## Dermatophyte Biology and Epidemiology **Key Point:** Dermatophytes are NOT obligate intracellular parasites. They are keratinophilic fungi that live in the stratum corneum, hair, and nails — all dead, keratinized tissue. They can survive outside the host in contaminated fomites, floors, and shared spaces for extended periods. ### Correct Statements Explained **Trichophyton rubrum dominance:** - T. rubrum accounts for ~60–70% of dermatophyte infections in India [cite:Inamadar & Palit, Indian J Dermatol] - Followed by T. mentagrophytes and M. canis **Tinea corporis presentation:** - Classic "ringworm" lesion: erythematous patch with central clearing (hypopigmentation) and raised, scaly, inflammatory border - Annular or serpiginous morphology is pathognomonic **Tinea pedis epidemiology:** - Warm, humid climate → increased maceration and moisture retention - Poor foot hygiene, occlusive footwear, and communal bathing areas are major risk factors in India ### Why Option 2 Is Wrong **High-Yield:** Dermatophytes are **keratinophilic** (not obligate intracellular) fungi. They: 1. Colonize dead keratinized tissue (stratum corneum, hair shaft, nail plate) 2. Can be cultured on Sabouraud dextrose agar in vitro 3. Survive on fomites (floors, towels, combs) for weeks to months 4. Do NOT invade living epidermis or dermis (unlike some Candida species) This distinction is critical for understanding transmission and environmental control measures. ## Mnemonic: DKF **D**ermatophytes = **K**eratinophilic **F**ungi (not obligate intracellular) **Clinical Pearl:** Dermatophyte infections spread via direct contact with infected skin, fomites, or infected animals — not via bloodstream or systemic dissemination in immunocompetent hosts.
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