A friend of mine had a horse with photosensitivity and literally in a couple days time his legs swelled up and the sking started sloughing off. It was bad. She had to keep in indoors as much as possible during sunlight hours and keep sunscreen on his white parts. I think they now make a few sun protective clothing items for horses as well, but never heard how well they work. My friends case was pretty extreme, I sure hope your horse is less sensitive. Good luck and sorry I wasn't much more help, I just know that's what she had to do.