Paper II
Question
Describe the etiological agents, clinical types and laboratory diagnosis of dermatophytes.
Answer
Etiological agents of dermatophytes: keratinophilic fungi belonging to three genera — Trichophyton (skin, hair, nails), Microsporum (skin and hair, not nails), and Epidermophyton (skin and nails, not hair). Common species: Trichophyton rubrum (the most common cause worldwide), T. mentagrophytes, Microsporum canis.
Clinical types (“tinea,” named by anatomical site):
- Tinea corporis — ringworm of the body; annular lesion with scaly, active advancing edge and central clearing.
- Tinea capitis — scalp infection, patchy hair loss/scaling, more common in children.
- Tinea pedis — “athlete’s foot,” interdigital scaling/maceration.
- Tinea cruris — groin infection.
- Tinea unguium (onychomycosis) — nail infection.
- Tinea barbae — beard area infection in adult males.
Laboratory diagnosis:
- Wood’s lamp examination — some Microsporum species (ectothrix infections) show apple-green fluorescence of infected hair under UV light; most Trichophyton infections (endothrix) do not fluoresce.
- Direct KOH (10–20%) microscopy — of skin scrapings/plucked hairs/nail clippings, demonstrating septate, branching hyphae and, on hair, arthroconidia arranged around (ectothrix) or within (endothrix) the hair shaft — the standard rapid diagnostic test.
- Culture — on Sabouraud dextrose agar (with cycloheximide/chloramphenicol), incubated at 25–30°C for 2–4 weeks; colony morphology and microscopic examination of macroconidia/microconidia (lactophenol cotton blue mount) allow definitive species identification.
- Molecular methods (PCR) — increasingly used for rapid, precise species identification in reference laboratories.