What is the pathophysiology, symptoms, diagnosis, management, and differential diagnoses of a dermatophyte (tinea) infection?

Dermatophyte (Tinea) Infections: Comprehensive Clinical Guide

Pathophysiology

Dermatophytes are fungi that require keratin for growth, causing superficial infections of the skin, hair, and nails through direct contact with anthropophilic (human), zoophilic (animal), or geophilic (soil) organisms, as well as indirectly through contaminated fomites. 1

• The infection spreads when fungal spores penetrate the stratum corneum and establish growth in keratinized tissues 1
• Transmission occurs through skin-to-skin contact with infected individuals, contact with infected animals, or exposure to contaminated surfaces in public facilities 2, 3
• The fungi proliferate in warm, moist environments, making certain body areas more susceptible 4

Clinical Presentation and Symptoms

The clinical manifestations vary by anatomical location but typically include pruritus, scaling, and characteristic annular lesions with raised borders:

Tinea Corporis (Body)

• Circular or ring-shaped lesions with central clearing and raised, scaly borders 5, 4
• Itching, burning, and scaling of affected areas 5
• Lesions may be single or multiple 4

Tinea Cruris (Groin/Jock Itch)

• Bilateral involvement of the inguinal folds with sharp, raised borders 5, 4
• Pruritus and burning sensation 5
• Sparing of the scrotum distinguishes it from candidal infections 4

Tinea Pedis (Athlete's Foot)

• Interdigital scaling, maceration, and fissuring 5, 4
• Moccasin-type distribution with diffuse plantar scaling 4
• Itching, burning, and cracking 5

Tinea Capitis (Scalp)

• Scaling, alopecia, and lymphadenopathy are strong predictive features 6
• Black dot pattern from broken hair shafts 6
• Kerion formation (boggy, inflammatory mass) represents delayed host response, not bacterial superinfection 2
• "Comma-shaped" hairs in white children with ectothrix infection; corkscrew hairs in Afro-Caribbean children 6

Tinea Unguium (Onychomycosis)

• Nail thickening, discoloration (yellow-white), and subungual debris 1
• Distal and lateral subungual pattern most common 1

Diagnosis

Diagnosis requires both clinical assessment and mycological confirmation through KOH microscopy and fungal culture before initiating therapy whenever possible. 3

Specimen Collection

• Scalp lesions: Use scalpel scraping, hair pluck, brush sampling, or swab; multiple sampling methods increase yield 6, 2
• Skin lesions: Collect scales from the active border using scalpel scraping 3, 7
• Nail infections: Obtain subungual debris and nail clippings 1

Laboratory Methods

• KOH preparation (10-30%): Mount specimens for light or fluorescence microscopy to visualize hyphae and arthroconidia; provides rapid diagnosis but has limited sensitivity 6, 2, 1
• Fungal culture: Inoculate Sabouraud agar with cycloheximide; incubate for at least 2 weeks (3 weeks if T. verrucosum suspected from cattle exposure) 6, 2
• Dermoscopy: Useful adjunctive tool for visualizing black dot hair stubs and characteristic hair patterns in tinea capitis 6
• Susceptibility testing: Not routinely indicated as resistance development is rare 6, 2

When to Start Treatment Without Confirmation

• Initiate therapy immediately if kerion, severe scaling, lymphadenopathy, or alopecia are present, as these are strong predictive factors 6, 2
• In high-risk populations, awaiting culture results (2-4 weeks) increases transmission risk 6

Management

Tinea Capitis (Scalp)

Oral therapy is mandatory for tinea capitis, as topical agents alone are ineffective; organism-directed therapy optimizes outcomes with terbinafine preferred for Trichophyton species and griseofulvin for Microsporum species. 6, 2

First-Line Systemic Therapy

For Trichophyton species:

• Terbinafine (preferred): 2
  • Children <20 kg: 62.5 mg daily for 2-4 weeks
  • Children 20-40 kg: 125 mg daily for 2-4 weeks
  • Children >40 kg and adults: 250 mg daily for 2-4 weeks

For Microsporum species:

• Griseofulvin (preferred): 2
  • Children <50 kg: 15-20 mg/kg/day for 6-8 weeks
  • Children >50 kg and adults: 1 g/day for 6-8 weeks

