Cytologic Evaluation

The first stop in approaching ear disease is to examine a cytologic preparation of the otic exudates. Examination of a prepared ear smear gives the clinician a starting point for treatment based on the presence of yeasts, cocci, or rod bacteria.

Obtaining samples and preparing a slide to examine constitute a simple procedure that should be a part of the minimum approach to every case of otitis presented to the veterinarian. A sample is obtained with the use of a small cotton-tipped applica­tor. The swab is inserted through a disinfected otoscope cone positioned near the horizontal canal. The swab is extended beyond the plastic cone, and pressure is applied to the ear canal epithelium as the swab is drawn back through the cone. Every attempt is made to sample from the horizontal canal epithelium only because the vertical canal is often contaminated with a number of commensal organisms unrelated to the ear disease.

The material collected on the swab is rolled onto a clean microscope slide, with the exudates from the left ear on the left part of the slide and the sample from the right ear on the right side. The slide is appropriately labeled with the patient’s name, the date of the collection, and which sample is from which ear. It is then heat fixed and stained with a modified Wright’s blood stain. A drop of slide-mounting medium is placed over the dried, stained material. Then a coverslip is placed on top of the drop, and the glue is allowed to set. The use of slide-mounting medium makes a permanent record of the cytologic characteristics, which can be stored for compari­son at subsequent examinations. Alternatively, immersion oil can be smeared along the stained slide and examined.

Examination of the slide under low-power magnification allows an overall view of the cellular debris. High-power examination can help identify and quantify organ­isms.

When infectious organisms are seen under high-power (400 ?) magnification, cocci are usually Staphylococcus, and rods are usually Pseudomonas or Proteus. Budding yeasts of Malassezia may be seen individually in the background on a roll smear, but large numbers of yeasts colonizing on exfoliated epithelium indicate secondary yeast infection. Staphylococcus and Malassezia are often found together in the same ear, and there is evidence to suggest that Malassezia growth is stimulated by Staphylococcus.

Bacterial culture and sensitivity testing of exudates may be useful in cases of resistant otitis externa. Many organisms have developed resistance to the routinely used antibiotics, and they should be identified. A limitation of bacterial sensitivity testing is that the organism’s sensitivity or resistance is reported by the laboratory on the basis of the minimum inhibitory concentration (MIC) of the antibiotic in the blood required to kill the bacteria. Topical antibiotics can achieve significantly higher concentrations in the ear canal than systemic antibiotics can in the blood. The high topical antibiotic concentration may actually be effective at killing a bacterium that was reported as resistant. Samples for culture should be taken from the hori­zontal canal if possible, so that contaminant bacteria are not mistaken for the offend­ing organism. Routine culture of exudates in all cases of otitis is often unrewarding and extremely misleading because four or five bacterial isolates are often reported.

Roll-smear cytologic evaluation becomes useful in determining the etiology of cases of otitis externa without secondary infection. Sheets of epithelial cells may indicate neoplasia as the cause of otitis externa, and the presence of numerous intact nonstaining epithelial cells may indicate a seborreic condition. Parasites such as Otodectes and Demodex, which may be present in the ceruminous exudates, may be found on cytologic preparations, but these parasites are better viewed on a direct mineral oil preparation; this type of preparation is made by rolling the ear swab sample in a drop of mineral oil on a slide. Negative findings on cytology may indi­cate the need for antiinflammatories only, without the antibiotic or antifungal therapy.