Many cases of M. canis
infection fluoresce apple green when
illuminated with a Wood's lamp (ultra-violet light, 365-366 nm). However, this
technique requires some experience to be interpreted correctly, and obviously
will detect all cases of dermatophytosis.
Hairs plucked from the periphery of a
lesion can be examined by direct light microscopy, after incubation in 10% potassium
hydroxide, for chains of arthrospores along hair shafts or in attached scale
material. However, technique also requires some experience if its results are
to be interpreted correctly.
Definitive diagnosis depends on culture
in Sabaraud dextrose agar or Dermatophyte Test Medium (DTM) and identification
of any organisms grow, by colony morphology and microscopic examination of their
spore producing macroconidia.
Samples for culture may be plucked hairs
and scrapings from the edge of lesions. The lesions should first be clipped and
then patted with an ethanol-soaked swab to reduce bacterial contamination. For
animals without clinical signs, careful brushings taken with a sterile toothbrush
(soak in 0.1% chlorhexidine solution for 30 minutes) are ideal; the bristles
are directly pressed into the agar.
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