Superficial Mycoses

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The superficial (cutaneous) mycoses are usually confined to the outer layers of skin, hair, and nails, and do not invade living tissues. The fungi are called dermatophytes. Dermatophytes, or more properly, keratinophilic fungi, produce extracellular enzymes (keratinases) which are capable of hydrolyzing keratin.

A. CLINICAL MANIFESTATIONS

Tinea means "ringworm" or "moth-like". Dermatologists use the term to refer to a variety of lesions of the skin or scalp.

Tinea corporis - small lesions occurring anywhere on the body.

Tinea pedis - "athlete's foot". Infection of toe webs and soles of feet.

Tinea unguium (onychomycosis) - nails. Clipped and used for culture.

Tinea capitis - head. Frequently found in children.

Tinea cruris - "jock itch". Infection of the groin, perineum or perianal area.

Tinea barbae - ringworm of the bearded areas of the face and neck.

Tinea versicolor - Characterized by a blotchy discoloration of skin which may itch. Up to 25% of the general population may have this lesion at any one time. Diagnosis is usually possible by direct microscopic examination of KOH-treated skin scrapings which show a typical aspect of mycelia and spores described as "spaghetti and meatballs." Caused by Malassezia furfur

B. ECOLOGY

The dermatophytes (skin plants) causing human infections may have different natural sources and modes of transmission:

anthropophilic - usually associated with humans only; transmission from man to man by close contact or through contaminated objects.

zoophilic - usually associated with animals; transmission to man by close contact with animals (cats, dogs, cows) or with contaminated products.

geophilic - usually found in the soil, transmitted to man by direct exposure. Knowledge of the species of dermatophyte and source of infection are important for proper treatment of the patient and control of the source.

Invasion by zoophilic or geophilic organisms may cause inflammatory disease in man.

Geographic distribution: Dermatophytes occur worldwide, but some species have geographically limited distribution.

C. ETIOLOGIC AGENTS

There are three genera of dermatophytes:

1. Trichophyton species (19 species)

These infect skin, hair and nails. Rarely can cause subcutaneous infections, in immunocompromised individuals. Take 2-3 weeks to grow in culture. The conidia are large (macroconidia), smooth, thin-wall, septate (0-10 septa), and pencil-shaped; colonies a re a loose aerial mycelium which grow in a variety of colors. Identification requires special biochemical and morphological techniques. Trichophyton rubrum is presently the most common cause of tinea in South Carolina.

2. Microsporum species (13 species).

These may infect skin and hair, rarely nails. Its prevalence has decreased significantly. When prevalent (15-20 years ago), this organism could be easily identified on the scalp because infected hairs fluoresce a bright green color when illuminated with a UV-emitting Wood's light. The loose, cottony mycelia produce macroconidia which are thick-walled, spindle-shaped, multicellular, and echinulate (spiny). Microsporum canis is one of the most common dermatophyte species infecting humans.

3. Epidermophyton floccosum.

These infect skin and nails and rarely hair. They form yellow-colored, cottony cultures and are usually readily identified by the thick, bifurcated hyphae with multiple smooth, club-shaped macroconidia.

D. THERAPY

Skin infections can be treated (more or less successfully) with a variety of drugs, such as:

Tolfnatate (Tinactin) available over the counter - Topical

Clotrimazole - Topical

Miconazole - Topical.

Ketoconazole seems to be most effective for tinea versicolor and other dermatophytes.

Itraconazole - oral

Terbinifine (Lamisil) - oral, topical.

For skin and Nail infections.

Morpholines - oral

For infections involving the scalp and particularly the nails, griseofulvin is commonly used. This antimycotic must be incorporated into the newly produced keratin layer to form a barrier against further invasion by the fungus. This is a very slow process requiring oral administration of the drug for long periods - up to 6 to 9 months for fingernail infections and 12 to18 months for toenail infections.

Itraconazole and terbinafine are the drugs of choice for onychomycoses.

E. THE IDENTIFICATION REACTION

Patients infected with a dermatophyte may show a lesion, often on the hands, from which no fungi can be recovered or demonstrated. It is believed that these lesions, which often occur on the dominant hand (i.e. right-handed or left-handed), are secondary to immunological sensitization to a primary (and often unnoticed) infection located somewhere else (e.g. feet). These secondary lesions will not respond to topical treatment but will resolve if the primary infection is successfully treated.