Adjunctive Topical Therapy

• Povidone-iodine, ketoconazole 2%, or selenium sulfide 1% shampoos reduce spore transmission but do not cure infection 6
• Topical therapy alone is not recommended for management 6

Special Considerations

• Kerion management: Do not delay systemic antifungal therapy; kerion represents inflammatory host response, not bacterial infection 2
• Topical or oral corticosteroids may provide symptomatic relief for severe inflammation 2
• Dermatophytid reactions: May occur after treatment initiation (cell-mediated response to dying dermatophytes); do not discontinue antifungals, treat symptomatically with topical corticosteroids 2

Tinea Corporis, Cruris, and Pedis (Skin)

Most localized skin infections respond to topical antifungal therapy; reserve oral agents for extensive disease, treatment failure, or immunocompromised patients. 2, 3

First-Line Topical Therapy

Topical terbinafine 1% cream once daily for 1-2 weeks is the preferred first-line treatment for localized infections. 3, 5

Alternative topical options:

• Naftifine 1% once daily for 1-2 weeks 3
• Clotrimazole cream twice daily for 2-4 weeks 8
• Miconazole cream twice daily for 2-4 weeks 8
• Ciclopirox 0.77% cream or gel twice daily for 4 weeks 2

Topical allylamines (terbinafine, naftifine) have higher cure rates and shorter treatment courses than azoles due to fungicidal rather than fungistatic action. 4, 1

Oral Therapy for Extensive or Resistant Disease

Terbinafine 250 mg daily for 1-2 weeks achieves 87.1% mycological cure rate and is particularly effective against T. tonsurans. 2, 3

Alternative oral option:

• Itraconazole 100 mg daily for 15 days: 87% mycological cure rate; useful when organism unknown or Microsporum suspected 2, 8, 3

Tinea Manuum (Hand)

Apply terbinafine 1% gel once daily for 1-2 weeks as first-line treatment for mild to moderate infection without nail involvement. 2

For systemic therapy:

• Terbinafine 250 mg daily for 2-4 weeks: 86% mycological cure rate at 8 weeks for Trichophyton species 2
• Itraconazole 100 mg daily for 15 days: 87% mycological cure rate 2

Tinea Unguium (Onychomycosis)

Oral antifungal therapy is the treatment of choice for onychomycosis, with terbinafine generally preferred over itraconazole due to superior efficacy and shorter treatment duration. 2

First-Line Oral Therapy

Terbinafine 250 mg daily: 2

• Fingernail infections: 6 weeks
• Toenail infections: 12-16 weeks

Itraconazole (alternative first-line): 2

• Continuous therapy: 200 mg daily for 12 weeks
• Pulse therapy: 400 mg daily for 1 week per month (2 pulses for fingernails, 3 pulses for toenails)

Confirm diagnosis by KOH microscopy, culture, or histologic examination before initiating therapy due to expense, duration, and potential adverse effects. 1

Treatment Monitoring and Endpoints

The definitive endpoint for adequate treatment must be mycological cure (negative microscopy and culture), not just clinical response. 2, 3

• Repeat mycology sampling at the end of standard treatment period 2, 3
• Continue monthly sampling until mycological clearance is documented 2, 3
• If clinical improvement occurs but mycology remains positive, continue current therapy for additional 2-4 weeks 2
• If no initial clinical improvement, switch to second-line therapy 2

Safety Monitoring

Obtain baseline liver function tests and complete blood count before initiating terbinafine or itraconazole, especially with pre-existing hepatic abnormalities or prolonged therapy. 2, 8, 3

Itraconazole is contraindicated in heart failure and has significant drug interactions: 2, 3

• Enhanced toxicity with warfarin, certain antihistamines, antipsychotics, midazolam, digoxin, cisapride, and simvastatin 2, 8

Emerging Resistance

Terbinafine-resistant Trichophyton indotineae is an increasing concern; consider alternative agents (topical clotrimazole 1% twice daily plus luliconazole 1% cream at night) for refractory cases. 9

• Antifungal susceptibility testing should be considered in treatment-resistant cases 9

Prevention of Recurrence

Implement comprehensive prevention strategies to avoid reinfection:

