THE MERCK VETERINARY MANUAL
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Overview of Dermatophytosis

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Dermatophytosis is an infection of keratinized tissue (skin, hair, and claws) by one of the three genera of fungi collectively called dermatophytes—Epidermophyton, Microsporum, and Trichophyton. (Also see Fungal Infections.) These pathogenic fungi are found worldwide, and all domestic animals are susceptible. In developed countries, the greatest economic and human health consequences come from dermatophytosis of domestic cats and cattle. A few dermatophyte species are soil inhabitants (geophilic), eg, M gypseum and T terrestre, and cause disease in animals that are exposed while digging or rooting. Other species are host-adapted to people (anthropophilic), eg, M audouinii and T rubrum, and infect other animals rarely. The most important animal pathogens worldwide are M canis, M gypseum, T mentagrophytes, T equinum, T verrucosum, andM nanum. These species are zoonotic, especially M canis infections of domestic cats and T verrucosum of cattle and lambs. The zoophilic species are transmitted primarily by contact with infected individuals and contaminated fomites such as furniture, grooming tools, or tack. Exposure to a dermatophyte does not always result in infection. The likelihood of infection depends on several factors, including the fungal species, host age, immunocompetence, condition of exposed skin surfaces, host grooming behavior, and nutritional status. Infection elicits specific immunity, both humoral and cellular, that confers incomplete and short-lived resistance to subsequent infection or disease. New information concerning dermatophytic virulence factors, notably secreted proteases involved in the invasion of keratin, aspects of host immune response against dermatophytes, and new molecular tools available for studying dermatophytes should hasten development of safe and effective vaccines against dermatophytosis in species without vaccination options.

Under most circumstances, dermatophytes grow only in keratinized tissue, and advancing infection stops when reaching living cells or inflamed tissue. Infection begins in a growing hair or in the stratum corneum, where threadlike hyphae develop from the infective arthrospores or fungal hyphal elements. Hyphae can penetrate the hair shaft and weaken it, which, together with follicular inflammation, leads to a common clinical sign of patchy hair loss. As the infection matures, clusters of arthrospores develop on the outer surface of infected hair shafts. Broken hairs infected with spores are important sources for spread of the disease. Under experimental conditions, the housefly, Musca domestica, can transmit M canis mechanically with its outer body surface for as long as 5 days. The clinical relevance is yet to be determined. As inflammation and host immunity develop, further spread of infection is inhibited, although this process may take several weeks. Thus, for most healthy adult hosts, dermatophyte infections are self-limiting. In young or debilitated animals and, to some extent, in Yorkshire Terriers and longhaired breeds of domestic cats, infection may be persistent and widespread.

Dermatophytosis is diagnosed by fungal culture, examination with a Wood's lamp, and direct microscopic examination of hair or skin scale. Fungal culture is the most accurate means of diagnosis. Dermatophyte test medium (DTM) may be used in a clinical setting. Selected lesions should have the hair clipped to a length of ~0.3 cm. The area should be gently patted with an alcohol-moistened sponge and then patted dry to reduce contamination with saprophytic fungi. Hair stubble and skin scale are collected for placement on the growth agar. The container is then loosely capped to prevent drying. Incubation at room temperature is sufficient except when culturing for T verrucosum from food and fiber animals, in which case incubation at 37°C is necessary. T equinum requires nicotinic acid if subcultured from primary growth, and some T verrucosum isolates require thiamine or thiamine and inositol.

Dermatophyte growth is usually apparent within 3–7 days but may require up to 3 wk on any type of DTM. Dermatophytes growing on DTM cause the medium to change to red at the time of first visible colony formation. Dermatophyte fungi have white to buff-colored, fluffy to granular mycelia. Saprophytic contaminant colonies are white or pigmented and almost never produce an initial color change on DTM. Definitive diagnosis and species identification require removal of hyphae and macroconidia from the surface of the colony with acetate tape and microscopic examination with lactophenol cotton blue stain. Dysgonic strains do exist, but lack of growth of suspected ringworm is most likely due to lack of expertise in culture technique and fungal identification or to disease processes other than ringworm that cause clinical signs.

The Wood's lamp is useful as a screening tool for M canis infections in cats and dogs. Infected hairs fluoresce yellow-green; however, only ≤50% of M canis infections fluoresce, and other fungal species in animals do not. Therefore, negative Wood's lamp examinations are not meaningful. False-positive examinations may occur and are especially likely in oily, seborrheic skin conditions. Fluorescing hairs should always be cultured to confirm the diagnosis.

Direct microscopic examination of hairs or skin scrapings may enhance clinical suspicion by demonstrating characteristic hyphae or arthrospores in the specimen. The technique is more useful in diagnosing dermatophytosis in large animals than in small animals. Hairs (preferably white ones) and scrapings from the periphery of lesions are examined for fungal elements in a wet preparation of 20% potassium hydroxide that has been gently warmed or incubated in a humidity chamber overnight.

An ELISA for the serodiagnosis of canine dermatophytosis has been researched but is not commercially available. The sensitivity and specificity is high and similar to that of fungal culture with DTM, but positive results can be seen after elimination of the dermatophyte infection. Cross reactivity between the various dermatophytes would not allow for species identification, which is important for identification of the source of infection.

Last full review/revision May 2013 by Sandra R. Merchant, DVM, DACVD

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