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61 MAY/JUNE 2017 ■ TVPJOURNAL.COM DERMATOLOGY DETAILS unilateral or bilateral otitis, but bilateral otitis is more common. Foreign bodies, neoplasia, and polyps are usually associated with unilateral otitis; however, bilateral problems have been reported. 3 Secondary Causes Secondary causes occur in combination with primary causes or predisposing factors. The most common secondary causes are infections. 1–3 Generally, secondary causes of otitis externa are easy to eliminate once identified. When they are chronic and difficult to treat, it is TABLE 1 Causes and Factors Associated With Otitis Externa in Dogs CAUSE EXAMPLES PRIMARY CAUSES: ACTUAL CAUSE OF OTITIS EXTERNA IN A NORMAL EAR Parasites Ear mites (Otodectes cynotis), demodicosis, sarcoptic mange, ticks Dermatophytes Microsporum canis, Trichophyton mentagrophytes Hypersensitivity disorders Environmental allergy, food allergy, flea allergy, contact allergy, drug reactions Keratinization disorders Idiopathic seborrhea, abnormal cerumen production Endocrine disorders Hypothyroidism, hypercortisolism, sex hormone imbalances Foreign bodies Plant material, hair, sand, hardened medication Autoimmune disorders Pemphigus complex, lupus erythematosus Glandular disorders Ceruminous, sebaceous, and apocrine gland hyperplasias; ceruminous cystomatosis Benign tumors Ceruminous gland adenoma, basal cell tumor, papilloma Malignant tumors Ceruminous gland adenocarcinoma, squamous cell carcinoma Non-neoplastic growths Inflammatory polyps Primary otitis media More common in brachycephalic breeds, such as Cavalier King Charles spaniel Idiopathic No identified cause despite attempt SECONDARY CAUSES: CONTRIBUTE TO OR CAUSE DISEASE ONLY IN AN ABNORMAL EAR Bacteria Various species: most common are Staphylococcus and Pseudomonas species Fungus/yeast Malassezia pachydermatis, Candida albicans (rare) PREDISPOSING FACTORS: INCREASE RISK FOR DEVELOPMENT AND PERSISTENCE OF OTITIS EXTERNA Conformation Pendulous pinna, congenital stenosis, excessive hairs in ear canal Breed disposition Cocker spaniel, German shepherd, poodle, basset hound Excessive moisture Swimmer's ear Immunosuppression Medications, immunosuppressive diseases Overtreatment Excessive cleaning and moisture, physical trauma (eg, cotton swabs) Respiratory viral diseases More common in cats and common cause of otitis media PERPETUATING FACTORS: RESULT FROM INFLAMMATION AND INFECTION AND PREVENT RESOLUTION OF OTITIS Progressive pathologic changes Altered epithelial migration, proliferation, hyperplasia, stenosis, and calcification Otitis media Infectious, sterile, proliferative, cholesteatoma, osteolysis, osteomyelitis Abnormal tympanic membrane Opacity, scarring, diverticulum, rupture, myringitis FIGURE 2. Otitis externa secondary to allergies.