Perpetuating Factors

The perpetuating factors that prevent the ear canal from effectively healing include infections with bacteria and yeasts, improper treatment of the ear, overtreatment of the ear with ear cleaners and medications, and otitis media.

Because of the changes in the ear canal that occur as a result of disease, bacteria and yeasts colonize and reproduce. Otitis externa is complicated by the growth of these infectious organisms, which is secondary to a primary disease process. Although antimicrobial therapy may temporarily relieve the symptoms of otitis externa, they may recur unless the underlying disease is identified and treated as well. Concomitant growth and colonization by these organisms are considered to be perpetuating factors in ear disease.

Improper treatment of infectious organisms is a problem and may be the most important reason for treatment failures. Applying ointment medications directly into an ear full of wax results in the ointment being layered on the wax and not penetrat­ing through to the skin of the ear canal, where the infection is present. Treatment of a yeast infection with an antibiotic alone results in failure. Treatment of pseudomonal ear infections with drugs such as cephalexin, clindamycin, and neomycin also results in failure. Treatment with effective systemic antibiotics and antifungals that cannot get into the ear canal to affect the organisms will result in failure.

Colonizing organisms associated with otitis externa include bacteria and yeasts. Members of the genera Malassezia, Staphylococcus, and Pseudomonas are the organisms most commonly isolated from the ears of dogs. Corynebacterium, Enterococcus, and Proteus are also frequently isolated. The prevalence of one organ­ism over another is determined by a variety of factors. For example, excessive ceru­men production from cerumen gland hyperplasia permits Malassezia growth.

Epidemiologic reports from investigators around the world indicate a wide vari­ety of prevalent organisms found in the ears of dogs with otitis externa. Malassezia pachydermatis is commonly found in the author’s Alabama practice, but a report from Spain indicates that Candida albicans is the most prevalent otic fungal organ­ism isolated there. Interpretation of the literature concerning the most prevalent organisms isolated from ear disease may therefore need to be regionalized. In addition, prevalent organisms may be determined by breed susceptibility to certain primary disease states that result in otitis. For example, acute otitis externa or otitis media in German Shepherds frequently is perpetuated by a secondary Pseudomonas infection.

Isolates from the external ear canal do not always correlate with the isolates from the middle ear. Pseudomonas may be found more often in the middle ear than the external ear, whereas Malassezia is rarely found in the middle ear when the eardrum in intact. Some organisms rarely isolated from the external ear canal of dogs, such as B-hemolytic streptococci, may be found as the prevalent organism in otitis media.

Frequently, otitis externa resists topical treatment because infection extends through the tympanic membrane into the tympanic bulla, causing otitis media. This secondary otitis media occurs in approximately 16% of cases of acute otitis externa and in as many as 50% of cases of chronic otitis externa. Primary otitis media (exten­sion of infection from the nasopharynx through the auditory tube to the tympanic bulla) is rare in the dog but is often seen in cats with nasopharyngeal polyps.

Studies in dogs have suggested that otitis media may be more common than previously recognized. In many dogs with otitis externa with intact eardrums, signif­icant bacterial populations may also be isolated from the middle ear. These dogs may have had eardrum rupture that healed, trapping bacteria in the tympanic bulla.