Medical Treatment of Otitis Externa
World Small Animal Veterinary Association World Congress Proceedings, 2003
Stephen D. White, DVM, DACVD
Professor School of Veterinary Medicine, University of California
Davis, CA, USA

Treatment of the ear may be divided into three stages: ear cleaning with management of predisposing causes, treatment of perpetuating causes, and identification and treatment of primary causes.

Often, good results with ear cleaning can be obtained at home by the owners, but it is necessary that the veterinarian spend time with the owner showing the proper method of cleaning. Most of cleaning products contain various types of acids often with a ceruminolytic or alcohol added. The author prefers a product containing salicylic acid, lactic acid, and propylene glycol (Epi-otic®, Allerderm/Virbac). Cleansing/drying solutions are usually instilled into the ear once or twice daily, preceding any treatment solution also prescribed.

If home cleaning is ineffective, the ears will need to be cleaned under general anesthesia. Intubation is necessary to prevent aspiration of flushing fluids that can drain through the eustachian tube. Instilling the ear with a ceruminolytic agent greatly helps the cleaning process. The ear is subsequently flushed using a mild disinfectant solution (dilute chlorhexidine works well) or just saline solution. This solution should then be removed via suction. After the cleaning procedure has been performed, an otic examination is indicated to ensure that all remaining wax and debris are removed. The use of an ear curette is particularly helpful in removing wax.

Some predisposing causes of otitis externa are relatively easy to determine and remedy. Ear canal maceration caused by excessive moisture is best treated with one of the cleansing/drying solutions mentioned above. Treatment errors such as ineffective treatments or hypersensitivity-type reactions are easily monitored by periodic rechecks of the patient once a therapy is selected. Obstructive ear disease and conformation abnormalities are best dealt, when practical, with surgical removal or correction, respectively.

Perhaps the most overlooked perpetuating cause of otitis externa is otitis media. Ultimately diagnosis will depend on diagnostic imaging, preferably a CT scan; culture and sensitivity testing should be performed.

The infectious perpetuating causes of otitis externa such as bacteria and yeast are usually treated with the use of topical medications, instilled in the ear twice daily. For most cases of cocci or yeast (seen on microscopic examination of ear exudates) the following topicals are effective: Tresaderm® [neomycin, thiabendazole and dexamethasone ] MSD AGVET Merck; Otomax® [gentamycin, betamethasone, and clotrimazole ] Schering. Ear/Skin Cleanser® [boric acid, acetic acid] DermaPet.

In Pseudomonas infection the following topicals may prove helpful; however, such infections must be treated systemically as well: Gentocin® [gentamycin, betamethasone];Otomax® [gentamycin, betamethasone, clotrimazole] Schering TrizEDTA® [Tris-EDTA]; Ear/Skin Cleanser® [boric acid, acetic acid] DermaPet; Baytril otic [enrofloxacin, silver sulfadiazine] Bayer; Cortisporin® solution [polymyxin B, neomycin, hydrocortisone] Glaxo Wellcome; Silvadene Cream® [1% silver sulfadiazine], Marion Laboratories (dissolved in water at a 1:10 ratio.) Timentin® [Ticarcillin-Clavulanate] SmithKline Beecham. One protocol advanced for the conversion of this injectable antibiotic to a topical: Dilute according to manufacturer's directions, then draw into 2ml aliquots, and freeze. Thaw and use each aliquot as 0.5 ml in each ear, twice daily.

In moderate to severe cases of otitis externa, or when the owner is unable to instill medication into the ear (aggressive patient, ear canal swollen or occluded) systemic medications are invaluable. When a systemic antibiotic is needed, pending culture, marbofloxacin (5.5 mg/kg/once daily) or enrofloxacin (10-15 mg/kg given once daily may be used. For systemic anti-yeast medication ketoconazole, 5 mg/kg, given once daily (in dogs) or itraconazole 5 mg/kg, given once daily (in cats) are used.

It is important to monitor patients treated for bacteria and/or yeast, preferably 2-3 weeks after initiating therapy. At that time, otic examination and microscopic examination of the ear exudate are repeated. If there has been no improvement and compliance has been good, a change in medications is indicated. The therapy should be continued until both otic and microscopic examination approach that of a normal dog. In the author's experience, this will take at least one month. An important point to remember is that frequently (although not always) bacteria and yeast are only perpetuating causes of otitis externa and if the ear examinations never become normal, or if the infections relapse upon discontinuation of therapy, the presence of otitis media or primary causes should be investigated (see below). Another important point is that the ear must be kept relatively clean, for the treatment to work. This often means continuance of a cleansing/drying solution in the treatment protocol.

