VMC 311: Systematic Veterinary Bacteriology and Mycology (2+1)

DIAGNOSIS

Diagnosis

• Based on Direct Examination
  • Fluorescence of hairs is useful for identification of hairs that may be infected with dermatophytes.
  • In 50% to 70% of cases, hairs and skin scales infected with M. canis or M. audouinii may emit a bright greenish fluorescence under ultraviolet light
• Based on Microscopic examination
  • Skin scrapings and hair are examined microscopically for the presence of hyphae and arthroconidia.
  • The scraping should include material from the margins of any lesion and the full thickness of the keratinized epidermis.
  • The hair is plucked, so as to include the intrafollicular portion.
  • The sample is placed on a slide, flooded with 10% to 20% KOH or NaOH , with a cover slip, and heated gently.
  • Microscopic examination should begin under low power and subdued light.
  • Infected hairs are encased in an irregular sheath of arthrospores that may double their normal thickness.
  • At higher magnification of such hairs, individual, spherical arthroconidia are recognizable.
  • In hairless skin, branching hyphae and chains of arthroconidia occur.
• Based on culture
  • Hair and skin scrapings should be inoculated to dermatophyte test medium (DTM) and Sabouraud dextrose agar medium, with and without cycloheximide and chloramphenicol, and incubated at 25-30º C for up to 4 weeks. Samples suspected of containing T. verrucosum are incubated at 37ºC.
  • The DTM medium will turn red as the dermatophyte is growing, and the fungus itself will usually be hyaline and fluffy.
  • Although selective, other molds may grow on DTM and therefore idenification should be confirmed by microscopy.
  • Very long, narrow hyphae with distinctive shapes and micro or macroconidia are indicative of dermatophyte infection.

Differential diagnosis

• Differentiate from Insect bites, Urticaria, bacterial infections, seborrheic dermatitis and interdigital dermatitis.