Advances In Treatment Of Chronic Otitis Externa And Media In The Dog

Terese C. DeManuelle, DVM, Diplomate ACVD
Allergy & Dermatology Veterinary Referral Center, Milwaukie, OR
Adjunct Professor of Dermatology,
Washington State University School of Veterinary Medicine, Pullman, Washington

 

INTRODUCTION

Many dogs with otitis externa require lifelong management of their disease.  Client education is an important part of the treatment plan or owners will become frustrated.  Long term maintenance therapy may be necessary.  Owners also must be educated about the complexity of chronic ear disease and realistic expectations.  Otitis is a pattern of cutaneous disease that has predisposing factors, perpetuating factors, primary causes and secondary causes.   

PRIMARY CAUSES

Primary causes of otitis externa include atopic dermatitis (most common allergic cause of otitis), food allergy, and ear mites (Otodectes).  Otitis externa is the only clinical sign in 3-5% of atopic dogs.  One study revealed that 87% of atopic dogs have historical or physical evidence of otitis.  In uncomplicated allergic otitis externa, inflammation may be limited to the vertical canal only.

Over 20% of dogs with food allergy manifest their disease with otitis externa only.  Labrador Retrievers and Cocker Spaniels are two breeds that more commonly present with food allergy symptoms manifested as otitis externa only. In dogs with food allergy, 80% have manifestations of otitis externa in conjunction with other dermatologic disease.  Ear disease may be unilateral. Food allergy should be a primary differential in young dogs presenting with otitis externa and in older dogs with an acute onset of otitis without previous history of otic disease.

Flea allergy does not usually cause otitis externa, but may be found in young flea allergic dogs or those with “total body disease”.       

Hypothyroidism allows an increase in mucin deposition in the dermis, epidermal hyperplasia, hyperkeratosis and alterations in cutaneous fatty acid composition.  Fatty acids can be irritating to the ear canal epithelium. 

Ear mites may be missed on otic examination and mites abandon the severely inflamed ear.  Prophylactic treatment for ear mites is essential to be certain that mites are not a component of chronic ear disease.  Systemic therapy is safer and easier with the approval of selamectin (Revolution, Pfizer). Selamectin is very effective for eliminating and preventing Otodectes.  Ivermectin continues to be used by veterinarians, primarily because of cost considerations, however, it is not approved for treatment of ear mites. 

SECONDARY CAUSES

Common secondary causes of otitis include Pseudomonas, Malassezia otitis media, and topical irritant reactions.  Therapeutic ingredients in ear medications such as propylene glycol may induce inflammation in the previously damaged aural epidermis.  Treatment of topical irritant reactions consists of discontinuation of topical therapies or limiting topicals to saline or aqueous based products.  Systemic therapy for topical irritant reactions is preferred.  Topical irritant reactions should be considered any time a case of otitis fails to respond or worsens with therapy.

Malassezia

Malassezia otitis externa is best treated with topical miconazole at 1% or higher concentrations.  Efficacy of miconazole may decrease when combined with other topicals because of a dilutional effect.  Malassezia otitis media is not uncommon and is treated with systemic antifungals such as ketoconazole (10 mg/kg once daily) or itraconazole (10 mg/kg once daily).

Pseudomonas

Pseudomonas otitis externa is usually sensitive to polymyxin B, ticarcillin, or enrofloxacin.  Polymyxin B is inactivated by purulent debris and must be applied only in clean ears.

 Previously, veterinarians have compounded enrofloxacin with a saline base (4 cc of 22.7 mg/ml enrofloxacin (Baytril, Bayer) with 8 to 12 mls of another liquid.  Topical enrofloxacin may be effective even when sensitivity indicates a resistance because of the greatly increased concentratration achieved when used topically.  Silver sulfadiazine (2%) solution is effective against Pseudomonas spp as well as Malassezia pachydermatis.  Enrofloxacin (5 mg/ml) combined with 1% silver sulfadiazene has recently been FDA-approved and is now available commercially (Baytril Otic®, Bayer).  The formulation should be effective for severe otitis externa when Pseudomonas and Malassezia organisms are involved.  The formulation does not contain corticosteroids.

Acetic acid (2%) is effective against Pseudomonas after 1 minute of contact time.  Higher concentrations of acetic acid may be irritating.  Aluminum acetate is also effective when used topically for Pseudomonas.  

PERPETUATING FACTORS

Perpetuating factors include inflammation and stenosis of the aural canal.

Chronic aural inflammation initiates development of numerous skin folds. The skin folds along with the inhibition of the normal cleaning mechanism of the ear canal results in perpetuation and protection of secondary microorganisms.  Folds, fibrosis and swelling lead to stenosis of the canal lumen. 

  EVALUATION OF CHRONIC OTITIS

             Dogs with chronic recurrent otitis externa should be evaluated for otitis media.  This requires heavy sedation or general anesthesia to properly evaluate the external ear canal and middle ear.  Procedures required may include MRI/CT evaluation, otoscopy, otic flushing, pneumo-otoscopy, tympanometry, impedance audiometry, acoustic reflex testing, endoscopy and myringotomy.  In a recent study, the optimal indicator of otitis media in the dog was by myringotomy.  This study compared myringotomy to several other techniques including bulla radiographs, otoscopy, pneumotoscopy, tympanometry, acoustic reflex testing, and endoscopy.  The study did not evaluate the sensitivity and specificity of magnetic resonance imaging or computer tomography evaluation to diagnose otitis media.  Vestibular syndrome or deafness may occur after otic flushing, even when no ototoxic drugs are utilized.  These side effects are uncommon.

