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”).
Trichophyton is a genus of fungi, which includes the parasitic varieties that cause tinea, including athlete’s foot, ringworm, jock itch, and similar infections of the nail, beard, skin and scalp.