Understanding macroscopic fungal colony morphology is an important first step in determining whether a dermatophyte is present.
Microsporum and Trichophyton species—the most common dermatophytes in dogs and cats—are white, light yellow, tan, or buff-colored cottony-to-powdery-appearing
colonies (Figures 3 & 4). Dermatophyte colonies are never black, green, or gray.
Figure 3. The dermatophyte culture plate exhibits Microsporum canis growth (the white-to-pale-yellow fungal colonies at the top of the culture plate) that is at risk of being overgrown by the
gray saprophytic fungal colonies on the bottom of the plate. Daily fungal culture observation with or without sampling suspect
dermatophyte colonies and inoculating them on a new culture plate is important to ensure that saprophytes do not overgrow
the dermatophytes and potentially cause a false negative culture result.
Additionally, with positive dermatophyte culture results, determining the number of macroscopic colonies gives you information
about the severity of infection and, in animals undergoing antifungal treatment, information about the response to therapy.4
Figure 4. Trichophyton mentagrophytes culture often produces a white-to- cream-colored powdery surface. This culture plate has been incubated with inadequate humidity,
causing cracking and separation of the culture media on the right side.
Microscopic evaluation of suspect fungal growth is also important since some environmental fungi can mimic dermatophytes in
gross colony morphology and in their ability to turn the media red1 and because some strains of Microsporum canis may not produce media color change.6 Microscopic examination can be done in the clinic, or the entire culture plate can be sent to a reference laboratory for
fungal identification (usually at a reduced cost compared with fungal culture).
Microscopic identification process
Because the organisms are zoonotic, wear gloves to avoid transmitting dermatophyte spores to your hands. Gently touch a small
piece of clear acetate tape to the surface of the fungal colony, and then apply the tape to a glass slide over a drop of blue
stain (methylene blue, lactophenol cotton blue, or the blue Diff-Quik solution [basophilic thiazine dye]) (Figure 5). Examine the slide under 100X to 400X magnification to identify the characteristic dermatophyte macroconidia.
Figure 5. To obtain a sample for microscopic fungal identification, touch the tape to the top of the fungal colony and then
carefully apply the tape to a slide on top of a drop of blue stain.
In the early stages of growth, only fungal hyphae with no macroconidia may be seen, especially in cases of Trichophyton species infections. Incubate these cultures longer to allow spore development for more reliable identification.
Microscopic dermatophyte characteristics
Microsporum canis spores are large, spindle-shaped, and thick-walled with six or more internal cells (Figure 6) and often have a terminal knob. If M. canis is identified, then other animals in the household should be screened via dermatophyte culture using the toothbrush technique
to determine whether they are asymptomatic carriers. All pets with positive culture results should be treated with topical
antifungal therapy, with or without systemic treatment. Culture-positive animals should be isolated from culture-negative
animals if possible.
Figure 6. Microsporum canis macroconidia and fungal hyphae (Diff-Quik, 400X).
Microsporum gypseum produces large spindle-shaped spores with thin walls, no terminal knob, and six or fewer internal cells (Figure 7).
Figure 7. Microsporum gypseum has numerous macroconidia with no terminal knob and thinner walls and fewer internal cells than M. canis has (Diff-Quik, 400X).
Trichophyton mentagrophytes produces long cigar-shaped macroconidia with thin walls (Figure 8). Spiral-shaped hyphae and numerous small grapelike clusters of microconidia are also characteristic of Trichophyton species (Figure 9).1
Figure 8. Trichophyton mentagrophytes is characterized by cigar-shaped macroconidia, which may be few in number, and numerous globose microconidia (Diff-Quik,
Saprophytic fungi will form hyphae and often small spores, but do not form macroconidia.
Figure 9. Spiral hyphae are often characteristic of T. mentagrophytes (Diff-Quik, 400X).
In cases in which the fungal species cannot be easily identified in the clinic, submit the dermatophyte culture to a veterinary
reference laboratory for fungal identification.