Dermatophyte

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.^[1] These anamorphic (asexual or imperfect fungi) 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. Toenail and fingernail infections 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.

Types of Dermatophyte Infections

Athlete's foot or tinea pedis

Contrary to the name, it does not affect just athletes. Tinea pedis affects men more than women. Frequently affects the webs between the toes first, before spreading to the sole of the foot in a "moccasin" pattern.

Jock itch or tinea cruris

Frequently, the feet are also involved. The theory is that the feet get infected first from contact with the ground. The fungus spores are carried to the groin from scratching, from putting on underclothing or pants. Frequently extend from the groin to the perianal skin and gluteal cleft.

Ringworm of the body or tinea corpora

Facial ringworm or tinea faciei

Can be misdiagnosed for other conditions like psoriasis, discoid lupus, etc. Can be aggravated by treatment with topical steroid or immunosuppressive creams.^[2]

Blackdot ringworm or tinea capitis

Infected hair shafts are broken off just at the base, leaving a black dot just under the surface of the skin. Scraping these residual black dot will yield the best diagnostic scrapings for microscopic exam. Numerous green arthrospores will be seen under the microscope inside the stubbles of broken hair shafts at 400x. Tinea capitis can not be treated topically, and must be treated systemically with antifungals.^[3]

Scalp ringworm or tinea capitis

Ringworm of the hands or tinea manuum

In most cases of tinea manuum, only one hand is involved. Frequently both feet are involved concurrently, thus the saying "one hand, two feet".^[4]

Ringworm of the nail, Onychomycosis, or tinea unguium

See Onychomycosis

Diagnosis and Identification

Rapid in office testing can be done with scraping of the nail, skin, or scalp. Characteristic hyphae can be seen interspersed among the epithelial cells. Trichophyton tonsurans, the causative agent of tinea capitis (scalp infection) can be seen as solidly packed arthrospores within the broken hairshafts scraped from the plugged black dots of the scalp.

Fungal culture medium is used for positive identification of the species. Usually fungal growth is noted in 5 to 14 days. 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.

A special media called Dermatophyte Test Medium (DTM) has been formulated to grow and identify dermatophytes.^[5] Without having to look at the colony, the hyphae, or macroconidia - one can identify the dermatophyte by a simple color test. The specimen (scraping from skin, nail, or hair) is embedded in the DTM culture medium. It is incubated at room temperature for 10 to 14 days. If the fungus is a dermatophyte, the medium will turn bright red. If the fungus is not a dermatophyte, no color change will be noted. If kept beyond 14 days, false positive can result even with non-dermatophytes. Specimen from the DTM can be sent for species identification if desired.

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, shoes,^[6] socks,^[6] towels, hotel rugs, sauna, bathhouse, 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.

Frequency of species

• About 76% of the dermatophyte species isolated are Trichophyton rubrum
• 27% are Trichophyton mentagrophytes
• 7% are Trichophyton verrucosum
• 3% are Trichophyton tonsurans
• Infrequently isolated (less than 1%) are Epidermophyton floccosum, Microsporum audouinii, Microsporum canis, Microsporum equinum, Microsporum nanum, Microsporum versicolor, Trichophyton equinum, Trichophyton kanei, Trichophyton raubitschekii, and Trichophyton violaceum.

Medications

• Topical medications like clotrimazole, butenafine, miconazole, and terbinafine.
• Systemic medications (oral) like fluconazole, griseofulvin, terbinafine, and itraconizole.
• Tea tree oil

Treatment

Tinea corpora (body), tinea manus (hands), tinea cruris (groin), tinea pedis (foot) and tinea facie (face) can be treated topically.

Tinea unguum (nails) usually will require oral treatment with terbinafine, itraconizole, or griseofulvin. Griseofulvin is usually not as effective as terbinafine or itraconizole. A lacquer (Penlac) can be used daily, but is ineffective unless combined with aggressive debridement of the affected nail.

Tinea capitis (scalp) must be treated orally, as the medication must be present deep in the hair follicles to eradicate the fungus. Usually griseofulvin is given orally for 2 to 3 months. Clinically dosage up to twice the recommended dose might be used due to relative resistance of some strains of dermatophytes.

Tinea pedis is usually treated with topical medicines, like ketoconazole or terbinafine, and pills, or with medicines that contains miconazole, clotrimazole, or tolnaftate. Antibiotics may be necessary to treat secondary bacterial infections that occur in addition to the fungus (for example, from scratching).

See also

• Hair perforation test
• National Center for Mycology

References

1. ^ "dermatophyte" at Dorland's Medical Dictionary
2. ^ http://www.dermnetnz.org/fungal/tinea-faciei.html
3. ^ http://www.emedicine.com/derm/topic420.htm
4. ^ http://dermnetnz.org/fungal/tinea-manuum.html
5. ^ "BBL Prepared Tubed and Bottled Medium for Detection and Presumptive Identification of Dermatophytes Dermatophyte Test Medium (DTM), Modified with Chloramphenicol". Becton, Dickinson and Company. http://www.bd.com/ds/productCenter/299701.asp. Retrieved 2008-12-07. 
6. ^ ^{a b} Ajello L, Getz M E (1954). "Recovery of dermatophytes from shoes and a shower stall". J. Invest. Dermat. 22 (1): 17–22. doi:10.1038/jid.1954.5. PMID 11237081. 

External links

• Images and descriptions of dermatophytes