Question
Dermatophytes and lab. diagnosis of dermatophytosis
Answer
Dermatophytes are keratinophilic fungi — genera Trichophyton, Microsporum, and Epidermophyton — that specifically infect keratinized tissue (skin, hair, nails), causing dermatophytoses (tinea/ringworm infections).
Classification by habitat: anthropophilic (human-adapted, e.g., T. rubrum, chronic/less inflammatory); zoophilic (animal-adapted, e.g., M. canis from cats/dogs, more acutely 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).
Laboratory diagnosis of dermatophytosis:
- Wood’s lamp examination — some Microsporum species show characteristic green fluorescence of infected hair under UV light (most Trichophyton species do not fluoresce), useful for tinea capitis screening.
- Direct KOH microscopy — of skin scrapings, hair, or nail clippings, demonstrating septate, branching hyphae.
- Culture — on Sabouraud dextrose agar (with cycloheximide to inhibit contaminating environmental fungi); colony morphology, pigmentation, and microscopic conidial morphology (macroconidia/microconidia shape) allow species identification.
- Specimen collection: scraping from the active advancing edge of a skin lesion; hair pluck (not cut hair) for tinea capitis; nail clippings/subungual debris for onychomycosis.
Treatment: topical antifungals for limited skin involvement; oral antifungals (terbinafine, itraconazole, griseofulvin) for extensive disease, tinea capitis, or onychomycosis.