Otitis is one of the most common reasons dog owners seek consultation with their family veterinarian. There is no set standard of treatment as ear infections in dogs can have a number of underlying causes. Typically, a work up for otitis will not only include identification and treatment of any microorganisms creating active infection, but also, and arguably more importantly, investigation of an underlying cause.
Etiology
Triggers for otitis are often split into three categories: primary, predisposing, and perpetuating causes with some overlap of the above.
Primary causes are factors that directly initiate an inflammatory response within the external ear such as parasites, foreign bodies, keratinization defects (i.e. secondary to endocrine disorders), contact reactions, and most commonly allergic dermatitis. Veterinary dermatologists anecdotally report that up to 90% of cases of canine otitis externa are secondary to a hypersensitivity reaction, either cutaneous adverse food reaction (food allergy) or atopic dermatitis (environmental allergy). Keep in mind that otitis need not be bilateral to fall under the category of hypersensitivity. Autoimmune diseases, although can affect ear canals, are rare causes of otitis externa.
Predisposing causes are typically factors that create an environment that harbours increased risk for developing otitis externa. Classic examples are breeds of dogs with excessive hair growth within ear canals, and congenital anatomic features such as small, stenotic ear canals or canals prone to overproduction of cerumen. Masses, either benign polyps or malignant adenocarcinomas, also fall under this category when they create obstruction to air flow and impede normal epithelial migration. Finally, iatrogenic-induced inflammation from overly exuberant hair plucking or owner’s use of irritating cleaning solutions can additionally act as predisposing causes of otitis.
Lastly, perpetuating causes of otitis are factors which do not initiate inflammation, but once established, will cause ear disease to propagate unless specifically addressed. Staphylococcus pseudintermedius and Malassezia pachydermatis are common microorganisms found to over-colonize inflamed ear canals. Although these are considered normal inhabitants of the skin, they are still considered opportunistic pathogens when they overgrow and need to be treated as such. In cases where the tympanic membrane has ruptured, any bacteria or yeast found within the tympanic bulla can serve as a nidus of infection.
The tympanic membrane can occasional appear to be intact on otoscopic examination even in cases of otitis media, therefore in dogs with recurrent infections, it is important to consider middle ear disease as a potential perpetuating cause.
Clinical signs
In the acute phase of otitis externa, pet owners may report a variety of clinical signs including head shaking, face or ear rubbing, otic discharge, decreased hearing, or changes to ear carriage. In cases where owners may not observe or recognize signs of otitis, other evidence may include malodour, the presence of aural hematoma, or evidence of self-trauma such as excoriations around the base of the pinnae. With chronicity, hyperplasia, stenosis, and eventually calcification may lead to complete occlusion of the ear canals. Rupture of the tympanic membrane and otitis media are common sequelae. Dogs may exhibit pain or be “head shy” when erosions are ulcers develop within the ear canals from tissue maceration.
Basic diagnostic approach
Taking a thorough history can often reveal primary and/or predisposing factors. Knowing the patient’s lifestyle, such as whether or not they are a swimmer, how frequently they are groomed, etc. can reveal clues to the underlying problem. General health questions may lead to suspicion of a possible endocrinopathy. A physical examination will similarly reveal evidence in other body sites if the patient is allergic or has an autoimmune skin disease.
Despite having access to a multitude of topical otic medications that contain both antifungal and antibacterial properties these days, it is still essential that ear cytology be performed each and every time a patient is presented for a recheck exam following a diagnosis of otitis.
Cytology is a quick and inexpensive method of tracking your patient’s progress as well as a method for troubleshooting when a lack of response is observed. Culture and susceptibility testing for bacterial otitis externa is not considered a first-line test because sensitivity results reflect serum antibiotic concentrations. When ear medications are instilled into the ear canal directly, a much higher concentration is achieved. Culture results therefore do not influence topical therapy selection. However, this information is very useful when it comes to systemic management of otitis as adjunct therapy to topical therapy. It is appropriate, often desirable, to refer acute or complex otitis cases to a veterinary dermatologist as dermatologists are typically equipped with a video otoscope and have access to advanced imaging techniques for a more comprehensive work up, if needed.
In summary, dogs with chronic, recurrent otitis are one of the most frustrating conditions to treat in general practice. In order to be successful in managing these patients, the inciting cause must be identified and completely addressed along with the secondary infectious component. Combining cytologic findings with a thorough history and physical examination will provide a good starting point in formulating a systematic approach to ear management.
References:
Morris DO 2004. Medial therapy of otitis externa and otitis media. Vet Clin Small Anim 34: 541-55.
Paterson S, Matyskiewicz W, 2018. A study to evaluate the primary causes associated with Pseudomonas otitis in 60 dogs. J Small Anim Pract 59(4): 238-42.
Rosser EJ 2004. Causes of otitis externa. Vet Clin Small Anim 34: 459-68.
Saridomichelakis MN, Farmaki R, Leontides LS et al. 2007. Aetiology of canine otitis extnera: a retrospective study of 100 cases. Vet Dermatol 18(5): 341-7.
Zur G, Lifshitz B, Bdolah-Abrams T, 2011. The association between the signalment common causes of canine otitis externa and pathogens. J Small Anim Pract 52(5): 254-8.
Dr. Andrea Lam, DVM, DACVD is a Board certified veterinary dermatologist. Dr. Lam loves learning and educating, and has trained several dermatology interns as well as dermatology residents. Her research interests include novel management strategies for the treatment of canine atopic dermatitis and applications for stem cell therapy.
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