Trichophyton equinum and T mentagrophytes are the primary causes of ringworm in horses, although Microsporum gypseum, M canis, and T verrucosum have also been isolated. Clinical signs consist of one or more patches of alopecia and erythema, scaling, and crusting, which are present to varying degrees. Early lesions may resemble papular urticaria but progress with crusting and hair loss within a few days. Diagnosis is confirmed by culture. Differential diagnoses include dermatophilosis, pemphigus foliaceus, and bacterial folliculitis. Transmission is by direct contact or by grooming implements and tack. Most lesions are seen in the saddle and girth areas (“girth itch”).
Treatment is generally topical because systemic therapy is expensive and of unproven efficacy. Whole-body rinses as described above for cattle may be recommended, and individual lesions treated with clotrimazole or miconazole preparations. Grooming implements and tack should be disinfected, and affected horses should be isolated.
Last full review/revision July 2011 by Sandra R. Merchant, DVM, DACVD