First thing I did was give the area a good scrub with hibiscrub, work it in well to the infected area plus 1-2 inches around the sores to get all the excess, rinse well and then dry with a towel or leave in a stable to dry. I kept that up for 2-3 days to ensure all the scurfy / scabby bits were up and kept him rugged from then on in. I need to keep mine rugged as he does suffer this but even with just a rain sheet on it has not come back. :)
 
Keep it dry as possible. Maybe consider clipping and rugging if it spreads. I took a pair of scissors to my mine which allowed the air to get to the area and heel it.
 
newrider.com