Malassezia otitis (Proceedings)

Malassezia spp. are second only to Staphylococcus in terms of the frequency recovered from canine ear cultures. In terms of cytological findings, they are certainly the most prevalent organism. M. pachydermatis accounts for 98% of Malassezia organisms recovered from the dog (M. globosus and others accounting for the rest). There are four distinct genetic types of M. pachydermatitis, types A – D. Type A is usually found on skin, but all four types are found in canine ear canals. Although common, M. pachydermatis is not a totally innocuous organism. It has been associated with neonatal intensive care deaths, with a pet-owning nurse identified as the likely source of the outbreak.

Otitis externa caused by Malassezia is typically associated with moderate pruritus, inflammation, and cerumen formation in the ear canal. Malassezia is considered a "secondary" cause of otitis externa; a predisposing or primary cause is required for Malassezia otitis externa to develop. The most commonly encountered primary causes are canine atopic dermatitis and food hypersensitivity. Examples of predisposing causes are pendulous pinnae, narrow ear canals, and high humidity. The lipid composition and cerumen quantity in the ear canal also influences the growth of Malassezia organisms.

Diagnosis of Malassezia otitis externa is straightforward. Cytology of exudate collected from the ear canal should be performed routinely. Each ear should be evaluated separately. Although heat fixing of slides has commonly been recommended, a recent report indicates that it is not necessary.

More challenging is the diagnosis of the underlying cause. Any dog with Malassezia otitis externa should be evaluated for food allergy and atopic dermatitis, as appropriate to the history and physical examination. In some cases, otitis externa is the earliest clinical sign of these conditions.

The treatment protocol should address the immediate, secondary infection, as well as the underlying cause. Ideally, the ear canal should be cleaned in the clinic. Don't overestimate the ability of owners to successfully clean a pet's ear canal, or the ability of cleaning solutions to break up or solubilize cerumen. When the tympanic membrane is not visualized during otoscopic examination, the ear cleaning or flushing procedure is both diagnostic and therapeutic. Video otoscopy is an excellent tool for educating pet owners regarding the importance of ear cleaning/flushing.

There are many ear cleaning solutions to choose from. When the status of the tympanic membrane is in question, saline or squalene-based solutions are recommended. Acetic acid and boric acid solution (MalAcetic®; DermaPet) is reportedly safe for the middle ear, while providing some antimicrobial activity. When a perforated tympanic membrane is found during an ear flush procedure, a final rinse with saline seems to minimize the likelihood of developing ototoxicity.

Treatment of the secondary Malassezia otitis should include antifungal and glucocorticoid ingredients in most cases. The most common antifungal ingredients used in the treatment of Malassezia otitis externa are miconazole, clotrimazole and silver sulfadiazine. The owner should clean the ear canals, as frequently as daily, if needed. Topical glucocorticoids are helpful in reducing the cerumen production associated with an inflamed ear canal, as well as making the pet more comfortable. Glucocorticoid-containing ear cleaning solutions may help prevent the recurrence of infections by controlling inflammation and subsequent cerumen production.