Paper II
Question
Describe the aetiology, clinical features and laboratory diagnosis of Dermatophytosis.
Answer
Dermatophytosis
Aetiology: caused by a group of keratinophilic fungi called dermatophytes, belonging to three genera — Trichophyton (infects skin, hair, and nails), Microsporum (infects skin and hair, not nails), and Epidermophyton (infects skin and nails, not hair). Common species include Trichophyton rubrum (the most common cause worldwide) and Trichophyton mentagrophytes.
Clinical features: dermatophyte infections are collectively called “tinea”, named by anatomical site:
- Tinea corporis — ringworm of the body; an annular lesion with a scaly, active, advancing edge and central clearing.
- Tinea capitis — scalp infection, causing 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, causing thickened, discoloured, brittle nails. All present with characteristic pruritic, scaly, erythematous lesions, with the annular, centrifugally spreading pattern being a distinctive feature.
Laboratory diagnosis:
- Wood’s lamp examination — certain Microsporum species (ectothrix infections) show characteristic 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 (obverse/reverse pigmentation, texture) and microscopic examination of macroconidia/microconidia (lactophenol cotton blue mount or slide culture) allow definitive species identification.
- Molecular methods (PCR) — increasingly used for rapid, precise species identification in reference settings.