Dermatophytosis: Introduction
(Ringworm) |  |
| Dermatophytosis is an infection of keratinized tissue (skin, hair, and claws) by one of the 3 genera of fungi collectively called dermatophytes—
Epidermophyton
,
Microsporum
, and
Trichophyton
. (See also
fungal infections,
Fungal Infections: Introduction.) 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 humans (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
, and
M
nanum
. These species can be spread to people, 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. Contact with a dermatophyte does not always result in infection. Whether infection is established depends on the fungal species and on host factors, including age, immunocompetence, condition of exposed skin surfaces, on 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. |
| Under most circumstances, dermatophytes grow only in keratinized tissue, and advancing infection stops on 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 patchy hair loss. As the infection matures, clusters of arthrospores develop on
the outer surface of infected hair shafts. Broken hairs with associated spores are important sources for spread of the disease. 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 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 agar, which is then lightly covered 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. Dermatophyte growth is usually apparent within 3-7 days but may require up to 3 wk. 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. |
| The Wood’s lamp is useful in screening examinations for
M
canis
infections in cats and dogs. Infected hairs fluoresce yellow-green; however, only 80% 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 allow early diagnosis by demonstration of 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. |
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