Question
Dermatophytes
Answer
Dermatophytes are keratinophilic fungi — genera Trichophyton, Microsporum, and Epidermophyton — that specifically infect keratinized tissue (skin, hair, nails), causing dermatophytoses (tinea/ringworm infections), since they lack the ability to invade deeper, living tissue.
Genus-specific characteristics: Trichophyton — can infect skin, hair, and nails; Microsporum — infects skin and hair, but not nails; Epidermophyton — infects skin and nails, but not hair.
Classification by habitat: anthropophilic (human-adapted, e.g., T. rubrum, T. tonsurans — chronic, less inflammatory infections); zoophilic (animal-adapted, e.g., M. canis from cats/dogs — more acute, highly inflammatory); geophilic (soil-dwelling, e.g., M. gypseum).
Clinical patterns (named by affected body site): tinea capitis (scalp), tinea corporis (body/trunk — annular lesion with central clearing and an active advancing edge), tinea cruris (groin), tinea pedis (feet, “athlete’s foot”), tinea unguium/onychomycosis (nails), tinea barbae (beard).
Laboratory diagnosis:
- Wood’s lamp examination — some Microsporum species show characteristic green fluorescence of infected hair under UV light (most Trichophyton species do not fluoresce).
- Direct KOH microscopy — of skin scrapings, hair, or nail clippings, showing septate, branching hyphae.
- Culture — on Sabouraud dextrose agar (with cycloheximide to inhibit contaminants); colony morphology, pigmentation, and microscopic conidial morphology allow species identification.
Treatment: topical antifungals for limited skin involvement; oral antifungals (terbinafine, itraconazole, griseofulvin) for extensive disease, tinea capitis, or onychomycosis.