Question
Dermatophytes
Answer
Dermatophytes are a group of closely related keratinophilic fungi — genera Trichophyton, Microsporum, and Epidermophyton — that specifically infect keratinized tissue (skin, hair, nails), causing dermatophytoses (tinea/ringworm infections).
Genus-specific characteristics: Trichophyton — can infect skin, hair, and nails; Microsporum — infects skin and hair, not nails; Epidermophyton — infects skin and nails, not hair.
Classification by habitat: anthropophilic (human-adapted, e.g., T. rubrum — chronic, less inflammatory); zoophilic (animal-adapted, e.g., M. canis — acute, highly inflammatory); geophilic (soil-dwelling, e.g., M. gypseum).
Clinical patterns: tinea capitis (scalp), tinea corporis (body — annular lesion with central clearing), tinea cruris (groin), tinea pedis (feet), tinea unguium/onychomycosis (nails), tinea barbae (beard).
Laboratory diagnosis:
- Wood’s lamp examination — some Microsporum species show green fluorescence under UV light (most Trichophyton species do not fluoresce).
- Direct KOH microscopy — of scrapings, showing septate, branching hyphae.
- Culture — on Sabouraud dextrose agar (with cycloheximide); colony morphology and conidial morphology allow species identification.
Treatment: topical antifungals for limited skin involvement; oral antifungals (terbinafine, itraconazole, griseofulvin) for extensive disease, tinea capitis, or onychomycosis.