Image Quiz: Dermatology-Patchy alopecia in a Chihuahua

Microsporum canis is correct!

Microsporum and Trichophyton species exhibit white, light-yellow, tan, or buff-colored cottony-to-powdery-appearing colonies on dermatophyte culture plates, usually after about seven to 10 days. On microscopic examination, M. canis spores are large, spindle-shaped, and thick-walled with six or more internal cells. While not evident in this photograph, they often have a terminal knob. Microsporum gypseum spores are also large and spindle-shaped, but they have six or fewer internal cells, thinner walls, and no terminal knob. Trichophyton mentagrophytes have cigar-shaped macroconidia and globose microconidia. Microsporum nanum is a rare cause of tinea in people and a frequent cause of ringworm in its natural reservoir, the pig.

In this case, identifying the causative dermatophyte species was important, as the puppy resided in a household with multiple dogs and cats. Results of dermatophyte cultures of all the pets revealed that one of the cats was an asymptomatic carrier of M. canis. The puppy and carrier cat were separated from the other pets, and both were treated with weekly lime sulfur dips and daily oral fluconazole until two negative dermatophyte culture results were obtained.