Question
Dermatophytes
Answer
Dermatophytes are a group of closely related keratinophilic fungi — belonging to three genera, Trichophyton, Microsporum, and Epidermophyton — that specifically infect keratinized tissue (skin, hair, nails), collectively causing the superficial fungal infections known as dermatophytoses (tinea/ringworm infections), since they lack the ability to invade deeper, living tissue.
Classification by natural habitat:
- Anthropophilic — adapted to humans (e.g., T. rubrum, T. tonsurans), transmitted person-to-person, tend to produce chronic, less inflammatory infections.
- Zoophilic — adapted to animals (e.g., M. canis from cats/dogs, T. verrucosum from cattle), transmitted from animals to humans, tend to produce more acute, highly inflammatory reactions in the accidental human host.
- Geophilic — soil-dwelling (e.g., M. gypseum), acquired from soil contact.
Clinical patterns (named by affected body site): tinea capitis (scalp), tinea corporis (body/trunk — the classical annular “ringworm” lesion with central clearing and an active, scaly, advancing edge), tinea cruris (groin), tinea pedis (feet, “athlete’s foot”), tinea unguium/onychomycosis (nails), tinea barbae (beard area).
Laboratory diagnosis: Wood’s lamp examination — some Microsporum species (notably M. canis, M. audouinii) show characteristic green fluorescence of infected hair under UV light (useful for tinea capitis screening; most Trichophyton species do not fluoresce); direct KOH microscopy of skin scrapings/hair/nail clippings — demonstrates 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.
Treatment: topical antifungals (azoles, terbinafine) for limited skin involvement; oral antifungals (terbinafine, itraconazole, griseofulvin) for extensive disease, tinea capitis, or onychomycosis, where topical therapy alone cannot adequately penetrate hair/nail keratin.