MTG is awesome stuff, it worked wonders on my horse I used to have. I have also tried the listerine trick, was suggested by my vet and worked well. If it is all over I would suggest giving a bath with an iodine based shampoo. We once had a horse who had a severe case of it, gave him an iodine bath and applied the MTG and was gone within a few days. You will need to do lots and lots of brushing, but be sure not to use that brush on any other horse. If you have a mild case and only need to focus on a few spots the listerine spray will work or just use the MTG. I learned that rain rot is caused by bacteria in the soil and when it rains and horses roll in it, it can enter the skin through pores. Similar to a staph infection people can get on their skin. I can't remember why horses with lighter pigment skin are more prone to it though. Hope my thoughts are helpful.