Also watch for signs of photosensitivity, Is Your Horse Getting Too Much Sun?
I am dealing with this, with my grade Paint. Something in the pasture last year set it off, he had extremely strange hair in the white patches, like the lines in the sand when waves wash up and recede, and the most frightening thing was his muzzle turned orange. And I mean orange too.
He has been grazing in same pasture for 3 years at that point, but something was different last year. Now he cannot go out in sunlight, and must be kept off of pasture until grass has died mostly, and sun is at lower angle in the sky.