CHAPTER 150 Fungal Skin Diseases
Superficial fungal infections involve the superficial layers of the skin, hair, and keratinized structures. The most common superficial fungal infections affecting horses are dermatophytosis and dermatitis caused by Malassezia spp. infection.
Dermatophytosis (“ringworm”) in horses is most commonly caused by Trichophyton equinum. Two varieties of T. equinum have been reported, T. equinum var. equinum and T. equinum var. autotrophicum. Less common agents of infection in horses are Trichophyton verrucosum and Trichophyton mentagrophytes and Microsporum equinum, Microsporum canis, and Microsporum gypseum. Dermatophytosis is overdiagnosed in general practice and is frequently confused with other conditions that have a similar clinical appearance, such as superficial bacterial folliculitis. The incidence and prevalence of dermatophytosis vary with the climate, being more common in hot and humid climates. Dermatophytosis usually affects immunocompromised, very young, or elderly horses. Therefore, it is prudent to pursue identification of possible underlying immunocompromising conditions when dermatophytosis is diagnosed.
Infection occurs by contact with contaminated equipment, such as brushes, tack, or direct contact with affected horses. Occasionally asymptomatic carriers may be a source of infection. Chronic glucocorticoid administration increases the risk for development of dermatophytosis.
Clinical signs consist of papules, pustules, or circular areas of alopecia surrounded by scaling (epidermal collarettes). Lesions spread from the periphery and may clear centrally. This pattern leads to partial hair growth in the center of the alopecic areas, whereas the periphery of the lesions appears erythematous. As the disease progresses, scaling and seborrhea develop. The girth and halter areas are frequently affected because transmission occurs by contact with infected equipment. In young horses, the disease frequently becomes generalized. Secondary bacterial infections commonly develop and cause increasing pruritus. In chronic cases, crusting and scaling with lichenification and hyperpigmentation develop.
Differential diagnoses for a papular or pustular eruption with crusting and scaling should include staphylococcal folliculitis, dermatophilosis (“rain rot”), pemphigus foliaceous, contact allergy, insect allergies (Culicoides spp. hypersensitivity), parasitic diseases (Psoroptes spp., Chorioptes spp.), sarcoidosis, and sterile eosinophilic folliculitis. In all horses that have this type of clinical presentation, the minimum database should include skin scrapings, cytologic analysis, and fungal culture. Skin scrapings help investigate the presence of parasites, and cytologic analysis (e.g., tape cytology) is performed to determine whether there are bacteria or acantholytic cells, which are frequently seen with pemphigus foliaceous.
Cytologic findings in horses with pyoderma, such as that caused by staphylococcal folliculitis, include cocci and degenerated neutrophils. However, normal cytologic findings do not rule out bacterial folliculitis because bacteria may not be present on skin surface but instead reside inside the hair follicles. For this reason, it is frequently appropriate to prescribe topical therapy for bacterial folliculitis, regardless of the outcome of cytologic analysis. It is also important to keep in mind that severe dermatophytosis may lead to formation of acantholytic cells; thus, cytologic identification of these cells does not necessarily imply a diagnosis of pemphigus foliaceous. Such diagnosis must be confirmed histologically.
Dermatophytosis is diagnosed on the basis of clinical signs coupled with positive results of fungal culture. Woods lamp evaluation is not recommended as a diagnostic tool because only a few strains of M. canis elicit fluorescence. To complicate assessment even further, topical therapy may cause false-positive reactions. For fungal cultures, hair is collected at the periphery of the lesion and placed on a dermatophyte test medium (DTM) plate. This medium is essentially Sabouraud’s dextrose agar containing cycloheximide, gentamicin, and chlortetracycline, with phenol red added as a color indicator. To encourage growth of T. equinum var. equinum, it is important to add vitamin B complex to the medium. This can be done by adding a few drops of injectable vitamin B complex to the medium. To minimize growth of saprophytes, the area should be pretreated with alcohol. It is important to wait until the area is completely dry before plucking hairs for culture to avoid false-negative culture results. Growth of either saprophytic fungi or dermatophytes can change the color of DTM medium to red. Dermatophytes usually grow in 3 to 5 days and utilize protein first. The resulting alkaline metabolites turn the medium to a red color in several days, concomitantly with colony growth. By contrast, saprophytic fungi utilize carbohydrates first and protein later and thus the change in color of the medium develops only after prolonged incubation (10 to 14 days). Plates should be checked frequently for fungal growth and color change. Inappropriate and frequent diagnosis of dermatophytosis results if the plate is not assessed daily and diagnosis is made solely on the basis of color change of the medium.
Dermatophytosis can also be diagnosed histologically when arthrospores are seen in conjunction with folliculitis or furunculosis. Other findings may include epidermal pustules and, in rare instances, acantholytic cells. For this reason, fungal staining (either periodic acid Schiff’s or Grocott’s silver) should be requested in every case in which acantholytic cells are found to rule out the possibility of dermatophytosis.
In healthy horses, dermatophytosis is self-limiting, and infection should resolve spontaneously within 3 to 4 months. If the horse is immunocompromised or unable to mount effective immunity against the dermatophyte, treatment is needed. Dermatophytosis in horses is usually treated topically because of the expense of systemic therapy and the paucity of studies evaluating appropriate doses of systemic antifungal drugs in horses. Although the literature reports the use of orally administered griseofulvin, to the best of my knowledge, no evaluation of efficacy or appropriate dose has been performed. Oral fluconazole administration can be used in horses in which the condition is refractory to topical treatment. Pharmacokinetic studies in horses indicate that the recommended regimen is a loading dose of 14 mg/kg, followed by 5 mg/kg every 24 hours.
Topical treatment should be undertaken in all horses with confirmed dermatophytosis. Topical therapy frequently includes weekly dips with lime sulfur (4 to 6 oz/gallon of water). Lime sulfur is highly effective in killing dermatophytes; it is also antipruritic and removes scales and crusts. Limitations of lime sulfur therapy include the odor, temporary yellow discoloration of white haircoats, and drying of the coat if applied excessively or used on horses with dry skin. For this reason, use of conditioners may be necessary. I frequently recommend adding a few tablespoons of baby oil to the gallon of lime sulfur solution. If application of lime sulfur is not an option, shampoos may be used. These are less desirable than a dip because of the lack of residual activity. Miconazole (2%) shampoo in combination with chlorhexidine (2%) can be used twice weekly and has been reported to be effective. Other horses in contact should also receive at least one treatment topically to minimize the number of arthrospores that can reinfect the affected horse, regardless of whether the other horses have skin lesions. Treatment of all horses in contact is important for a successful outcome. Length of treatment of infected horses should be based on mycologic cure rather than clinical cure, as many horses look better before they are fully cured. It is important not to discontinue treatment too soon, and length of treatment should not be based on clinical improvement alone.