Candace A. Sousa, DVM
Dermatophytosis (dermatomycosis, “ringworm”) is an infection of the dead, keratinized part of skin tissue (hair, nails and stratum corneum) with one of several fungi belonging to a group referred to as the dermatophytes. Dermatophytes usually don’t invade and can’t survive in living tissue or in areas of intense inflammation. Approximately 99% of feline dermatophytosis is caused by Microsporum canis. Cats can be asymptomatic carriers. Microsporum gypseum is found naturally in the soil. Trichophyton mentagrophytes is usually contracted by contact with rodents or contaminated soil. Factors that predispose to infection include age (both the very young and the very old), compromised immune status (including the use of corticosteroids), poor nutrition, debilitating disease, and stress.
Dermatophyte infections are commonly over diagnosed and infected animals may be presented with a variety of clinical conditions. The classical lesion is a ring with a circular patch of hair loss, scale and/or crust. Some animals will have lesions consisting of irregular alopecia, erythema, and/or pruritus. Lesions can be localized or occur as a generalized papular, crusting dermatitis (miliary dermatitis-like). Pigmentary disturbances of the hair or skin can also be a sign of dermatophytosis. Highly inflammatory lesions are called a kerion. A mycetoma (a nodule with fistulous tracts discharging purulent material with granules) can also develop, especially in cats. Onychomycosis can accompany infections of the extremities. And many animals are asymptomatic carriers of dermatophytes.
The diagnosis of dermatophytosis can be made in several different ways. A Wood's light examination of hairs is a good screening test but does not give a definitive diagnosis. The ultraviolet light detects fluorescent metabolites produced by some strains of M. canis in growing hair. The fluorescence is bright yellow-green. Only about 50% of M. canis strains fluoresce. False positive fluorescence may be caused by topical medications. Suspected fluorescing hairs should always be cultured to confirm the presence of a dermatophyte infection.
Some veterinarians choose to perform a direct microscopic examination of hair and scale after digestion with 10% KOH or chlorphenolac. Arthrospores are seen surrounding the hair shafts. This procedure takes time, training, and experience.
Culture and identification of dermatophytes is the method of choice for diagnosing an infection. The hairs to be sampled should be clipped to about to 0.5cm in length. They are then cleaned with a non-antiseptic soap or alcohol and air-dried. Hair shafts and bulbs (Wood's light positive if present) are plucked with a hemostat and scales are lightly scraped from the surface. If there are no visible lesions, a new toothbrush should be used to vigorously brush over all parts of the haircoat for 2 to 3 minutes to collect material. Samples are cultured on Dermatophyte Test Media (DTM® Sabouraud's dextrose agar with cyclohexamide, gentamicin, chlortetracycline, and a phenol red color indicator). The DTM cap or lid should be loosely capped and the culture left at room temperature. The culture should be examined daily for a minimum of 10 days. A red color change in the medium with the first sign of white colony growth should be noted. Dermatophytes first metabolize protein in the media releasing alkaline metabolites, which causes the color change. Once the colony is mature, lactophenol cotton blue or new methylene blue can be used for microscopic identification of the fungal colony. Occasionally, but not consistently, fungal elements will be identified with histopathology.
Many cases of dermatophytosis are self-limiting with spontaneous clearing within 1 to 3 months. The major aim of treatment is to shorten the course of the infection and to minimize dissemination of infectious material to other animals or humans. Topical therapy in some form is indicated in every case. The affected areas (whole body in some cases) should be clipped. In man, infective spores can be cultured as far as 10cm from the visible lesion therefore spot treatment is usually not recommended. Caution should be exercised in disposing of the contaminated hair. (Table 1)
Griseofulvin is the drug of choice if oral medication is used. It is a fungistatic, not fungicidal, antibiotic whose spectrum is limited to dermatophytes. Occasional resistant dermatophytes are isolated. It should be prescribed if the animal has multifocal lesions, is immunosuppressed, has long hair, or has onychomycosis. Dosage recommendations vary. Microsize (Fulvicin-U/F®) is dosed at 20 -120 mg/kg/day. Ultramicrosize (Gris-PEG®) should be used at 5-20 mg/kg/day. The medication is given q 24 hours or divided q 12 hours. Absorption is enhanced when given with a high fat meal. Treatment should be continued until the animal is culture negative (average 4 to 6 weeks; up to 6 to 12 months for onychomycosis). Griseofulvin is highly teratogenic and must not be used in pregnant animals. Some animals, especially cats, experience gastrointestinal disturbances. Myelosuppression is a severe and unpredictable side effect that is not dependent on the dose of medication, length of therapy, or breed of animal but is seen more often in cats who are FIV positive. Periodic CBC's are strongly recommended in animals that are treated for long periods of time. Hepatotoxicity, drug eruptions, and neurologic side-effects have also been reported.
Ketoconazole (Nizoral®) is a broad-spectrum fungistatic synthetic imidazole. Its use has not been well evaluated for the treatment of dermatophytosis. It should be used only in severe cases caused by griseofulvin resistant isolates or in patients that cannot tolerate griseofulvin. It is dosed at 10 mg/kg PO q 24 to 48 hours for cats or q 12 to 24 hours for dogs. Administering ketoconazole with food aids its absorption. It can be expensive and there are reports of side effects, especially anorexia in cats.
Itraconazole (Sporanox®) can be used for cases that have failed griseofulvin therapy or in animals that cannot tolerate griseofulvin. It possibly has better efficacy against dermatophytes than ketoconazole. It is very expensive and the recommended dosage is 5 to 10 mg/kg PO q 24 hours. Terbinafine (Lamisil®) has been recently reported to be effective against dermatophytes when used at 20 mg/kg q 24 hours. Fungal vaccines are widely used in Europe but they are highly controversial and have unproved merit.
A recent report has described the use of oral lufenuron to treat dermatophyte infections in both dogs and cats. Dogs can be treated once with 54 to 68 mg/kg PO. The recommended dosage for cats is 51-266 mg/kg (a minimum of 100 mg/kg for cats in a cattery). This medication should be administered with a meal to increase absorption.
Dermatophytosis affects the reputation, economic status and emotional state of catteries and their operators. If one cat, particularly a kitten, in the cattery is infected, virtually all cats in the cattery are carrying the fungus on their haircoats. A recommended protocol to eradicate a dermatophyte infection from a cattery is:
Preventative measures should be instituted for the uninfected cattery.
The greatest threat is from newly introduced cats and show cats. Any new
cats, cats returning from a show, or cats returning from breeding loan
should be cultured and kept isolated until the results are negative. The
cat should be shampooed and dipped after culturing prior to introducing
them into the cattery.
TOPICAL THERAPY FOR DERMATOPHYTOSIS