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 often start as a papular eruption followed by crusts, alopecia, and erythema. The amount of inflammation is dependent upon the pathogen.
Diagnosis can be rapidly confirmed by direct examination of hairs and scales or by fungal culture. Lesions should not be wiped with alcohol, because this can cause a negative culture. Early lesions should be sampled and provided to a reference laboratory, along with scales, hairs, and crusts. Differential diagnoses include dermatophilosis, pemphigus foliaceus (rarely), and bacterial folliculitis.
Transmission is by direct contact or by traumatic inoculation of the pathogen via contaminated grooming implements and tack. Most lesions are seen in the saddle and girth areas (“girth itch”).
Systemic treatment with oral antifungal drugs is cost prohibitive. The treatment of choice is twice weekly, whole body, leave-on rinses with lime sulfur 1:16 or enilconazole 1:100. Bleach may be irritating and a human health hazard and so should not be used. Adjuvant topical treatment on non-rinse days can be used. Effective products include enilconazole in a spray form (stable for 7 days) or a spray formulation of 2% chlorhexidine/1% ketoconazole or 2% chlorhexidine/1%–2% miconazole. Lesions around the eyes can be treated with 1%–2% miconazole vaginal cream once daily. Infected animals should be isolated. Tack should be thoroughly cleaned by first removing gross organic debris and then washing thoroughly with a detergent soap. Disinfection can be accomplished with any over-the-counter bathroom cleaner labeled as antifungal for Trichophyton spp. Fabric leads or blankets should be washed twice in a washing machine.
Also see Pet Health content regarding ringworm in horses.