Dermatophytes

Dermatophyte Basics - Skin Invaders

• Keratinophilic fungi causing superficial mycoses (tinea/ringworm).
• Invade dead, keratinized tissues: stratum corneum (skin), hair, nails.
• Do not typically invade living tissue.
• Three Anamorph Genera:
  • Trichophyton (T): Infects skin, hair, nails.
  • Microsporum (M): Infects skin, hair.
  • Epidermophyton (E): Infects skin, nails. (📌 Mnemonic: E for Epidermophyton, Excludes hair)
• Ecological Classification (Sources):
  • Anthropophilic: Humans; chronic, mild inflammation (e.g., T. rubrum).
  • Zoophilic: Animals; acute, intense inflammation (e.g., M. canis).
  • Geophilic: Soil; moderate inflammation (e.g., M. gypseum).

⭐ Dermatophytes possess keratinases, enzymes that digest keratin, allowing them to colonize and derive nutrients from skin, hair, and nails.

Clinical Tineas - Ringworm Ruckus

Lab Diagnosis - Scope & Spores

• Specimen Collection: Skin scrapings, nail clippings, plucked hairs (infected part).
• Microscopy (KOH Mount):
  • Reagent: 10-20% KOH (dissolves keratin).
  • Visualizes: Septate hyaline hyphae, arthroconidia.
  • Parker ink / Calcofluor white stain enhances visibility.
• Culture:
  • Medium: Sabouraud Dextrose Agar (SDA) with cycloheximide & chloramphenicol.
  • Incubation: 25-30°C for 1-4 weeks.
  • Identification: Colony morphology (obverse/reverse pigment), microscopic features (macroconidia, microconidia).
• Wood's Lamp (UV light ~365nm):
  • Microsporum audouinii, M. canis: Bright green fluorescence (pteridine).
  • Trichophyton schoenleinii: Pale green fluorescence.
  • Not all dermatophytes fluoresce.

⭐ Arthroconidia are the infectious propagules formed by fragmentation of hyphae, characteristic of dermatophytes in tissue.

Dermatophyte Treatment - Fungal Fighters

• Topical Antifungals (Localized/Mild Infections):
  • Azoles: Clotrimazole, Miconazole, Ketoconazole, Luliconazole, Sertaconazole.
  • Allylamines: Terbinafine cream (highly effective).
  • Ciclopirox olamine.
  • Duration: 2-4 weeks, continue for 1-2 weeks after clinical resolution.
• Systemic Antifungals (Extensive/Nail/Hair/Resistant Infections):
  • Terbinafine: 250 mg OD. Tinea capitis (4-8 weeks), onychomycosis (6-12 weeks for fingernails, 12-24 weeks for toenails).
  • Itraconazole: 100-200 mg OD/BD. Pulse therapy for onychomycosis (200 mg BD for 1 week/month).
  • Fluconazole: 150 mg once weekly for 4-6 weeks (tinea corporis/cruris); 50 mg OD for tinea pedis.
  • Griseofulvin: (Mainly for tinea capitis in children) 10-20 mg/kg/day.
    • 📌 Mnemonic: "Greasy Griseofulvin" - take with fatty meal for ↑ absorption.

⭐ Terbinafine is generally considered fungicidal against dermatophytes, while azoles are primarily fungistatic.

• Adjunctive Therapy:
  • Keratolytics (Salicylic acid) for hyperkeratotic lesions.
  • Keep area dry; loose cotton clothing.
• ⚠️ Avoid topical steroid monotherapy (can worsen infection - tinea incognito).

High‑Yield Points - ⚡ Biggest Takeaways

• Dermatophytes (Trichophyton, Epidermophyton, Microsporum) are keratinophilic fungi infecting skin, hair, and nails.
• Cause various Tinea infections (ringworm), classified by anatomical location (e.g., Tinea pedis, Tinea cruris).
• Trichophyton rubrum is the most common etiological agent worldwide.
• Diagnosis relies on KOH microscopy (revealing septate hyphae) and Wood's lamp (fluorescence for some Microsporum species, e.g., M. canis).
• Arthroconidia are the infective elements responsible for transmission.
• Treatment includes topical antifungals; oral agents like terbinafine, itraconazole, or griseofulvin are used for extensive or nail/hair infections.