Although there are many pathogenic dermatophytes, the 3 most common pathogens encountered in small animal practice are Microsporum canis, M gypseum, and Trichophyton.
On occasion, other pathogens may be isolated but this is rare. The use of mediums such as dermatophyte test medium (DTM) is encouraged because it contains a color indicator and antimicrobial agents that inhibit most, but not all, contaminant growth. The number of colonies that need to be sampled and examined microscopically can be limited by focusing on white- to buff-colored colonies producing a red color change in the medium; the red color change must occur simultaneously with the appearance of grossly visible colony growth. Plates should be examined daily-once the entire plate turns red, the value of the color indicator as a "red flag" is gone. Some contaminants will have the right gross appearance and produce the simultaneous red color change, so all suspect colonies should be checked microscopically. M canis, M gypseum, and Trichophyton have rather characteristic microscopic morphologies; however, if help is needed in identification, see www.doctorfungus.org. For future use in identification, pictures of organisms can be photographed directly through the microscope eyepiece using a digital camera. Many of the pictures in this article were taken with this technique.