The dermatophytes are a group of fungi that invade and grow in the dead keratin of skin, hair, and nails. Dermatophytes are, by far, the most prevalent of the 3 major classes of superficial infections.  Less frequently, superficial skin infections are caused by nondermatophyte fungi (eg, Malassezia furfur in tinea versicolor) and Candida species.
Several species of dermatophytes commonly invade human keratin, and these belong to the Epidermophyton, Microsporum, and Trichophyton genera. They tend to grow outwards on skin, producing a ringlike pattern, hence the term "ringworm". They are very common and affect different parts of the body. Clinically, dermatophytosis infections, also known as tinea, are classified according to the body regions involved.
The type and severity of the host response is often related to the species and strain of the dermatophyte causing the infection. The dermatophytes are the only fungi that have evolved a dependency on human or animal infection for the survival and dissemination of their species. The infection may spread from person to person (anthropophilic), animal to person (zoophilic), or soil to person (geophilic). The most common of these organisms are Trichophyton rubrum, Trichophyton tonsurans, Trichophyton interdigitale and/or Trichophyton mentagrophytes, Microsporum canis, and Epidermophyton floccosum.
Table. Ecology of Common Human Dermatophyte Species. Table reprinted with permission from David Ellis, Bsc (Hons), MSc, PhD, FASM, FRCPA (Hon), Affiliate Associate Professor, The University of Adelaide (http://www.mycology.adelaide.edu.au/mycoses/cutaneous/dermatophytosis). (Open Table in a new window)
|T rubrum||Humans||Very common|
|T interdigitale||Humans||Very common|
|Trichophyton violaceum||Humans||Less common|
|Microsporum audouinii||Humans||Less common|
|Microsporum ferrugineum||Humans||Less common|
|T mentagrophytes||Mice, rodents||Common|
Dermatophytes are keratinophilic fungi and have the ability to invade keratinized tissue (eg, hair, nails, any area of the skin). They invade, infect, and persist in the stratum corneum of the epidermis and rarely penetrate below the surface of the epidermis and its appendages. Humid or moist skin provides a very favorable environment for the establishment of fungal infection. At a minimum, the skin responds to the irritation of the superficial infection by increased proliferation in the basal cell layer, which causes scaling and epidermal thickening. Clinically, tinea infections are classified according to the body region involved/infected, as follows:
Tinea capitis - Scalp (see the image below)Tinea capitis; gray patch ringworm. Gray patch refers to the scaling with lack of inflammation, as noted in this patient. Hairs in the involved areas assume a characteristic dull, grayish, discolored appearance and are broken and shorter.
Tinea manuum and tinea pedis - Palms, soles, and interdigital webs
Tinea corporis - Body (shown in the image below)Annular plaque (tinea corporis).
Tinea cruris - Groin
Tinea barbae - Beard area and neck (shown in the image below)Wax model of kerionlike tinea barbae. Courtesy of the Museum of the Department of Dermatology, University of Medicine, Wroclaw, Poland.
Tinea faciale - Face
Tinea unguium ( onychomycosis) - Nail
Tinea capitis (caused by the species of genera Trichophyton and Microsporum) is the most common pediatric dermatophyte infection. The age predilection is believed to result from the lack of certain florae and fungistatic sebum in this age group. It usually takes 2 weeks to produce clinically visible changes. The natural course of tinea capitis is of a spontaneous cure at puberty, once sebum production begins. Hair invasion is divided into several types. The site of formation of spore-forming bodies classifies the species causing the invasion, as follows:
Ectothrix species: Conidia form on the exterior of the hair shaft; the cuticle is destroyed and involved areas fluoresce a green-yellow under a Wood lamp; this is caused by M canis, Microsporum distortum, M ferrugineum, M audouinii, as well as nonfluorescent T rubrum, T verrucosum, Trichophyton megninii, T mentagrophytes, M gypseum, and M nanum
Endothrix species: Conidia form within the hair shaft, and each is filled with hyphae and spores; the cuticle is not affected, and hairs do not fluoresce under a Wood lamp; this is caused by anthropophilic ( T rubrum, Trichophyton gourvilii, T tonsurans, T violaceum, Trichophyton yaoundei, T soudanense) organisms
Favus species: Hyphae arrange within and around the hair shaft; this is a rare and severe form resulting in favuslike crusts or scutula and hair loss with honey comb destruction pattern of the follicles; this is caused by T schoenleinii (see the image below)Tinea favosa of the scalp shows erythematous lesions with pityroid scaling. Some hairs are short and brittle.
Kerion: Thick plaques and boggy skin that form often with bacterial infection superimposed; mainly caused by M canis  ; this pattern develops in such a manner that it is often believed to be a response to the dermatophyte (see the image below)Typical lesions of kerion celsi on the vertex scalp of a young Chinese boy. Note numerous bright yellow purulent areas on skin surface, surrounded by adjacent edematous, erythematous, alopecic areas. Culture from the lesion grew Trichophyton mentagrophytes. Courtesy of Skin Diseases in Chinese by Yau-Chin Lu, MD. Permission granted by Medicine Today Publishing Co, Taipei, Taiwan, 1981.
The specific etiologic agent is often associated with a specific region of infection. Further elaboration of the discussion below can be found in the tinea articles of the Medscape Reference Dermatology volume (Tinea Barbae, Tinea Capitis, Tinea Corporis, Tinea Cruris, Tinea Faciei, Tinea Nigra, Tinea Pedis, Tinea Versicolor).
A recent study found tinea capitis present in more than 30% of children at certain grade levels in some urban areas of the United States. 
High prevalence rates of tinea pedis and onychomycosis have been linked to increased urbanization, community showers, sports, and the use of occlusive footwear.  These factors are thought to contribute to the high prevalence of tinea pedis in certain occupational groups, including marathon runners (22-31% prevalence), miners (21-72.9% prevalence), and soldiers (16.4-58% prevalence). Several of these studies also found high rates of onychomycosis presenting with tinea pedis. Outbreaks of infections can occur in schools, households, and institutional settings.
Although dermatophytes are found throughout the world, the most prevalent strains and the most common sites of infection vary by region. Hot, humid climates and overcrowding predispose populations to skin diseases, including tinea infections. Developing countries have high rates of tinea capitis, while developed countries have high rates of tinea pedis and onychomycosis.
Low socioeconomic conditions are strongly linked to higher prevalence rates for skin infections, including tinea infections. A review of 18 studies representing large geographical areas determined that tinea capitis is present in up to 19.7% of the general population in developing countries.
While mortality due to dermatophytes is very low, there is significant morbidity associated with these infections, particularly in the armed forces and active adults.
Cellulitis in the lower extremities, which causes a breach in the skin and allows the inoculation of opportunistic bacteria, is a frequent complication of interdigital fungal infection.
In patients with impaired cell-mediated immune function, atypical and locally aggressive presentations of dermatophyte infection may occur. These include extensive skin disease, subcutaneous abscesses, and dissemination.
Fungal infection affects all races; however, the prevalence of organisms varies by country.
Both sexes are affected by fungal infection. Tinea cruris is much more common in males because of the male anatomy, which allows moisture to accumulate in the crural folds.
In the United States, tinea pedis is the most common in adults and tinea capitis is the most common in children. Tinea corporis is present in all ages, although it is more frequent in adolescents and pregnant females.
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