• Wear protective footwear in public bathing facilities, gyms, and hotel rooms 2, 3
• Apply antifungal powders (miconazole, clotrimazole, tolnaftate) in shoes and on feet 2, 3
• Screen and treat all household contacts, as over 50% may be affected with anthropophilic species like T. tonsurans 8, 3
• Clean all fomites (combs, brushes, towels, clothing) with disinfectant or 2% sodium hypochlorite solution 2, 3
• Avoid skin-to-skin contact with infected individuals 2, 7
• Keep skin dry and cool at all times 7
• Do not share towels, clothing, or hair accessories with infected individuals 7

Patient Counseling

Key counseling points:

• Complete the full treatment course even if symptoms improve, as clinical clearing does not guarantee mycological cure 2, 3
• Continue treatment for at least one week after clinical clearing of infection 4
• Expect 2-4 weeks for culture results if obtained 6
• Dermatophytid reactions (pruritic papular eruptions) may occur after starting treatment; this represents immune response to dying fungi and does not require stopping antifungals 2
• For tinea pedis, treatment duration is longer (4 weeks with azoles, 1-2 weeks with allylamines) compared to tinea corporis/cruris (2 weeks) 4
• Avoid sharing personal items and practice good hygiene to prevent transmission 7
• For onychomycosis, nail appearance may not fully normalize until new nail grows out completely (6-12 months) 1

Differential Diagnoses

Consider these alternative diagnoses when evaluating suspected dermatophyte infections:

For Annular Lesions (Tinea Corporis)

• Pityriasis rosea: Herald patch followed by Christmas tree distribution; KOH negative 7
• Nummular eczema: Coin-shaped plaques without central clearing; KOH negative 7
• Psoriasis: Silvery scale, extensor surfaces, nail pitting; KOH negative 7
• Granuloma annulare: Non-scaly annular lesions; KOH negative 7
• Erythema migrans (Lyme disease): History of tick exposure, expanding erythema; KOH negative 7

For Groin Lesions (Tinea Cruris)

• Candidiasis: Involves scrotum, satellite lesions; budding yeasts on KOH 4
• Erythrasma: Coral-red fluorescence under Wood's lamp; caused by Corynebacterium 4
• Inverse psoriasis: Well-demarcated plaques without scale; KOH negative 4
• Intertrigo: Maceration without distinct border; KOH negative 4

For Foot Lesions (Tinea Pedis)

• Dyshidrotic eczema: Vesicles on sides of feet and hands; KOH negative 4
• Contact dermatitis: History of exposure, bilateral distribution; KOH negative 4
• Psoriasis: Silvery scale, nail involvement; KOH negative 4

For Scalp Lesions (Tinea Capitis)

• Seborrheic dermatitis: Greasy yellow scale, no hair loss; KOH negative 6
• Alopecia areata: Smooth patches without scaling; exclamation point hairs; KOH negative 6
• Bacterial folliculitis: Pustules without scaling; bacterial culture positive 6
• Trichotillomania: Irregular patches, broken hairs of varying lengths; KOH negative 6

For Nail Lesions (Onychomycosis)

• Psoriatic nails: Pitting, oil spots, skin psoriasis elsewhere; KOH may show parakeratosis 1
• Lichen planus: Pterygium formation, thinning; KOH negative 1
• Traumatic onycholysis: History of trauma, single nail; KOH negative 1

Common Pitfalls and How to Avoid Them

• Starting treatment without mycological confirmation: Always obtain KOH or culture when possible to avoid misdiagnosis and inappropriate therapy 3, 7
• Using topical therapy alone for tinea capitis: This will fail; oral therapy is mandatory 6
• Stopping treatment at clinical clearing: Continue until mycological cure is documented to prevent relapse 2, 3
• Treating kerion with antibiotics alone: Kerion is inflammatory, not infectious; requires systemic antifungals 2
• Discontinuing antifungals for dermatophytid reactions: These are immune responses; treat symptomatically and continue antifungals 2
• Ignoring household contacts: Screen and treat family members to prevent reinfection, especially with anthropophilic species 8, 3
• Failing to address moisture and hygiene: Exacerbating factors must be controlled for treatment success 4
• Not considering drug interactions with itraconazole: Check for contraindications and interactions before prescribing 2, 3
• Assuming treatment failure is due to resistance: First assess compliance, drug absorption, and reinfection sources 3