Canine otitis externa is one of the most common dermatologic problems encountered in small animal practice. Otitis may be an extension of a whole body skin disease (eg, allergic skin disease), due to unresolved overgrowth of microbes, or caused by problems limited solely to the ears (eg, foreign bodies). However, the core diagnostic approach is the same: otic examination, dermatologic examination, and otic cytology. After these core diagnostics are completed, therapeutic measures and/or additional diagnostic testing can be planned. The most important core diagnostic test is otic cytology. In this study, 83 dogs with either acute or chronic unilateral or bilateral otitis externa had each affected ear swabbed twice using separate cotton-tipped swabs. The samples were rolled onto a glass slide, labeled, air dried, and stained with a modified Wright’s stain. For this study, 2 parallel smears were made on each slide (2 from each swab). Initial examination was done at 400× to identify representative fields; then 6 representative fields were examined under 1000× magnification from each parallel smear. Of 83 dogs, 68 had bilateral otitis and 15 had unilateral otitis; a total of 151 ears were examined. Yeast was found in 141 ears; in 129 ears (91%), yeast was found in both samples. Cocci were found in 123 ears; in 98 (80%), they were found in both samples. Rods were found in 19 ears; in 11 (58%), they were found in both ears. A Wilcoxon’s matched pair test found no statistical difference between the first and second samples with respect to number of microorganisms. A statistical test for qualitative agreement of the 2 successive swabs (negative or positive) was determined using a k test. For rods and cocci, there was substantial agreement between the 2 swabs and moderate agreement for yeast. Rods were never found without the presence of other organisms and there was a trend for more organisms to be found on the second swab; however, the latter was not statistically significant.
Commentary: There is little question that ear swab cytology is valuable in the management of canine otitis externa. It is an easy and inexpensive in-house diagnostic test that can yield a great deal of information regarding initial treatment, necessary next-step diagnostic tests (ie, ear culture), and/or monitoring of treatment. The findings in this study are important in a clinical setting because they demonstrate that repeat swabbing of the ear for diagnostic specimens provides reproducible results. In other words, one does not have to ponder whether the first specimen from the ear should be a culture swab or a cytology swab. You can take a swab for cytology to examine and, if a culture is needed, it can be obtained with confidence that the culture sample will be representative of the cytological specimen.
Reproducibility of ear cytology in dogs with otitis externa. Lehner G, Sauter Louis C, Mueller RS. Vet Rec 167:23-26, 2010.