Skin Diseases Caused by Fungal Organisms

Introduction

1. Superficial mycoses - confined to the stratum corneum, hair, and nails
2. Subcutaneous mycoses - involving dermis and subcutaneous tissue
3. Deep/systemic mycoses - hematogenous spread, often in immunocompromised patients

Classification of Causative Organisms

I. Dermatophytes (Most Common)

• Anthropophilic (humans) - e.g. T. rubrum, T. tonsurans, E. floccosum
• Zoophilic (animals) - e.g. M. canis (cats/dogs), T. verrucosum (cattle)
• Geophilic (soil) - e.g. M. gypseum

II. Non-Dermatophyte Yeasts

• Malassezia furfur - causes Pityriasis/Tinea versicolor
• Candida albicans - cutaneous candidiasis

Major Clinical Entities

1. Tinea Capitis

• Dermatophytosis of the scalp; mainly in children
• Patchy lesions with erythema, scaling, crust formation, and hair loss
• Causative agents: T. tonsurans (US), M. canis
• Types of hair invasion: ectothrix (Microsporum) vs endothrix (Trichophyton)
• Treatment requires oral antifungals (griseofulvin, terbinafine)

2. Tinea Corporis (Ringworm of the body)

• Most common superficial fungal infection of glabrous skin
• Affects all ages, especially children
• Predisposing factors: excessive heat, humidity, exposure to infected animals
• Clinical: Expanding round plaque with elevated, scaling, erythematous border and central clearing ("ringworm" appearance)
• Histopathology: Mild spongiotic (eczematous) dermatitis with intraepidermal neutrophils and neutrophilic microabscesses in the stratum corneum

3. Tinea Pedis (Athlete's foot)

• Most prevalent form; affects 30-40% of the population at some point
• Diffuse erythema and scaling, initially in toe web spaces
• Predisposing factors: occlusive footwear, communal bathing areas

4. Tinea Cruris (Jock Itch)

• Inguinal areas; predominantly obese males in warm weather
• Moist red patches with raised, scaly, well-defined borders
• Predisposed by heat, friction, and maceration

5. Tinea Unguium / Onychomycosis

• Dermatophytosis of the nails
• Nail thickening, discoloration, onycholysis (separation of nail plate)
• Requires prolonged oral antifungal therapy

6. Tinea Barbae

• Dermatophyte infection of the beard area in adult males
• Relatively uncommon

7. Tinea Versicolor (Pityriasis Versicolor)

• Caused by Malassezia furfur (a yeast, NOT a dermatophyte)
• Located on the upper trunk; highly distinctive
• Lesions: groups of macules of varied color (hypo- or hyperpigmented) with fine peripheral scale
• "Spaghetti and meatballs" pattern on KOH mount - short curved hyphae + round spores
• Hypopigmentation due to azelaic acid produced by Malassezia, which inhibits tyrosinase

8. Cutaneous Candidiasis

• Caused by Candida albicans
• Sites: intertriginous regions (axillary, inguinal, inframammary folds), oral mucosa, genitalia, nail folds
• Characteristic: Erythematous plaques with satellite lesions (small peripheral pustules/papules)
• Predisposed by diabetes, immunosuppression, antibiotic use, obesity, corticosteroid therapy

Histopathology of Dermatophyte Infections

• Properties of the organism
• Host immune response
• Degree of secondary bacterial superinfection

1. Mild spongiotic (eczematous) dermatitis in the epidermis
2. Intraepidermal neutrophils and neutrophilic microabscesses - especially in the stratum corneum
3. Superficial perivascular lymphocytic infiltrate in the dermis
4. Fungi located in the anucleate cornified layer (stratum corneum) of lesional skin, hair, or nails

Fig: (A) Tinea corporis - erythematous plaque. (B) Histology showing spongiotic dermatitis with neutrophilic abscesses. Inset (PAS stain): deep red/pink hyphae within stratum corneum. [Robbins & Cotran Pathologic Basis of Disease]

Special Stains for Diagnosis

Culture & Laboratory Diagnosis

• Sabouraud Dextrose Agar (SDA) with cycloheximide + chloramphenicol: standard isolation medium; incubated at 20-25°C for up to 4 weeks
• Dermatophyte Test Medium (DTM): Contains phenol red indicator; turns red with dermatophyte growth (proteolytic activity raises pH)
• Wood's Lamp: Microsporum species fluoresce green (except M. audouinii/M. canis)
• KOH Preparation: Demonstration of septate hyphae confirms dermatophytosis

Pathogenesis

1. Attach to and invade keratinized tissues (skin, hair, nails)
2. Use keratin as a nutrient source
3. Evoke innate immune response (neutrophil recruitment, epidermal spongiosis)

Treatment Summary

Summary Table

• Robbins & Cotran Pathologic Basis of Disease, Superficial Fungal Infections, Ch. 25
• Fitzpatrick's Dermatology, 9th Ed., Ch. 160 - Dermatophytes & Superficial Mycoses
• Sherris & Ryan's Medical Microbiology, 8th Ed., Ch. 6