Dermatophytes

• Dermatophytes are moulds capable of invading keratin of skin, hair and nails → causing dermatophytosis (tinea)
• Several species infect humans - most belong to the genera Trichophyton, Microsporum and Epidermophyton
  • Other new taxonomic genera*: Nanizzia, paraphyton, Lophophyton, Arthroderma*
• Infection is classified by the body site involved:
  • Tinea capitis – scalp, hair
  • Tinea corporis – trunk, limbs (ringworm)
  • Tinea manuum/pedis - palms and soles (athlete’s foot)
  • Tinea cruris - groin (jock itch)
  • Tinea barbae - beard area and neck
  • Tinea faciale - face
  • Tinea unguium / onychomycosis - nails
• Organisms may be anthropophilic (primarily / exclusively infect humans), zoophilic, or geophilic (inhabit soil)
  • Can be transmitted:
    • human → human (direct contact not necessary - eg athlete's foot from walking barefoot in changing rooms)
    • animal → human
    • soil → human,
• All favour humid or moist skin
• Trichophyton rubrum is the most commonly isolated dermatophyte in the UK, comprising about 70% of isolations

Epidemiology

• Dermatophyte infections are exceedingly common worldwide, with a higher incidence in hot and humid regions
• Huge burden of morbidity - eg. fungal nail infection is estimated to affect 2.7-4.7% of adults in the UK

Pathogenesis

• Transmitted by hardy arthrospores
  • Type of fungal spore formed by fragmentation of the hyphae
• Fungal cells adhere to keratinocytes where they germinate and invade
• Risk factors: moist conditions, communal baths, athletic activities causing abrasions, atopy
• Invasive infections may occur in immunocompromised individuals with defective cellular immunity - particularly seen with Trichophyton spp.

Clinical features

Precise appearance varies with associated site, fungal species involved and host immune response.

May have atypical appearance if topical steroids inappropriately applied.