Dermatophytes

Background

Microbiology

• Dermatophytes are molds that cause superficial skin, hair, and nail infections
• The four most common genera Epidermophyton, Microsporum, Trichophyton, and Nannizzia
• Fungi may be anthropophilic, zoophilic, or geophilic (soil origin)

Epidemiology

• Risk depends on the specific mold and its natural environment (humans, animals, or soil)

Anthropophilic dermatophytes

• Most common worldwide is Trichophyton rubrum
• Others include Trichophyton concentricum, Trichophyton interdigitale, Trichophyton schoenleinii, Trichophyton soudanense, Trichophyton tonsurans, Trichophyton violaceum, Microsporum audouinii, Microsporum ferrugineum, and Epidermophyton floccosum
• Risk factors include areas where people share common facilities, such as military camps and factories

Zoonotic dermatophytes

• Of zoonotic dermatophytes, Microsporum canis is the most common worldwide
• Other zoonotic dermatophytes include Trichophyton benhamiae (guinea pigs), Trichophyton erinacei (hedgehogs), Trichophyton quinum (horses), Trichophyton mentagrophytes (rodents), Trichophyton quinckeanum (mice), Trichophyton simii (monkeys), Trichophyton verrucosum (cattle ringworm), Nannizzia nana (pigs), and Nannizzia persicolor (bank voles)

Geophilic

• Uncommon causes of disease
• Nannizzia gypseum is more common in western Pacific and Central America
• More common in gardeners and farmers

Clinical Presentation

• Ringworm
  • Most active form is in the periphery of the ring
  • Can invade immunocompromised patients, e.g. Majocchi's granuloma and kerion
• Named by anatomic location
  • Tinea capitis: head
  • Tinea corporis: body
  • Tinea cruris: groin
  • Tinea unguun: nails

Diagnosis

Specimen collection

• Hair: need the root, with sterile forceps
• Skin: disinfect with alcohol or sterile water, then scrape the border with a blunt scalpel
• Nails: alcohol gauze to clean, then scrape subungual debris

Laboratory Testing

• Direct microscopy: KOH (potassium hydroxide) calcofluor of skin scraping, under UV light
• Culture with Littman agar w streptomycin, Sabouraud agar with chloram+gent, Sabouraud agar with CG+cyclohexamide
  • Incubate for 2-3 weeks at 30ºC

Indentification

• Chemical properties
  • Growth at 37ºC
  • Urease production/urea hydrolysis
  • BCP-MSG reaction (important)
  • Hair perforation test
  • Vitamin requirements with trichophyton agars T1 to T7
  • Growth on polished rice grain
• Colony features
  • Speed of growth
  • Colour on surface
  • Texture
  • Colour on reverse
• Microscopic features
  • Macroconidia, microconidia, septae