A 28-year-old woman from Mumbai presents with a 3-week history of pruritic, scaly patches on her forearms and lower legs. Examination reveals well-demarcated, erythematous plaques with central clearing and a raised, scaling border. KOH mount of scale shows branching septate hyphae. She reports recent contact with stray cats in her neighborhood. Which dermatophyte is most likely responsible for this infection?

## Clinical Diagnosis: Microsporum canis Infection ### Key Clinical Features **Key Point:** Microsporum canis is a zoophilic dermatophyte with a strong epidemiological link to animal contact, particularly cats and dogs. The patient's presentation—well-demarcated plaques with central clearing, raised scaling border, and recent contact with stray cats—is classic for M. canis infection. The branching septate hyphae on KOH mount confirm a dermatophyte. ### Microsporum canis Characteristics | Feature | Detail | |---------|--------| | **Ecology** | Zoophilic (animals → humans) | | **Common source** | Cats, dogs, other mammals | | **Lesion pattern** | Circular with raised border, central clearing | | **Fluorescence** | Blue-green under Wood's lamp (40–50% of strains) | | **Culture** | Flat, white to buff, with radial grooves | | **Body sites** | Exposed areas (arms, legs, face) | | **Transmission** | Direct contact with infected animals | ### Dermatophyte Ecology & Epidemiology **High-Yield:** Dermatophyte classification by ecology determines epidemiological clues: 1. **Zoophilic** (animal → human): M. canis, M. audouinii, T. mentagrophytes var. mentagrophytes 2. **Anthropophilic** (human → human): T. rubrum, T. mentagrophytes var. interdigitale, E. floccosum 3. **Geophilic** (soil → human): M. gypseum, T. verrucosum **Clinical Pearl:** Animal contact history is the strongest epidemiological clue for zoophilic species. M. canis accounts for ~10–15% of dermatophyte infections in India and is the most common zoophilic cause. ### Why M. canis Over Other Zoophilic Species - **M. audouinii**: Rare in India; historically important but now uncommon; causes scalp infections predominantly - **T. mentagrophytes (zoophilic var.)**: Also zoophilic but more commonly from rodents/rabbits; less common from cats - M. canis is the most frequent zoophilic dermatophyte from feline contact ### Diagnostic Confirmation 1. **KOH mount**: Branching septate hyphae (✓ already done) 2. **Culture on Sabouraud dextrose agar**: White, flat, radial grooves 3. **Wood's lamp**: Blue-green fluorescence (not all strains) 4. **Microscopy of culture**: Spindle-shaped macroconidia with thick walls ### Management **Key Point:** Topical azoles (miconazole, clotrimazole) for localized lesions; systemic terbinafine (250 mg daily × 2–4 weeks) for extensive involvement. **Clinical Pearl:** Advise patient to avoid contact with infected animals and ensure animal treatment to prevent recurrence.