Stable in wet conditions. You can't really rug as the bacteria get into the rug weave and as soon as the horse warms up will simply re-infect the skin. Wash all brushes etc in an antibacterial. A full clip helps - at least you can keep the skin clean. Burdock, Clevers, calendula and nettles are thought to help.
Feeding linseed (oil or micronised) is supposed to help improve water repellency of their coat... For my mares recent rain scald I applied Manuka honey & it cleared up within a week!