Paper II
Question
Classify Dermatophytes. Describe the laboratory diagnosis.
Answer
Classification of dermatophytes:
- By genus: Trichophyton (can infect skin, hair, and nails), Microsporum (skin and hair, not nails), Epidermophyton (skin and nails, not hair).
- By habitat: anthropophilic (human-adapted, e.g., T. rubrum, T. tonsurans — chronic, less inflammatory infections), 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 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 contaminants); colony morphology, pigmentation, and microscopic conidial morphology (macroconidia/microconidia shape) allow species identification.
- Slide culture technique — for undisturbed examination of fruiting structures.
- 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 disease; oral antifungals (terbinafine, itraconazole, griseofulvin) for extensive disease, tinea capitis, or onychomycosis.