Malassezia Otitis Externa - Etiology And Treatment
Louis N. Gotthelf, DVM
Animal Hospital of Montgomery
Montgomery, AL
Malassezia otitis externa is perhaps the most common ear condition seen in a veterinary practice. Dogs seem to present more often with this condition than cats, perhaps because of the increased numbers of ceruminous glands in the dog and the increased reactivity of the dog's ear canal to inflammation.
Many species of the yeast Malassezia have been identified in animal and human skin diseases. In the dog, Malassezia pachydermatis, one of the non-lipophilic yeasts is most often isolated, while in the cat, Malassezia sympodialis, a lipid dependent yeast is most often found. Malassezia is a surface pathogen and relies on adhesin molecules composed of water soluble glycoproteins to allow the yeast to stick to the corneocytes lining the ear canal. Increased humidity of the ear canal is a favorable growth factor for Malassezia. The otic inflammation that results from an overgrowth of yeast is due to the metabolic products of yeast fermentation including many cytokines. With maceration of the ear canal surface epithelial cells and the loss of the moisture barrier, Malassezia antigens are exposed to the immune system in the dermis resulting in antigenic stimulation.
Cerumen in the ear is a combination of many ingredients. Sebaceous cellular secretion provides the bulk of the lipid content of cerumen in the form of triglycerides, providing the moisture barrier to protect the ear canal epithelium. The cerumenous glands contribute a secretion lower in lipids that seems to keep the cerumen more liquid. Cerumen also contains desquamated epithelial cells and immunoglobulins.
Alterations in cerumen lipid composition caused by underlying skin diseases such as adverse food reactions, atopy or endocrinopathies like hypothyroidism play a role in allowing colonization of the opportunist Malassezia yeasts. It has been shown that over 50% of atopic dogs have elevated Malassezia populations on their skin.
With inflammation, the cerumenous glands (straight apocrine glands) begin to increase in size and tortuosity. These glands dilate and fill with material higher in moisture and lower in triglygerides, but also higher in free fatty acids. The excessive secretion from the hyperplastic cerumenous glands decreases the overall lipid concentration in the ear canal, which removes the protective moisture barrier present in non-inflammed ears. Low levels of free fatty acids in surface lipids coupled with increased levels of surface triglycerides favors Malassezia infections.
Inflammatory diseases causing otitis externa may result in either increases or decreases in the amount of sebaceous secretion. However, inflammation and increased blood supply to the apocrine glands usually increases the number of apocrine glands as well as the amount of secretion from the apocrine (cerumen) glands.
The diagnosis of Malassezia otitis externa relies on the demonstration of significant numbers of yeasts present on a stained roll smear of otic secretions. In the dog, numbers of yeasts exceeding 5-8 per high power field are significant, while in cats, numbers exceeding 12-15 per high power field are significant. Many dogs with high numbers of Malassezia identified from the ear(s) also have Malassezia present elsewhere on the body, namely in the interdigital area, the axillae and chest, or on the perineum. Often these dogs are very erythematous and pruritic with thick waxy otic secretions, red ears, and there may or may not be hyperkeratosis on the concave pinna.
Since most Malassezia otic infections involve an underlying hypersensitivity reaction, the underlying adverse food reaction or atopic dermatitis must also be identified and treated. In young puppies, Malassezia otitis may be the result of an adverse food reaction or as a sequella to ear mite infestation. In middle age dogs, atopy or endocrinopathy should be considered as well as adverse food reactions. With food trials, corticosteroid therapy or hyposensitization, long term success in the treatment of Malassezia otitis can be expected. Anecdotal reports of the use of Cyclosporine-A modified (Atopica, Novartis) have not been encouraging in the treatment of Malassezia otitis externa.
In order to effectively treat Malassezia otitis externa, several things need to be done. First, the underlying skin disease must be identified and treated. Second, the lipids have to be removed from the ear canal. And third the ear canal has to be dessicated to either remove or bind water molecules.
To remove the lipid substrates from the ear and to treat otitis externa complicated by Malassezia, the author prefers to clean the ear in the hospital first using a cerumenolytic type of ear cleaner, such as the detergent dioctyl sodium succinate (Klearwax, DermaPet, Inc). This detergent is placed into the ears of the affected patient and water is added to the detergent to enhance its lipid emulsifying properties. Detergents must be flushed from the ear completely to prevent irritation and maceration of the ear canal epithelium. After degreasing the ear canal, it is dried using suction.
Home use of an acetic acid/boric acid solution (Malacetic Otic, DermaPet, Inc) by the owner keeps the lipids and moisture from accumulating in the ear canal. Acetic acid degreases the ear canal, removing surface lipids. Boric acid, by its binding of water molecules, keeps the epithelium relatively dehydrated which interferes with the ability of the adhesin molecules on the yeast surface to attach to the corneocytes. In two studies, the use of 2%acetic acid and 2% boric acid for one week once daily at home prevented colonization of the ear canals with yeast for 2 weeks in over 80% of the dogs treated and decreased the symptoms of otitis for the same period. In cases where there is significant otic inflammation, a 1% solution of acetic acid and boric acid containing 1% hydrocortisone (Malacetic HC, DermaPet, Inc.) may be used.
Other studies using at home ear cleaners resulted in decreased numbers of organisms, so therapeutic ear cleaning is sometimes all that is required to control Malassezia populations.
A topical solution of miconizole or clotrimizole may be used in the ear canal after cleaning. However, the use of these topical antifungal drugs in waxy ears is not recommended. In addition to ear cleaners , systemic antifungals like oral ketoconazole or itraconizole are useful for refractory yeast otitis externa and/or otitis media cases or for yeast otitis cases where there is also stenosis. These systemic compounds may reduce the pruritis associated with the yeasts, but they have not been shown to reduce otic yeast numbers.
In mild cases of Malassezia otitis externa, the external canal can be cleaned by the owner at home to facilitate removal of excessive exudate accumulation associated with otitis externa/media. The ear cleaner or flush is used daily for 7-10 days by filling the ear canal to overflowing, massaging the base of the ear, and allowing the solution to remain in the ear canal for 5 minutes. The loosened debris is wiped off of the concave pinnal surface with a dry cotton ball. This procedure is repeated once daily. When the ear canal is clean, the cotton ball will remain fairly white when the solution is wiped away. At that time, home ear cleaning is reduced to once weekly.
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