I had 2 horses COVERED by rainrots. Looked horrible. Those were not my horses (I'd never let it go like that), but the owner just didn't care. What I did was pulling off those little bumps with fingers (as gentle as possible) and wipe the spot with alcohol right after. Took me 3 days one horse, 2 days other, but they didn't develop the sores after that and I could ride them when I was done with no problems.