Dermatophytosis in Dogs and Cats
[Ringworm]
By
Karen A. Moriello, DVM, DACVD, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison
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Dermatophytosis is superficial fungal infection of the skin and hair and less commonly of the claw or hoof. The most important pathogens of veterinary importance are:
Microsporum canis [affects cats, dogs, and to a lesser extent large animals]
Trichophyton mentagrophytes, T verrucosum, andT erinacei [affects hedgehogs]
M gypseum [soil organism that causes inflammatory lesions]
The genera Microsporum and Trichophyton are being reclassified into the genus Arthroderma.
Dermatophytosis is a self-curing disease and will resolve without treatment in otherwise healthy animals. It is considered zoonotic, as it causes skin lesions in people that are easily treated. Transmission is by direct contract with an infected animal, but mere exposure does not always result in disease. Transmission from the environment is inefficient if it involves spores alone; microtrauma is needed [eg, clipping of hair coat with contaminated clippers, tack]. The prevalence is difficult to determine because it is not a reportable disease in pets, and many studies reporting prevalence do not distinguish between fomite carriage and true infection. Overall, it is an uncommon disease, with reported prevalence of true disease 20 hairs and crusts].
Treatment
Dermatophytosis is a self-curing disease in most animals
In animals that are treated, a systemic antifungal drug will eliminate active infection in hair follicles
Animals can be treated to shorten the course of the disease and minimize contagion to other susceptible animals or people. Infected small animals should be kept isolated from other pets until there is clear evidence of clinical cure. Young animals should not be overly confined or undersocialized, or lifelong behavioral problems may occur.
Cats can be treated with itraconazole [5 mg/kg, PO, once daily on a week on/week off schedule]. Most infections are resolved after 3 or 4 cycles. An evidence-based review found that itraconazole is well tolerated in cats and was not associated with liver toxicity or vasculitis. Compounded itraconazole should not be used however. Poor bioavailability has been documented for such formulations, so a commercial veterinary liquid formulation should be used.
Small dogs can be treated with oral itraconazole [5 mg/kg, once daily]; pulse therapy is likely to be effective, but this has not been documented. Other dogs can be treated with ketoconazole [5 mg/kg, once daily] or terbinafine [3040 mg/kg, once daily].
Other points to keep in mind regarding drug therapy for dermatophytosis include:
Ketoconazole should not be used in cats because it causes anorexia.
Fluconazole should not be used because this is the least effective drug for dermatophytes.
Griseofulvin is no longer recommended because itraconazole and terbinafine are superior drugs.
Lufenuron is ineffective.
In addition to systemic treatment, topical therapy is required because it disinfects the hair coat. This is important because infective spores are the source of contagion and transmission, and disinfection minimizes environmental contamination. A whole body rinse [lime sulfur 1:16 or enilconazole 1:100] should be used twice a week; these products have residual activity. Shampoo containing 2% chlorhexidine and 2% miconazole is effective and may be the only option in countries where lime sulfur or enilconazole is not available; shampoo therapy has no residual activity. Even if systemic therapy is stopped, topical therapy should be continued until a mycologic cure is demonstrated.
Adjunct focal topical therapy can be used for lesions in hard-to-treat locations such as the ears and face. A 1%2% vaginal miconazole cream can safely be used on the face. For the ears, otic products that contain clotrimazole or miconazole/chlorhexidine or ketoconazole/chlorhexidine combinations are available.
Environmental cleaning removes infective material from the environment. Spores do not multiply in the environment and do not invade the environment like mildew. Spores are a normal dormant life stage of dermatophytes and other microorganisms and are easily removed from the environment. Mechanical removal of organic material and hair followed by washing of the surface with a detergent until visibly clear is the most important step for environmental cleaning/disinfection. After cleaning, a disinfectant should be used. Any bathroom disinfectant labelled as effective against Trichophyton will kill any spores not removed by mechanical cleaning. Disinfectant use alone will not remove environmental contamination. Thorough cleaning once or twice a week is adequate. Between cleanings, remove organic material with wipes or other means. Soft surfaces such as bedding or towels can be disinfected via washing; bleach is not needed. Carpets can be disinfected via carpet shampooing or steam cleaning.
The end point of treatment includes both clinical and mycologic cure. Clinical cure is the resolution of all lesions and the lack of any new lesions. A Wood's lamp examination can be used to look for areas of residually infected hairs in animals with M canis infections. Once lesions have resolved and there is a clear clinical cure, a fungal culture or PCR can test for mycologic cure. One negative PCR test supports a mycologic cure. Unless the animal has systemic illness, one negative fungal culture also supports mycologic cure.
Key Points
Dermatophytosis is caused by several Microsporum and Trichophyton spp and can affect a wide range of species.
Infections in otherwise healthy animals are self-limiting and do not require treatment.
Animals can be treated to shorten the course of the disease and minimize contagion to other susceptible animals or people.
In animals that are treated, a systemic antifungal drug will eliminate active infection in hair follicles.
In addition to systemic treatment, topical therapy is required because it disinfects the hair coat.
For More Information
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