Ringworm is an infection of the skin or hair caused by a type of fungus. In horses, Trichophyton equinum and Trichophyton mentagrophytes are the primary causes of ringworm, although other fungi have also been found in ringworm infections. All of these spread easily in the environment, and some can be transmitted from horses to people.

These fungi live in the soil and cause disease in animals that are exposed while digging, rolling, and lying down. They can also spread by contact with infected individuals and contaminated objects such as stalls or grooming tools. Broken hairs with associated spores are important sources for spread of the disease. Contact does not always result in infection. Whether infection is established depends on the fungal species and on such factors as the age, health, condition of exposed skin surfaces, grooming behavior, and nutrition of the animal.

Under most circumstances, dermatophytes grow only in the dead cells of skin and hair, and infection stops on reaching living cells or inflamed tissue. As inflammation and host immunity develop, further spread of infection stops, but this process may take several weeks.

Infected animals will develop circular, bald, scaly patches with broken hairs. Common areas for ringworm to occur are the withers and saddle area, but the infection may spread to the neck, flanks, chest, or head. Occasionally 1 or more legs may also be involved.

Ringworm is diagnosed by fungal culture and direct microscopic examination of hair or skin scale. Fungal culture of hairs and scrapings from the affected areas is the most accurate method. Direct microscopic examination of hairs or skin scrapings may allow early diagnosis.

Ringworm infections usually clear up without treatment. Treatment with medicated shampoos can speed recovery in some cases. Such treatments are not always effective, however. Your veterinarian can provide you with information about any treatment that may be appropriate for your horse and advise you regarding precautions you should take to avoid ringworm infection in yourself and members of your family.

Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Carol S. Foil, DVM, MS, DACVD; John E. Lloyd, BS, PhD; Bertrand J. Losson, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Patricia D. White, DVM, MS, DACVD; Thomas R. Klei, PhD; David Stiller, MS, PhD; Stephen D. White, DVM, DACVD