Otitis externa is a common disorder affecting 10% to 20% of dogs and 2% to 6% of cats seen by veterinarians [1-3]. Although not a directly life-threatening condition, otitis diminishes the quality of life of affected patients. Failure to manage otitis externa appropriately often results in recurrent pain, inflammation, and infection, eventually progressing to chronic end-stage disease. Successful management of otitis in veterinary patients requires a detailed understanding of the multifactorial pathogenesis of the disorder. In each case, veterinarians should pursue two goals vital to effective treatment of otitis externa: identification and management of the primary cause (eg, atopy, adverse food reaction, Otodectes cynotis, neoplasia, foreign objects) and identification and management of concurrent perpetuating factors (eg, bacterial or yeast infection, edema, glandular hyperplasia, disruption of normal epithelial migration, otitis media) [4,5]. Simply chasing infection and inflammation without addressing the primary disease consistently results in treatment failure. Likewise, elimination of the primary disease without effective management of concurrent infection does not result in satisfactory resolution of clinical signs. Cytologic evaluation of otic exudate is a simple, practical, and inexpensive diagnostic test that assists veterinarians pursuing both goals. Additionally, the information derived from cytology is available to the veterinarian immediately, permitting rational therapeutic decisions at the time of the initial consultation. The principal value of otic cytology is identification and characterization of microbial overgrowth or infection that contributes to clinical signs and perpetuates inflammation. This information strengthens interpretation of culture and susceptibility data, guides rationale therapeutic decisions, and permits more accurate monitoring of response to treatment. Cytology may also assist in diagnosis of some of the primary causes of otitis externa, although its value in this regard is limited relative to other diagnostic tests, such as specific allergy testing, elimination diet trials, and parasite treatment trials. Practitioners may be tempted to make conclusions based on the odor and appearance of the otic exudate rather than on cytology. For instance, O cynotis is classically associated with dry, grainy, black discharge, sometimes described as "coffee grounds." In contrast, yellow or light brown discharge is reported to indicate bacterial infections, whereas waxy honey-colored or brown exudate is associated with Malassezia [6]. Unfortunately, these observations are not consistent or reliable [7,8]. Veterinarians are cautioned to avoid relying on the physical character of discharge, odor, or past experience when making a diagnosis or selecting therapy. Rather, these decisions should be based on evidence established by careful microscopic evaluation of exudate. Failure to do so may result in inappropriate use of antimicrobial agents, failure to recognize and treat relevant pathogens, and an inability to monitor changes in pathogens on subsequent examinations. The ultimate result is poor quality of case management, prolongation of treatment, or even treatment failure and progression of disease. Veterinarians are encouraged to view cytology as a mandatory test for every patient presenting for clinical signs of otitis externa.
