Fungal Skin Infections

Epidemiology

The epidemiology of dermatophyte infection is different for each clinical presentation and geographic area. For example, tinea pedis and nail infections are uncommon before puberty, whereas tinea capitis is primarily a disease of childhood. Dermatophyte infections are enhanced by heat and humidity, and travelers to the tropics may note an exacerbation of a pre-existing infection. Sources of dermatophyte infection include soil, animal reservoirs, and human-to-human transmission.

Clinical Features

Tinea capitis , or “ringworm” of the scalp, is mainly a disease of children caused by Microsporum or Trichophyton species. Trichophyton tonsurans , Microsporum canis , and Microsporum audouinii are most common.

The condition presents with hair loss (alopecia), usually with scale. Patchy areas of broken hairs covered by white scales can resemble seborrheic dermatitis. When tinea capitis invades the hair shaft the scalp shows a characteristic “black-dot” pattern, in which hairs broken off at scalp level resemble comedones within patches of alopecia. Lymphadenopathy is frequently present and can help differentiate from non-infectious causes of hair loss.

Inflammatory tinea capitis occurs in the setting of tinea infection coupled with a brisk host inflammatory response. A boggy inflamed plaque (kerion) may form, which is associated with systemic symptoms, including fever, pain, and regional lymphadenopathy. Permanent hair loss may occur.

Tinea favosa ( favus ) is a special type of chronic and progressive inflammatory infection on the scalp most frequently caused by Trichophyton schoenleinii . This variant is seen in Asia, Africa, the Middle East, and South America. It is characterized by permanent hair loss and inflammation of the scalp, which becomes covered by matted hair with dense, yellow, follicular, cup-shaped crusts (scutula) that have an unpleasant odor.

Tinea corporis , or “ringworm,” affects all ages and is recognized by annular, thin plaques, with erythema and scale most prominent at the advancing border ( Fig. 38.1 ). Lesions spread outward with central clearing. There may be multiple areas involved, but widespread involvement is unusual. The plaques vary in size from a few millimeters to many centimeters in diameter. Pruritus is variable. In some cases there is follicular involvement presenting with papules and pustules, especially at the advancing border. Differential diagnosis of tinea corporis includes psoriasis, nummular eczema, pityriasis rosea, subacute cutaneous lupus, and secondary syphilis.

Tinea imbricata is an unusual variant of tinea corporis caused by Trichophyton concentricum that is endemic in the South Pacific, Southeast Asia, and Central and South America. This presentation is chronic and results in the development of multiple large, loosely adherent scales covering large areas of the body that coalesce to form lacy concentric patterns similar to wood grain.

Tinea cruris , or “jock itch,” usually occurs in adult males and may be accompanied by tinea pedis. It is caused by Epidermophyton floccosum , Trichophyton rubrum , or Trichophyton mentagrophytes. The infection begins as an erythematous scaling patch involving intertriginous areas of the groin folds and inner thighs. Although it may extend on the lower abdomen and gluteal region, the scrotum is usually spared. There is mild erythema, with a well-demarcated scaly border. Differential diagnosis includes seborrheic dermatitis, erythrasma, intertrigo, candidiasis, and psoriasis.

Tinea pedis , or “athlete's foot,” is the most common location for dermatophyte infections. Clinical variants of tinea pedis include maceration and scale of the interdigital spaces and mild diffuse erythema and scale of the foot in a moccasin distribution. A vesicular presentation with 1-2 mm dried vesicles of the plantar surface, commonly on the mid foot, may occur. Differential diagnosis includes contact dermatitis and psoriasis.

Onychomycosis , or dermatophyte infection of the nail, results in nail plate thickening, dystrophy, and subungual debris of one or several nails. Toenails are affected more often than fingernails. Many organisms may cause onychomycosis, including nondermatophytes. When dermatophytes are the culprit, then concurrent tinea pedis is usually the source of the nail infection. Differential diagnosis includes psoriasis, lichen planus, candidiasis, and hereditary nail dystrophies. In diabetic individuals dermatophyte infections of the nails and feet create a portal of entry for bacterial infections, putting them at risk for cellulitis, especially due to streptococci.

Treatment

Epidemiology

Tinea nigra is a rare condition found in warm and humid climates, thought to be contracted from soil or decaying vegetation. The disease has been reported mainly in Central and South America, although cases have been identified in the southern United States, Africa, and Southeast Asia. Inoculating the organism into the skin with minor trauma has produced experimental infections, and this is believed to be the probable mechanism for natural infection. Lesions slowly develop over years and have been observed in travelers to endemic areas.

Clinical Features

The typical lesion is a well-demarcated, asymptomatic, brown to black patch of the palmar or plantar skin, resembling a stain. The lesions may resemble junctional nevi or acral lentiginous melanoma; however, the pigment may be partly removed by shaving off the most superficial stratum corneum layer of the skin. It occurs most commonly on the palms, but feet and other areas can be involved.

Diagnosis

KOH preparation of skin scrapings reveals hyphae. If a skin biopsy is done, the pigmented organisms can be seen in the stratum corneum.

Treatment

Tinea nigra is effectively treated with twice daily applications of imidazole or ciclopirox. Topical tolnaftate and oral griseofulvin are reported to be ineffective.

Tinea Versicolor

Tinea versicolor (pityriasis versicolor) is a common, usually asymptomatic superficial fungal infection that thrives under conditions of warmth and increased moisture.

Clinical features

The lesions of tinea versicolor are round or oval macules that coalesce into larger patches. They have a fine scale that sometimes is evident only when the lesion is scraped during the physical examination. In untanned Caucasians lesions may be subtly fawn brown and go unnoticed. The yeast blocks melanin synthesis in the skin and also produces a skin bleaching agent. With ultraviolet exposure a hypopigmented spotted appearance is enhanced due to contrast with the darkened surrounding skin ( Fig. 38.2 ). Lesions are typically distributed over the shoulders, chest, and back, and occasionally on the neck. Pruritus is usually absent.