Dermatophytosis (ringworm) in horses is a superficial fungal skin disease. It presents primarily, but not exclusively, as a crusting and scaling disease. It is diagnosed via direct microscopic examination, fungal culture, and, in some cases, skin biopsy. The most cost-effective treatment is topical therapy.
Trichophyton equinum and T mentagrophytes are the primary causes of dermatophytosis in horses, although Nannizzia gypsea (formerly Microsporum gypseum), M canis, and T verrucosum have also been isolated.
Transmission is by direct contact or by traumatic inoculation of the pathogen via contaminated grooming implements and tack. Most lesions are observed in the saddle and girth areas (“girth itch”).
Clinical signs often start as a papular eruption, followed by crusts, alopecia, and erythema. The amount of inflammation depends upon the particular pathogen.
Diagnosis can be rapidly confirmed by direct microscopic examination of hairs and scales or by fungal culture. Lesions should not be wiped with alcohol because doing so can lead to a negative culture result. 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.
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, or 2% miconazole/2% chlorhexidine shampoo. Continue topical therapy until mycological cure (eg, a negative result of fungal culture or PCR assay). Bleach should not be used, as it can be irritating and is a human health hazard.
Adjuvant topical treatment on nonrinse 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 every 24 hours.
Infected animals should be isolated. Tack should be thoroughly cleaned by first removing gross organic debris and then washing thoroughly with 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.
For More Information
Young A. Ringworm in horses (dermatophytosis). 2022.
Durham A. Diagnosing and treating ringworm in horses. 2020;42(4):233-237.
Also see pet owner content regarding ringworm in horses.
