Dermatophytosis (ringworm)

Dermatophytosis (ringworm) is a skin disease common in dogs and cats younger than 6 months. Three different fungal organisms can cause ringworm. Microsporum gypseum is usually acquired from the soil. Trichophyton mentagrophytes may be acquired from other animals, usually rodents, or from soil that is contaminated with rodent hair. Microsporum canis infections are acquired from infected dogs and cats. Many adult cats carry Microsporum canis and can infect other animals but have no symptoms themselves. People who handle animals with this organism can become infected.

The most common symptom of ringworm infection in people and animals is a circular patch of crusty skin with hair loss. Rarely the lesions may show up as nodules or abcess like swellings. It occurs most commonly in young stray kittens and will usually start on the head, feet and tail.

Dermatophytosis is diagnosed by removing hairs from the edge of a lesion and placing them on a DTM culture tube. If an animal is suspected of carrying ringworm but does not have any visible lesions then a "sterile" brush is used to collect hairs from all over the body which are placed on the culture tube. Once a fungal organism can be seen growing in the culture tube (3-14 days) a sample can be examined under the microscope and the species of fungus can be identified. Microsporum canis sometimes causes the infected hairs to fluoresce (glow) under a black light but since the other two organisms do not cause fluorescence this is not always a reliable test.

Spontaneous remission of dermatophytosis in dogs and cats often occurs within one to three months, whereas treatment is undertaken to reduce transmission to other animals, including humans, and to eradicate the infection. Topical treatment should be instituted in every case of dermatophytosis. The primary reason for this recommendation is to reduce ongoing environmental contamination and human exposure. Lesions should be clipped widely, or whole-body clipping should be done when lesions are numerous, especially in long-haired cats. It should be noted, however, that clipping may cause the lesions to become more inflamed and appear more numerous. Various topical preparations are available for use on local or widespread dermatophytosis. Currently, lime sulfur rinses, miconazole and ketoconazole shampoos, and enilconazole rinses are recommended as most likely to be effective. Caution should be exercised in using the topical imidazoles on kittens or very young puppies, as irritant dermatitis may develop.

Systemic therapy needs to be used only in selected cases. Oral griseofulvin or itraconazole or ketoconazole is the preferred treatment for Microsporum canis infections. Griseofulvin is teratogenic (causes birth defects), however, when given in the first two thirds of pregnancy. Griseofulvin can be used safely in young puppies and kittens once they are eating on their own, but the dose should be calculated carefully according to weight, and the lower end of the dose range given. Puppies should be treated for at least four to eight weeks, at which time treatment may be stopped if all clinical signs have resolved. Treatment of kittens should be monitored with culture, using a sterile brush to sample the haircoat. It may be very difficult to clear the infection from a long-haired cat. The haircoat should be kept clipped short until dermatophyte test medium cultures are negative.

Elimination of dermatophytosis from multiple cat households is of concern to breeders of long-haired cats. An ideal treatment plan for elimination of dermatophytes from a multiple-cat household is labor intensive and expensive. One approach is to isolate kittens immediately after weaning and treat them as outlined previously until their cultures are negative. Another approach is to begin griseofulvin treatment of pregnant cats that repeatedly produce infected kittens. Treatment is started during the last week of pregnancy and continues until weaning. Thereafter, the kittens should be isolated and treated individually until cultures are negative. Once again, the treatment of young kittens should be based on a very careful weight/dose assessment.