Most healthy horses with strong immune systems can fight fungal infections on their own over a six-to-eight-week period. Cool, wet or unsanitary conditions promote fungal infections. Also, horses are more susceptible to infection if their immune systems are compromised by poor nutrition, harsh environmental conditions, cramped living quarters, and diseases. For these reasons, fungal infections occur most often in the winter. Other factors such as excessive sweat left on the skin can also allow fungal infections to occur.
Most fungal infections are not very strong and can be cleared up by removing the crusts to expose the fungus underneath and then treating the affected areas with antifungal agents such as Betadine, chlorhexadine, miconazole, or captan. Sunlight and the horse's own immune systems are also key to helping fight a fungal infection.
Chronic infections that can't be killed are always a red flag--they signify that there is some other internal problem that is lowering the horse's immune system.
Since fungi are very contagious, brushes, tack and blankets should be disinfected with a diluted bleach solution.
Note people with lower immune systems are much more susceptible to fungal infections and shouldn't be handling infected horses.
This article written here with permission from Horse Illustrated, May 2003.