This condition is caused by the anaerobic bacterial organism Dermatophilus congolensis . It is not a fungus. Don't clip a horse with wet skin. Prevent transmitting the infection through isolating the horse's tack, blankets, and grooming tools. Horses can develop summer sores, caused by stomach worms.
Sweet itch is an allergic reaction to bites of midges.
General itching, due to bites of stable fly, black fly, or mosquito, can cause a horse to rub and so lose patches of hair. In the spring, the horse might show signs of abnormal shedding. Manufacturers offer various topical solutions to combat skin problems and hair loss. You can choose among shampoos, sprays, lotions, and creams. You might have to treat fungal infections or the lesions of rain rot.