Horses can get two skin diseases related to ultraviolet light exposure. One is sunburn and the other, called photosensitization, looks like sunburn but is more serious. Sunburn can occur in horses and is usually seen on the muzzle and around the eyes in horses with white coats and thin hair. Photosensitization is related to eating a plant that causes photosensitization, from eating plants that affect the liver, or from other causes of liver dysfunction. Sunburn can be treated by applying zinc oxide to the affected areas or by using a fly mask that will shade some of the area.
However, if you have a horse that has peeling skin on the white areas of the coat in places other than the head, it is likely the horse has photosensitization. Photosensitization occurs when the horse eats a plant containing a photodynamic agent or the horse already has liver damage. If a horse eats a plant containing the photodynamic agent, the agent enters the blood stream and reacts to ultraviolet light from the sun as the blood courses through superficial blood vessels on horses with white coats. The second type is when the horse eats a plant that causes liver damage or the horse has liver damage from another source. Plants contain chlorophyll and a metabolite of chlorophyll is a compound called phylloerythrin that builds up with liver disease and reacts with ultraviolet light on white skin. So if you have a horse with a white coat and the skin is inflamed, it could just be sunburn but could also be more involved; it could also indicate your horse may have a liver problem.