We tried everything under the sun - Listerine, MTG, Head and Shoulders, Kopper Care -- turns out it was a vitamin deficiency -- they needed Vitamin A both horses cleared up within 2 weeks of giving it to them. You can get this at your local Co-Op very inexpensive it's an injectable sold for Cattle (do not inject in horses) you give it to them on their grain or pellets 5 cc per week until it is cleared up. Please talk to Cherie on here she's the one that gave us the advice and it worked so well :)
Cheapest and BEST way to fix it is to give him a big boost in Vitamin A.
Here is an old post.
Rain Rot is totally a result of a horse running out of stored Vitamin A. That is why they always get it in the winter and early spring before there is good green grass and hay is getting too old to have any Vitamin A left in it.
If a horse has sufficient Vitamin A coming in or stored in their livers, you cannot give them rain rot or lice.
I would think that your horse may have lice instead of rain rot -- but the cure is the same. Rain rot is most frequently seen on the top of a horse and on the areas that rain hits -- which is how it got its name. It also usually has little scabs attached to clumps of hair that come off leaving sores, sometimes bloody, underneath them. It is more often caused by bacteria than fungus and it IS NOT contagious. You just cannot give it to a horse with healthy skin and a good immune system.
Using topicals only temporarily stops it as it comes right back if the horse's diet has not improved to get it more Vitamin A or Beta Carotene that turns into Vitamin A. It is a malfunction of the horse's immune system.
We quit treating it over 40 years ago when I was informed by a very good nutritionist that it was a Vitamin A thing. He was right. Add ing Vitamin A has cleared up 100% of the cases I have had come in ever since.
You can feed a good supplement with a high level of Vitamin A (like Mare Plus made by Farman) or you can 'jump start' the process by giving a horse 5cc of an injectable Vitamin A sold for cattle. You just squirt it in the horse's mouth and DO NOT give it a shot. Do it once a week and you will see rain rot, lice, goopy or crusty eyes and several other things tied in to adequate Vitamin A levels completely disappear.
Several people have come on this forum and told how it worked for them with no messy or expensive topical treatments. We have not treated one in more than 40 years and 100% of them have cleared right up.
I started feeding a supplement with high levels of vitamin A (adm grostrong) last year on Cherie's advice and I have thrown out all the listerine I was using to treat it. I have had no skin issues since. I don't feed my guys any feed only pasture, I just leave a bucket of the supplement out loose for them to get as much as they need. I like it because it has all the vitamins and minerals they need, so all my lazy butt has to do is keep the bucket filled.
A pretty simple way to get rid of it is clean it and take the scabs off, you can use an iodine (sp?) solution with water and scrub it off then just stay on top of it. It's not good to put too much stuff of it because it is caused by moisture. You want to dry it out best as possible and just stay on top of it. My horse got it pretty bad this year. Most horses did since there was no winter really. Cleaning it everyday helped and I tried a little derma gel on the open parts from where the scabs came off and that helped a little. I also got a paiste from the vet that is usually used on cattle and it worked okay, but what worked best for my horses and my horses in the past is clean the area's at least once a day. When your horse comes in from t/o hose the legs down really good to get the excess mud off and to soften up the scabs this will also help keep the other area's of the leg clean.
Hose the legs, clean and scrub once a day or more if you can get out to the barn more, and make sure to dry the areas very well after it is done. And stay on top of it!