Douglas J. DeBoer, DVM, DACVD
In most healthy animals, dermatophytosis is a self-curing disease and eventually will spontaneously resolve even without therapy. However, proper treatment accelerates cure and limits contagion. The best treatment protocol is a combination of three approaches: topical treatment, to kill infective material and prevent its dissemination into the environment; systemic treatment, to shorten the time of infection in the individual animal; and environmental treatment, to help prevent recurrence of infection or spread to other animals or people in the household.
"Spot-treatment" with topical drugs has limited effectiveness--whole-body dipping (rinsing) is the best method. The best topical rinses include twice-weekly lime sulfur solution or enilconazole (Imaverol®); combinations of chlorhexidine plus miconazole or ketoconazole are also helpful. For a single-animal household, clipping the haircoat it is not necessary; for a multiple-animal household or cattery, it is preferred--but primarily to facilitate topical treatment. Clipping removes many infected hairs that would otherwise fall off and contaminate the environment. Clip the hair short but very gently: very small (invisible) trauma from the clipper blade may help to spread the infection.
Griseofulvin is still occasionally used, but is difficult to obtain in many countries. It is highly teratogenic and must not be used in pregnant animals; this limits its use in a breeding colony. In cats, bone marrow suppression is a severe and unpredictable side effect. It is not dependent on dose, breed, or length of therapy. Blood counts (especially white blood cell) are recommended every month when using this drug in cats. Test the cat for FeLV and FIV infection before use, as this is a proven cofactor for these adverse reactions. Dosing should be continued until the infection is cured visually and, preferably, until the fungal culture is negative. This usually means 6 to 10 weeks of treatment. For cats, use the microsize (for example, Fulvicin U/F®) at 25 mg/kg/day; the ultramicrosize in PEG base (for example, Gris-PEG®), 5-10 mg/kg/day.
Ketoconazole is not a good alternative for cats, as this drug has a high prevalence (~25%) of hepatotoxicity in this species. Itraconazole (Sporanox®, Itrafungol®) is a triazole antifungal that is extremely beneficial for animal dermatophytosis. Because it persists in the stratum corneum, hair, and nails after dosing, the drug can be used effectively on an every-other-week schedule, thus saving cost. The dose is 5 mg/kg once daily on alternate weeks. Uncommonly, some cats may require 10 mg/kg once daily, every day, if they are not responding to treatment. Toxicity problems are very rare. Fluconazole is not well-studied for dermatophytosis; in humans, its major use is for yeast pathogens. Several recent in vitro studies have shown that MICs of fluconazole against dermatophytes are much higher than the MICs of itraconazole, suggesting that itraconazole is the superior drug. Terbinafine (Lamisil®), in initial studies, also seems to be effective in some situations, but currently appears to offer no advantages over itraconazole. In one report, cats with M. canis apparently resistant to azole drugs were successfully treated with terbinafine; the best dose appears to be 30-40 mg/kg/day. If you use this drug, monitor liver enzymes; it may elevate ALT in cats, though no clinical toxicity may be seen. The drug reaches especially high concentrations in the sebum and stratum corneum and may persist for several weeks in skin. Pulse-dosing regimens are used in humans, and are under development for animals.
Regardless of the specific drug chosen, systemic treatment should be continued at least until the cat is visibly free of lesions, though it is highly preferable to repeat a fungal culture at the end of treatment to verify mycologic cure. Typically, this entails 6-10 weeks of treatment. A very small number of cats, seemingly mostly long-haired breeds, have difficulty mounting the appropriate cell-mediated immune response to the fungus and take a very long treatment time to cure--sometimes 6 months or more.
Historical case reports at one time caused speculation that lufenuron may be of benefit in treatment of feline or canine dermatophytosis. It was theorized that this chitin synthesis inhibitor may inhibit growth of the fungal cell wall (which contains chitin). In three separate controlled studies at the University of Wisconsin, using feline experimental infection models, lufenuron at 40-100 mg/kg orally once monthly did not prevent establishment of ringworm infections in cats, and did not result in faster cure than placebo once the infections were established. In addition, there was no additive effect seen when lufenuron was used along with terbinafine. Thus, we do not recommend use of lufenuron for dermatophytosis in animals.
Environmental Disinfection and Other Measures of Prevention
Studies over the past several years have clearly demonstrated that many disinfectants sold and labelled as effective for killing dermatophytes in the household or veterinary clinic are, in fact, not apparently effective for this purpose. Initial and continuing studies that simulate the actual conditions of use demonstrated that, of many different disinfectants tested, only chlorine laundry bleach and enilconazole environmental spray are very effective. Chlorine bleach is typically supplied as a 5% solution, which should be diluted 1:100 for use. Disinfection of the environment has less importance in single-animal households, because at the time the patient is presented, exposure to all individuals in the household has already occurred. It becomes critical when dealing with an animal shelter or cattery, and must be thoroughly performed every 4-6 weeks along with topical and systemic treatment.
To date, treatment or prevention of dermatophytosis with a vaccine has been dramatically effective in cattle, somewhat effective in horses, and minimally effective in cats. Current products marketed for cats may be effective as ancillary treatment, but prophylactic/preventative effects of these vaccines has NOT been demonstrated in cats.