Bathe the vent in a warm Epsom salt soak for 15 minutes daily. Trim soiled feathers. Apply an antifungal cream (miconazole). Probiotics and apple cider vinegar in water help rebalance the gut. A vet may prescribe nystatin in stubborn cases.
Prevention
Keep the coop and run dry. Avoid high sugar treats and moldy feed. Add probiotics after antibiotic courses. Trim vent feathers on fluffy breeds.