Which fungal infection causes pustular lesions on the scalp?

Tinea Capitis (Scalp Ringworm)

The fungal infection that causes pustular lesions on the scalp is tinea capitis, specifically the inflammatory variant known as kerion, which is caused by dermatophyte fungi—most commonly Trichophyton species (T. tonsurans, T. violaceum) in urban settings, or zoophilic species like T. mentagrophytes and T. verrucosum. 1

Clinical Presentation

Pustular lesions on the scalp represent the inflammatory variant of tinea capitis, which manifests in two main patterns:

Kerion (Most Common Pustular Presentation)

• Painful, boggy, inflammatory mass studded with pustules
• Matted with thick crust
• Associated alopecia (hair loss)
• Regional lymphadenopathy is common
• Often misdiagnosed as bacterial abscess
• Represents delayed host inflammatory response to dermatophytes 1

Diffuse Pustular Pattern

• Scattered pustules or low-grade folliculitis
• Diffuse, patchy alopecia
• Painful regional lymphadenopathy 1

Causative Organisms

The specific dermatophytes causing pustular tinea capitis include:

• Endothrix species (now predominant in urban areas): T. tonsurans, T. violaceum 1
• Zoophilic ectothrix species: T. mentagrophytes, T. verrucosum (historically more common for kerion) 1
• Less commonly: T. soudanense 2

Diagnostic Approach

When pustular lesions are present with scaling, lymphadenopathy, or alopecia, start treatment immediately while awaiting laboratory confirmation 1. These clinical features are strong predictive factors for tinea capitis.

Laboratory Confirmation

• Specimen collection: Scalp scraping with blunt scalpel, hair pluck, brush, or swab 1
• Microscopy: Mount in 10-30% potassium hydroxide to identify hyphae/arthroconidia 1
• Culture: On Sabouraud agar for 2-3 weeks to identify specific dermatophyte 1
• Susceptibility testing is NOT indicated 1

Treatment Algorithm

Oral antifungal therapy is mandatory—topical therapy alone is insufficient 1. Treatment choice depends on the causative organism:

First-Line Therapy

For Trichophyton species (most common in pustular presentations):

• Terbinafine is more efficacious 1
  • <20 kg: 62.5 mg daily for 2-4 weeks
  • 20-40 kg: 125 mg daily for 2-4 weeks

  • 40 kg: 250 mg daily for 2-4 weeks

For Microsporum species (if identified):

• Griseofulvin is more effective 1
  • <50 kg: 15-20 mg/kg/day for 6-8 weeks

  • 50 kg: 1 g/day for 6-8 weeks

  • Take with fatty food to enhance absorption 1

Second-Line Therapy

If first-line treatment fails:

• Itraconazole 5 mg/kg/day (effective against both Trichophyton and Microsporum) 1
• Consider non-compliance, suboptimal absorption, or reinfection first 1

Adjunctive Measures

• Antifungal shampoos (ketoconazole 2%, selenium sulfide 1%, or povidone-iodine) reduce spore transmission 1
• Do NOT stop treatment if a pruritic papular "id" eruption develops—this represents successful immune response to therapy 1

Critical Pitfalls

1. Misdiagnosis as bacterial abscess: Kerion is often confused with bacterial infection, but it requires antifungal therapy, not just antibiotics 1
2. Using topical therapy alone: This will fail because antifungals cannot penetrate the hair shaft adequately 1, 3
3. Wrong drug for wrong organism: Terbinafine fails against Microsporum because it cannot reach scalp surface arthroconidia in prepubertal children 1
4. Delaying treatment: In high-risk populations with typical clinical features, waiting 2-4 weeks for culture results increases spread and complications 1

Treatment Goals

The aims are eradication of organism, clinical and mycological cure, symptom relief, prevention of permanent scarring alopecia, and reduction of transmission 1. Prompt recognition and appropriate systemic antifungal therapy are essential to prevent irreversible scarring, particularly with kerion presentations.