Microsporum canis is a cosmopolitan zoophilic dermatophyte usually acquired from infected dogs (hence canis) or cats. Most prevalent in children, it has been implicated in infections of the scalp and skin and occasionally nails. A scalp infection could be visualized by examining plucked hairs under a Wood’s (UV/black light) lamp for fluorescence.
Surface growth has been described as downy to woolly, to fluffy, hairy and silky. Typically it exhibits a light yellowish pigment at the periphery and growth shows closely spaced radial grooves. The reverse is pale tan to yellowish (yellowish-orange –media dependent) which tends to turn brownish as it ages.
Microsporum canis has septate hyphae that produce numerous macroconidia. The macroconidia are rather long (10-25 X 35-110 µm), spindle or fusoid in shape and are thick walled with a echinulte (rough) texture. The ends typically taper to a knob-like end that may be somewhat recurved at the apex. The macroconidia usually have six or more compartments when mature and few smooth walled club shaped macroconidia may be observed along the hyphae. Smooth walled, club shaped microconidia are infrequently seen forming along the length of the hyphae.
(all photos taken with DMD-108 digital microscope except where noted)
Microsporum canis growing at edge of cover slip from a slide culture preparation (LPCB X100)
(Click on any photo to enlarge for better viewing)
Microsporum canis - Macro & microconidia (LPCB X400)
(note 100 µm bar at upper right of photo)
Microsporum canis needs no special growth factors or cultural requirements. I found that on the relatively nutritious Sabouraud Dextose agar, my isolate became sterile on repeated subcultures. Macroconida were produce when grown on Corn meal agar. The macroconidia are produced as a survival mechanism and are induced when conditions are not as rich or favourable for growth.
Microsporum canis can be inoculated onto sterile (autoclaved) polished rice grains where they produce a yellow pigment.
A hair perforation test (positive) can be performed in vitro. As previously stated, hairs infected with Microsporum canis will fluoresce under a Wood’s UV lamp.
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*10% KOH - Potassium Hydroxide, kills and clears the preparation.