CULTURE AND SENSITIVITY TESTING

                  The primary indication for culture and sensitivity testing is otitis media or severe proliferative changes with bacterial rods when systemic therapy is indicated.   Cytologic evaluation in these cases usually demonstrates white blood cells.  Because multiple potentially pathogenic organisms can be cultured, it is important to combine cytologic examination with culture results.  The bacteria that appear most numerous may be determined and it allows the clinician to choose a more appropriate antibiotic regimen for the patient.  Laboratories that report antibiotic sensitivities with MIC information rather than the standard Kirby-Bauer susceptibility test allow for better determination of antibiotic dosages.  A recent study reported that 60% of Pseudomonas organisms described as intermediate or sensitive to enrofloxacin by Kirby-Bauer testing were resistant on MIC testing.  

SYSTEMIC ANTIMICROBIALS

                Systemic therapy is indicated for otitis media, severe otitis externa, topical irritant reactions, poor response to topical therapy, when marked proliferative changes are present and when owners cannot administer topical treatments.  Antibiotics that are known to penetrate bone, that are concentrated within inflammatory cells, or that have an excellent record in treatment of otitis media should be selected and given at doses that are at the high end of the recommended dosage range.  Antibiotics and antifungals should be used for 14 days after a complete clinical cure.  Examples of antibiotics that are useful for otitis media and proliferative otitis include: cephalexin, 22 mg/kg q 12 hrs; clindamycin (Antirobe®, Upjohn) 7 - 10 mg/kg q 12 hrs; sulfadimethoxine-ormetoprim 55 mg/kg day 1 and 25 mg/kg q. 24 hour on subsequent days; enrofloxacin (Baytril®, Bayer) 5-20 mg/kg q 24 hrs; and orbifloxacin (Orbax®, Schering-Plough) 2.5-12.5 mg/kg q 24 hrs.  In general fluoroquinolones are needed at higher doses for Pseudomonas aeruginosa infections such as 10-20 mg/kg enrofloxacin q 24 hrs; 5-12.5 mg/kg orbifloxacin q 24 hrs.  Marbofloxacin (Zeniquin®, Pfizer) at 5 mg/kg q 24 hrs has shown encouraging results for treating Pseudomonas otitis.  

OTITIS MEDIA, OTIC FLUSHING AND MYRINGOTOMY

            If the tympanic membrane is intact and otitis media is suspected, a myringotomy is necessary.  Evidence of inflammation to the tissues surrounding the middle ear or the inner ear usually indicates that otitis media has occurred. A recent study demonstrated that 71% of dogs with chronic otitis externa also have otitis media accompanied by an intact tympanic membrane.  The study also found that the tympanic membrane appeared intact but abnormal when completely visualized by fiber optic video enhanced otoscopy in dogs with otitis media.  The entire tympanic membrane is difficult to completely visualize by otoscopic evaluation.  Myringotomy allows sampling of the middle ear cavity and has been recommended as the best method of detecting otitis media.  After the myringotomy, the middle ear is flushed with warm sterile saline (200-300 mls).  The method of choice for flushing the ear canal when the tympanum is ruptured utilizes video enhanced endoscopy.  This technique allows visualization of anatomical structures and ensures a safe flushing technique.  The more delicate structures are located in the middle and dorsal areas of the middle ear cavity.  A serous, slightly sanguineous discharge may be observed for a few days after myringotomy.  Postoperative analgesia should be provided for at least 2-3 days.  Usage of NSAID’s (Rimadyl, Pfizer) at the recommended label dosage is an effective analgesic, however, NSAID’s cannot be used in conjunction with corticosteroids.  Commercially available topical drops or ointments should not be used for 5-7 days post-myringotomy.  Damage to the facial nerve can occur as a result of otitis media or when cleaning, medicating, or performing diagnostic procedures in an ear with a ruptured tympanum.  A missed diagnosis of otitis media often leads to treatment failure. 

INTRALESIONAL THERAPY

            Stenosis of the vertical canal occasionally may be nonresponsive to systemic corticosteroids to reduce the proliferative tissue.  In these uncommon cases, intralesional triamcinolone acetonide (4 mg/ml) may be helpful in widening the vertical canal lumen.  Triamcinolone acetonide is effective in inhibiting fibroblasts and reducing collagen production.  After cleaning the ear, multiple 0.1 cc injections are made as deep as possible in the canal.

SURGICAL INTERVENTION

             In some dogs with chronic recurrent otitis externa, surgical intervention is indicated.  Instances that may require surgery include:

(1)     Progressive pathologic changes of the ear (calcification, proliferation, and stenosis) that result in permanent ear canal occlusion that is nonresponsive to intralesional therapy;

(2)     Otitis media that fails to respond to myringotomy, ear canal flushing, and aggressive medical management;

(3)     Inadequate response of otitis externa to medical management (oral antimicrobials, oral corticosteroids, topical antimicrobials, topical corticosteroids) due to poor owner compliance or the presence of a resistant organism.

In summary, management of chronic recurrent otitis externa requires diagnosis and control of the predisposing factors, primary causes, and perpetuating factors.  Cleaning the ear canals and middle ear along with the use of oral antimicrobials, oral corticosteroids, topical antimicrobials, and topical corticosteroids may be necessary.  Abnormal epithelial migration can occur in response to inflammation.  Dogs with otitis media may have adnexa and stratified epidermis growing in the middle ear, which is believed to have migrated from the external ear canal.  Since permanent changes in the anatomy of the ear canal may be present, lifelong ear cleaning and drying agents may be required to prevent relapse.