occur in the skin and hair surrounding normal nevi, the primary sites of melanomas, cutaneous nodules of metastatic melanoma, and over the surface in the form of vitiligo in those with melanomas. It is generally assumed that vitiliginous changes are associated with .he local destruction of normal melanocytes on either an autoimmune or cytotoxic basis because of the presence of a benign nevus or a melanoma. In a report by Millikan et al. (1) on pigs with melanocytic nevi that might be melanomas, changes of pigment loss identical to that observed in some human subjects was described. Gray horses ordinarily have a high incidence of melanomas. Dark brown at birth, they probably developed melanomas after turning white and not before. There may be a human counterpart to this phenomenon. Because knowledge of the relationship of changes in hair and skin color during the course of a melanoma is likely to be important in developing a method to control the growth of a melanoma, the animal models should be investigated in detail. The Arabian horse described in this report is one animal with a high predisposition to develop a melanoma. We examined horses at the 2800-acre Al-Marah Farm in Maryland, the world's largest Arabian horse farm. The horses are descendants of animals that have lived in Asia Minor in Arabia for over 3000 years. The first thing one notices on scanning the fields is that the mares are white and the colts and fillys are dark brown. It is clear that although a few of the Arabian horses remain chestnut in color most of the horses change color with age, being dark brown initially and gradually becoming gray and white. Part of the nose usually has no pigment at birth. The onset of puberty is about 14 months. At 2 years of age the animals begin to gray and by 9 years they are very white. The white patch on the nose remains unchanged. Loss of pigment about the eyes and anus begins after 2 years of age. Not all animals develop vitiligo patches. In those that do partial loss recurs and extends. The life expectancy of the Arabian horse is 12-15 years although some live beyond 20. By 5 or 6 years of age many of the horses have developed melanomas, with tumors beginning about the anus. The animals handle the neoplasm well and go on for many years without apparent difficulty.