Zeke, I don't know if it would have anything to do with it, but the OP is from South America. I don't know what kind of training the vets down there get.
I also agree that it appears to be some kind of allergic reaction to something. Gabs, do you use any kind of shampoo/conditioner/fly spray/liniment when you bathe them or do you just use clean water? Have you checked their paddock for poisonous plants?
PS, does anyone know if horses even have reactions to plants like poison ivy? LOL, I don't know.