Hi, I'm from midwest too and had this last year from July through January, despite regular vet attention. Like you, only one of my five horses was affected. By October's wet season it had flared up and by December we were still dealing with it in huge patches in all 4 legs up to knees/hocks. In December she ended up with IV antibiotics. Good news is that she did heal, finally, and is fine today (but we had a close call this spring, which I mention below.)
I agree with the other posters - less is more. DO NOT even rub or scuff the skin - don't pick scabs off if they're stuck. I loosened them with mineral oil which helps them fall off but in retrospect I tried to pick her legs clean of all scabs (internet advice) and that was too aggressive. I started with a combo of antibacterial cream with antifungal cream with Desitin...and it has always worked before but in her case, there was something else going on, too.
In my mare's case, we got rid of the original infection or fungus in October or November, by wrapping with antibacterial and antifungal ointments only to have pseudomonas aeruginosa take hold in early December. We got ahead of it with IV Gentamycin.
Before we had the pseudomonas diagnosed through skin biopsies and cultures, and after we got rid of it, he recommended that we use Nolvasan ointment for 3-4 days a month (wrapped) for prevention. I did that this spring and she rubbed herself raw under the wraps...so we were dealing with scratches again under the bandages.
In a last-ditch effort to heal the rubs and stop the spread of the scratches, I did 3 things - left her in a stall with her legs open to the air (clipped legs, kept dry, no bandages to further irritate her skin); sprayed with Vetericyn gel, twice a day, liberally; and fed her Omega-3 supplements and free choice minerals to make sure that she got everything she needed to get healthy skin from the inside out. I did not touch the scabs until I could see they were falling off (hair growing underneath) and watched her legs like a hawk to make sure there weren't new spots (e.g., not spreading). If they had spread, I knew I would have needed to get my vet out again.
I don't know if I just got lucky or what, but the Vetericyn has been my new go-to for skin issues since this spring. We have not had any problems with scratches since I started using it and the horses have been out 24/7 on pasture. It has worked great - and even on this sensitive-skinned mare it has not irritated her. It's helped keep stuff from spreading, dried up oozy spots, and I've had the usual cuts and scrapes where it's been great as well.
My advice is that if the Desitin or other ointments don't do the trick in 3-5 days, switch over to Vetericyn and give it a shot. Watch for swelling, especially if it's accompanied by lameness. Use Desitin without aloe gel - the equine dermatologist that helped with my mare said that a lot of horses with skin issues are found to be allergic to aloe.
Good luck - I hope you find the right combination!