This image of the fungus Microsporum canis was obtained from a skin scraping of a patient with ringworm on the neck acquired from her infected cat. The fungus is identified as a dermatophyte by this calcofluor stain of the skin scrapings viewed at 500X magnification. The calcofluor dye binds to the chitin in the fungus and fluoresces under a fluorescent light.

Microsporum canis, a fungus, produces septate hyphae (as seen in image)and macroconidia (not seen in image), which are spindle-shaped, with an asymmetrical apical knob. The sporangium is 6 to 15 celled, long, rough and have thick outer cell walls (http://www.doctorfungus.org/thefungi/microsporum_canis.htm). M. canis can be grown on Sabouraud’s dextrose agar, and the texture is woolly to cottony and flat to sparsely grooved (http://www.mycology.adelaide.edu.au/). The color is white to yellowish when viewed from the top of the plate and deep yellow to yellow-orange from the reverse.

M. canis is a zoophilic dermatophyte of worldwide distribution and a frequent cause of Tinea corporis, more commonly known as ringworm, in humans, especially children. It invades the hair, skin, and rarely nails (http://www2.provlab.ab.ca/bugs/webbug/mycology/etiol.htm). Ringworm, or tinea corporis, is classically signaled by lesions which have active, erythematous (reddish) and spreading borders with central clearing. The infection is limited to the stratum corneum (uppermost layer) of the epidermis (www.diseasesdatabase.com). Tinea corporis can be present on any area of the body. Transmission of Tinea corporis may occur from direct contact with infected animals, such as cats and dogs, which are the main sources of infection, infected humans or contaminated fomites, such as clothes and furniture. Like many other fungal skin infections, warmth and humidity favor the occurrence of the infection. The natural habitats of the fungus are: animals, humans and soil (http://www.doctorfungus.org/thefungi/microsporum_canis.htm).

The main diagnostic tool is to look for the characteristic ringworm rashes and lesions on the skin. The key characteristic and diagnostic tool of tinea corporis is that the fungus involves the glabrous (relatively hairless) skin. Diagnosis may be made by microscopic examination of biopsied skin. Topical antifungal preparations may be used to treat ringworm.

Legend written by:
Rebecca Klein
Colorado State University
Fort Collins, Colorado