I have also had good luck with mouthwash. You want the clear, unscented and uncolored stuff if you can find it. Soak the affected areas daily or as often as you can. It helps to kill the bacteria and isn't harsh on the skin like soap so it helps keep new infections out as well. Leave it on don't wash it off. Monitor your horse carefully though as sometimes things like this can cause allergies and exacerbate the issue in certain cases. I have never had it happen but I have heard of allergic horses so just keep that in mind.