Epidermophyton

• TRANSMISSION ROUTE: Direct contact

Definition

Epidermophyton is a genus of filamentous fungi (molds). One species, floccosum, is a primary cause of human infections of the outer layer of the skin and the beds of the nails, including tinea pedis (athlete's foot), tinea cruris (jock itch), tinea corporis (ringworm), and onychomycosis (nail infections).

Natural Habitat and Features

Both species of Epidermophyton are distributed worldwide. Floccosum is anthropohilic, meaning its natural habitat is the human body. Stockdaleae is geophilic, meaning its natural habitat is the soil. Floccosum, but not stockdaleae, causes human infection.

Infection can be transmitted by direct human-to-human contact, from objects that came into contact with infected persons, or from self-inoculation. Human-to-human transmission occurs most frequently within families or among children in daycare or school settings. Infected inanimate objects (fomites) that can contribute to transmission include wet floors in gyms or locker rooms, shared towels, and contaminated tools in barber shops and hair and nail salons. Self-inoculation occurs when a person first touches an infected area of the body and then touches a noninfected, vulnerable area elsewhere on the body.

Colonies of Epidermophyton incubated on Sabouraud’s dextrose agar at 77° to 86° Fahrenheit (25-30° Celsius) mature within seven to fourteen days. From the front, the colonies appear greenish-brown or khaki. The suede-like surface is raised and folded in the center and is otherwise flat, with a submerged fringe of growth at the outermost edges. From the reverse, the colonies are usually deep yellowish-brown.

Microscopic examination of colonies of Epidermophyton reveals septate, hyaline hyphae (partitioned and transparent tube-like filaments). Smooth, thin-walled, fusiform (spindle-shaped) macroconidia (large multicelled spores) are seen singly or in clusters growing directly from hyphae. The macroconidia of floccosum are shorter than those of stockdaleae. Microconidia (small one-celled spores) are typically not observed in Epidermophyton species. This absence differentiates Epidermophyton from Microsporum and Trichophyton, the two other genera of fungi responsible for dermatophytosis (infections of the outer layer of the skin, nails, and hair). In older lab cultures, chlamydoconidia (round thick-walled spores) and arthroconidia (jointed spores) are observed.

Pathogenicity and Clinical Significance

Floccosum, along with species of Microsporum and Trichophyton, is among the most common causes of superficial fungal infections of the skin and nails in otherwise healthy persons. Unlike Microsporum and Trichophyton species, however, it is not implicated in causing infections of the hair. Floccosum is highly adapted to the nonliving outer layer of the skin (the stratum corneum) and to nail beds.

Infection develops when fungi penetrate minor lesions, such as paper cuts and blisters, of the surface layer of the skin. The infection spreads sideways, with sharp, advancing margins. It remains limited to the nonliving outer layer of skin because floccosum cannot penetrate living tissue. Floccosum is a common cause of tinea corporis (ringworm of the body), tinea cruris or jock itch (ringworm of the groin), tinea manuum (ringworm of the hands), tinea pedis (athlete’s foot), and tinea unguium or onychomycosis (ringworm of the nail). All the infections caused by floccosum are found worldwide, although tinea corporis is more common in tropical and subtropical regions.

Many infections caused by floccosum are asymptomatic and, even when symptomatic, are self-limiting and resolve spontaneously. Such infections do not require medical treatment. For most persons who require treatment, a topical agent (cream, ointment, or solution) is sufficient. Some infections, especially in immunocompromised persons, may require more aggressive treatment with oral agents alone or in combination with topical agents.

Drug Susceptibility

Susceptibility methods for testing agents used to treat dermatophytosis caused by floccosum in infected persons (in vivo) have not been standardized. Limited testing and comparisons have shown that specific newer agents have lower minimum inhibitory concentrations (MICs) than do earlier agents, such as griseofulvin, amphotericin B, and fluconazole (formerly the drugs of choice). These newer agents also carry a lower risk of side effects.

Fluconazole, the first azole, has been replaced by newer, broad-spectrum azoles with much lower MICs. These include clotrimazole, econazole, imidazole, miconazole, and sulconazole, all available as topical agents. Butenafine, ketoconazole, conazole, sulconazole, ciclopirox, and tolnaftate may also be used. Along with topical formulations of the allylamine drugs naftifine and terbinafine, these are the drugs of choice in otherwise healthy persons with infections caused by floccosum.

For persons with spreading, persistent, or recurring infections, oral drugs may be required in addition to or in place of topical agents. Oral terbinafine, which has an especially low MIC, is the drug of choice in these cases. Oral formulations of azoles, including itraconazole, ketoconazole, and voriconazole, may also be used. All these drugs are more effective and convenient than griseofulvin and do not have the risks associated with their use. However, griseofulvin may be required to treat extensive infections of the nail beds, although terbinafine and itraconazole are more commonly used due to higher rates of effectiveness and shorter treatment duration.

Bibliography

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Ya-Nin, Nokdhes, et al. "Prevalence and Characteristics of Epidermophyton Floccosum Skin Infections: A 12-year Retrospective Study." Mycoses, vol. 67, no. 2, 2024, p. e13702, doi:10.1111/myc.13702. Accessed 16 Oct. 2024.