Feline Hair Loss

A recently adopted, long-haired, 2-year-old male neutered cat is presented for unilateral hair loss on the margin of the left pinna.

History. Other than this recent problem, the cat has been healthy. The hair loss on the margin of the left pinna of the ear (Figure 1) has been present for the past 6 weeks. The owner noticed the cat scratching at its ears a few days ago. The cat does not go outside but does have access to a screened, three-season porch. There are no other pets in the household, and the owner has no skin lesions.

Physical Examination. Patchy hair loss is present on the margin of the left pinna. Careful examination of the ear reveals that the hair loss is spreading to the caudal aspect of the pinna.

Laboratory Work

Skin scrapings/mineral oil examination of hair shafts: negative for mites; note hair in Figure 2Flea combings: negative for ectoparasitesWood's lamp examination: negative for fluorescenceEar swab cytology: normal; no organisms seen14-day-old fungal culture plate; a dual plate with dermatophyte test medium (DTM) culture and plain Sabaroud's dextrose agar: toothbrush fungal culture from ear pinnae (Figure 3)

Diagnosis. Microsporum canis dermatophytosis. The gross colony morphology on the DTM plate is compatible with M. canis-the most common cause of dermatophytosis in cats. M. canis colonies produce a yellow color change on the fungal culture. This is visible if you turn the culture plate over and look at the bottom. This is commonly referred to as a "reverse yellow color." The gross colony morphology is pale white. The red color change on the Sabouraud's dextrose agar and DTM is suggestive of a dermatophyte but is not diagnostic. Many species of contaminants can mimic dermatophytes both in gross morphology and by causing a red color change on the DTM. Definitive diagnosis requires microscopic examination of the fungal culture colonies, which is done using lactophenol cotton blue stain. Positive identification of M. canis is made by finding spindle-shaped bodies of M. canis macroconidia with thick walls comprising many cells (Figure 4). Finding hyphae and/or ectothrix spores on hair shafts is diagnostic of dermatophyte infection. In this case, the toothbrush fungal culture technique was used to obtain a haircoat sample. Although the visible lesions were localized to the ears, it is important to note that the infection is generalized; thus, the cat requires systemic antifungal treatment and ideally concurrent topical antifungal therapy.

"Clearing agents," such as potassium hydroxide or chlorphenolac (a custom compound clearing agent), can be used to help visualize hyphae and spores. These agents cause the cell debris in the background to swell and become less visible. The hairs, spores, and hyphae become more refractile, making it easier to identify them. However, a skilled clinician can identify hyphae and spores using mineral oil for a mounting media without needing to clear the specimen. In this case, the ectothrix spores and infected hair shaft were an incidental finding on the skin scraping. In practice, it wastes time and money to directly examine hair or hair shafts, unless you are examining Wood's-positive hairs.

After proper staff training, microscopic diagnosis of dermatophytosis via fungal culture and/or direct examination of hairs from lesions can be done in-house. Diagnostic laboratories that specialize in mycology and/ or veterinary pathogens should be used if your facility lacks the in-house expertise to examine and monitor fungal culture plates.

ASK YOURSELF ...

• Is the fungal culture plate by itself diagnostic?• Is the direct examination of the hair shaft by itself diagnostic?• Does the negative Wood's lamp examination confound the diagnosis?

DID YOU ANSWER ...

• No. DTM is not diagnostic for a dermatophyte infection. The diagnosis must be confirmed by microscopic examination of fungal colony growth.• Yes. Direct examination of the hair and/or hair shaft is diagnostic by itself. Hyphae invasion of the hair has occurred, and an external cuff of ectothrix spores is present.• No. A negative Wood's lamp examination does not rule out M. canis infection.