I'd like to ask a question about my horse's skin problem–depigmentation. He is a 10-year-old chestnut Hanoverian gelding with dark eyes and eyelids. But now there is a pink spot on his left upper eyelid and I think it's getting larger. The skin hasn't changed, only the pigmentation. At first I thought it was an injury, but his keeper disagreed. I have read about lack of copper, but his blood copper level tested OK. He also is a little bit thin, but he used to be thinner. I tried to find information about the skin problem, but I found only the following statements: "It exists but nobody knows why," or "It can disappear or not." Could you tell me something more about this problem?

The name of the disorder is vitiligo. Like most disorders of pigmentation, vitiligo is only a cosmetic problem and not significant for the horse. Vitiligo is, in my opinion, an idiopathic (no apparent cause) loss of pigment especially in hairless or thin-haired areas. Others define vitiligo as depigmented spots and larger, poorly defined, areas of depigmentation appearing on the body being either idiopathic or resulting from primary damage to melanocytes (skin cells that produce pigment). However, in my opinion, depigmentation in areas with hair is better called leukoderma (partial or total loss of skin pigment) or leukotrichia (loss of pigment in hair).

Whatever definition you use, in the form described in the question there is no known cause. However, some breeds and individual families may be more affected, suggesting that there is a genetic basis to the condition. The skin is otherwise normal, but there is a gradual, often profound loss of pigment often around the muzzle, face, and eyes, and/or sometimes around the genitals. The depigmentation may wax and wane in intensity, but is usually permanent. There is no known treatment. Regrettably, there is no more information available.

I understand your worry that it seems to deteriorate over time. This is quite possible and happens often; it may end with white rings around the eyes. Lucky for your horse, it's just a depigmentation and doesn't do any harm