Dermatophyte

A dermatophyte is a parasitic fungus (mycosis) that infects the skin. The term embraces the imperfect fungi of the genera Epidermophyton, Microsporum and Trichophyton.

Dermatophytes (name based on the Greek for 'skin plants') are a common label for a group of three types of fungus that commonly causes skin disease in animals and humans. These anamorphic (asexual or imperfect) genera are: Microsporum, Epidermophyton and Trichophyton. There are about 40 species in these three genera. Species capable of reproducing sexually belong in the teleomorphic genus, Arthroderma, of the Ascomycota. (See Teleomorph, anamorph and holomorph for more information on this type of fungal life cycle).

Dermatophytes cause infections of the skin, hair and nails due to their ability to obtain nutrients from keratinized material. The organisms colonize the keratin tissues and inflammation is caused by host response to metabolic by-products. They are usually restricted to the nonliving cornified layer of the epidermis because of their inability to penetrate viable tissue of an immunocompetent host. Invasion does elicit a host response ranging from mild to severe. Acid proteinases, elastase, keratinases, and other proteinases reportedly act as virulence factors. The development of cell-mediated immunity correlated with delayed hypersensitivity and an inflammatory response is associated with clinical cure, whereas the lack of or a defective cell-mediated immunity predisposes the host to chronic or recurrent dermatophyte infection.

Some of these infections are known as ringworm or tinea. Toe- and fingernail infection are referred to as onychomycosis. Dermatophytes usually do not invade living tissues, but colonize the outer layer of the skin. Occasionally the organisms do invade subcutaneous tissues, resulting in kerion development.

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Identification

Microscopic morphology of the micro and macroconidia is the most reliable identification character, but a good slide preparation is needed, and also needed is the stimulation of sporulation in some strains. Culture characteristics such as surface texture, topography and pigmentation are variable so they are the least reliable criteria for identification. Clinical information such as the appearance of the lesion, site, geographic location, travel history, animal contacts and race is also important, especially in identifying rare non-sporulating species like Trichophyton concentricum, Microsporum audouinii and Trichophyton schoenleinii.

Transmission

Dermatophytes are transmitted by direct contact with infected host (human or animal) or by direct or indirect contact with infected exfoliated skin or hair in clothing, combs, hair brushes, theatre seats, caps, furniture, bed linens, towels, hotel rugs, and locker room floors. Depending on the species the organism may be viable in the environment for up to 15 months. There is an increased susceptibility to infection when there is a preexisting injury to the skin such as scars, burns, excessive temperature and humidity. Adaptation to growth o­n humans by most geophilic species resulted in diminished loss of sporulation, sexuality, and other soil-associated characteristics.

Classification

Dermatophytes are classified as anthropophilic (humans), zoophilic (animals) or geophilic (soil) according to their normal habitat.

• Anthropophilic dermatophytes are restricted to human hosts and produce a mild, chronic inflammation.
• Zoophilic organisms are found primarily in animals and cause marked inflammatory reactions in humans who have contact with infected cats, dogs, cattle, horses, birds, or other animals. This is followed by a rapid termination of the infection.
• Geophilic species are usually recovered from the soil but occasionally infect humans and animals. They cause a marked inflammatory reaction, which limits the spread of the infection and may lead to a spontaneous cure but may also leave scars.

Classification

• About 58% of the dermatophyte species isolated are Trichophyton rubrum
• 27% are T. mentagrophytes
• 7% are T. verrucosum
• 3% are T. tonsurans
• Infrequently isolated (less than 1%) are Epidermophyton floccosum, Microsporum audouinii, M. canis, M. equinum, M. nanum, M. versicolor, Trichophyton equinum, T. kanei, T. raubitschekii, and T. violaceum.

Medications

• Lamisil
• Tea tree oil

See also

• National Center for Mycology