Which of these dermatophytes does not typically affect the scalp?

Epidermophyton Is the Dermatophyte That Does Not Affect the Scalp

Epidermophyton is the only dermatophyte genus among the options that does not cause tinea capitis (scalp infection). 1

Dermatophyte Genera and Their Relationship to Scalp Infections

The British Association of Dermatologists' guidelines for the management of tinea capitis clearly identify which dermatophyte genera are involved in scalp infections:

• Trichophyton species (including T. tonsurans and T. rubrum):

  • T. tonsurans is a major cause of tinea capitis, accounting for 50-90% of dermatophyte scalp isolates in the UK 1
  • T. tonsurans causes "black dot" pattern tinea capitis with endothrix hair invasion 1
  • T. rubrum, while less common in scalp infections than T. tonsurans, can still cause tinea capitis 2

• Microsporum species (including M. audouinii):

  • M. audouinii is specifically mentioned as causing "grey patch" tinea capitis with ectothrix hair invasion 1
  • Microsporum infections of the scalp typically produce characteristic fine scaling with patchy circular alopecia 1

• Epidermophyton:

  • Notably absent from all discussions of tinea capitis in the guidelines 1
  • Epidermophyton is recognized as a dermatophyte genus that causes skin and nail infections but not scalp infections 3, 4

Clinical Patterns of Tinea Capitis by Causative Organism

Tinea capitis presents in different clinical patterns depending on the causative organism:

1. Grey patch pattern: Caused by Microsporum species (including M. audouinii)

   • Characterized by fine scaling with patchy circular alopecia
   • Dull grey appearance due to arthrospores coating affected hairs 1

2. Black dot pattern: Caused by Trichophyton species (including T. tonsurans)

   • Relatively non-inflammatory patches of alopecia with fine scale
   • Broken-off, swollen hair stubs create characteristic "black dot" appearance 1

3. Inflammatory variants: Can be caused by both genera

   • Kerion: Boggy inflammatory mass with pustules and crusting
   • Favus: Chronic inflammatory pattern with "scutula" (yellow crusted lesions) 1

Why Epidermophyton Does Not Affect the Scalp

Epidermophyton has distinct biological characteristics that explain its inability to cause scalp infections:

• Epidermophyton floccosum is the only species in this genus and has a predilection for glabrous (non-hairy) skin and nails 3, 4
• Unlike Trichophyton and Microsporum species, Epidermophyton lacks the enzymatic capability to effectively invade hair follicles 4
• Research studies consistently show Epidermophyton's absence from scalp infections, with it primarily causing tinea corporis, tinea cruris, and tinea pedis 2, 5

Common Pitfalls in Identifying Dermatophyte Infections

• Misdiagnosing inflammatory tinea capitis (like kerion) as bacterial abscess, leading to inappropriate management 1
• Failing to recognize that some dermatophytes have specific tissue tropism - Epidermophyton specifically does not invade hair 4
• Overlooking that while T. rubrum is the most common dermatophyte overall (causing various skin infections), T. tonsurans is more common specifically in tinea capitis 1, 5

In clinical practice, accurate identification of the causative organism through proper sampling and laboratory diagnosis is essential for appropriate management of dermatophyte infections 1.