Photosensitivity can be fatal as it will involve the liver, and not only certain plants, such as aslike clover or St. John's Wort can work with the sun, but medications and grooming supplies as well.
I had a Paint that I battled this with, he was, by the time I realized something was going on, was literally orange in his muzzle from nostrils down. His white hair, was very brittle and dry and looked like sand on a beach when waves have washed over it and receded.
Liver enzyme tests showed we had caught it before damage was done, but the yellowing was jaundice.
Had to keep him off pasture during sunlight hours and in dry lot, only out in dark.
Horses make me a better person.