Corticosteroids are important to use in the treatment of otitis externa to relieve the inflammation present (and its concurrent discomfort), especially if treating obstructive ear disease due to progressive pathologic change. Such change is initially due to tissue swelling, and progresses to fibrosis and fibroproliferative pathology. Calcification of the cartilage of the external ear canal may result. Corticosteroids are helpful in controlling or even reversing these changes, in the early stages. Often, topical steroids alone will suffice. The author will start with a high potency corticosteroid such as those included in the aforementioned topical preparations. Use of corticosteroids in the ear may lead to increases in the levels of serum enzymes such as alkaline phosphatase, a hypoadrenal (Addisonian) response on adrenal function tests, and rarely to visible Cushingoid signs (alopecia, abdominal distension). In cases where systemic corticosteroids are indicated (compliance problems, swollen- closed external canal opening, or chronic Pseudomonas infection) prednisone may be given orally, usually started at 1 mg/kg and eventually tapered to the lowest possible dose and frequency to be effective.

A frequent concern of veterinarians is what effect a ruptured tympanic membrane has on any of the recommendations for therapy. One should use solutions rather than ointments or creams, and avoid the use of topical aminoglycosides, if possible. However, the actual incidence of ototoxicity due to the aminoglycosides (or any other medications) in dogs and cats is unknown (and probably uncommon). Therefore, the veterinarian should not avoid using aminoglycosides in a patient with a ruptured t.m. if they are the only option for the animal.

Primary causes

Parasites. The most common parasitic cause of otitis externa is the ear mite O. cynotis. Typically it presents with a brown-black crumbly otic exudate. Pets may be infested with these mites but show no clinical signs; in addition, the mites may live on the body outside the ears. For these reasons the author treats with a systemic acaricide (including all in-contact pets). Selamectin (Revolution®, Pfizer) works well as a cutaneously-absorbed ectoendoparasiticide applied to the skin between the shoulder blades, and is the treatment of choice. Topically, Acarexx®: Idexx Pharmaceuticals (a 0.01% ivermectin solution) is quite effective as a one-time treatment

Demodex sp in both dogs and cats have been noted as otic parasites, and in cats the condition may resemble otitis due to O. cynotis. The ears may be the only place on the body affected by Demodex in cats, but in dogs demodectic otitis is usually a manifestation of generalized demodicosis. Therapy for the ears, therefore, must be part of a generalized treatment plan. Acarexx® may be helpful in treatment.

Foreign bodies. Alligator forceps are extremely useful for removal. It should be remembered that wax and other debris may act as a foreign body in its ability to elicit an inflammatory response; this is one of the reasons why keeping the ear clean is so important. Even if a foreign body is successfully removed from one ear, the other ear should always be examined; the first ear may simply have been the one that was the most discomforting to the patient.

Hypersensitivity diseases. In the opinion of the author, chronic otitis externa in the dog should be considered secondary to hypersensitivity until proven otherwise. This includes atopic dermatitis and food allergy, and less commonly contact and drug allergies. In most cases the ears are not affected alone; frequently the feet (especially interdigitally) and sometimes the face and axilla are also involved. Thus the importance of a thorough history and physical examination.

Keratinization/cornification disorders. Idiopathic seborrhea is the most common keratinization disorder causing otitis externa in the dog. The yellowish exudate fails to take up stain on microscopic examination. The otitis almost always occurs with seborrhea elsewhere on the body. Endocrinopathies, as hypothyroidism, may mimic such a presentation, and should always be investigated. In otitis externa due to idiopathic seborrhea the veterinarian will often need to use long-term topical corticosteroid therapy. The author's preference is a fluocinolone-DMSO (dimethyl sulfoxide) preparation (Synotic®, Diamond Laboratories), provided no infection is present; if it is, a topical antimicrobial must be used first. Rarely, a dog with sebaceous adenitis (idiopathic granulomatous inflammation targeting the sebaceous glands) may have an otitis externa as the initial sign of the disease. Ideally such a diagnosis is made via a biopsy of the ear canal. More commonly, a dry scaling of the medial pinna in association with an otitis externa is seen concurrently with other skin signs of the disease (severe seborrhea, alopecia, etc.).

References

1.  Cole LK, Kwochka KW, Kowalski JJ, Hillier A. Microbial flora and antimicrobial susceptibility patterns of isolated pathogens from the horizontal ear canal and middle ear in dogs with otitis media. J Am Vet Med Assoc. 1998 212: 534-8.

2.  Colombini S, Merchant S R and Hosgood G. Microbial flora and antimicrobial susceptibility patterns from dogs with otitis media. Veterinary Dermatology 2000, 11:235-40.

3.  White SD. Otitis externa. Waltham International Focus, 2:2-9, 1992

Speaker Information
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Stephen D. White, DVM, DACVD
Professor, School of Veterinary Medicine, University of California
Davis, CA, USA


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