Atopic Dermatitis

The ear canal is an invagination of epidermis forming a hollow skin tube in the inside of the head that begins at the eardrum. Pathological mechanisms affecting the skin of the animal have the same effect in the epidermal tube lining the ear canal.

It has been estimated that almost 75% of all canine ear disease is related to atopic dermatitis. Atopy seems to be prevalent in many breeds, giving evidence that this disease may have a genetic origin. Most dogs show clinical signs after the first year of life. Atopic patients probably have a high immunoglobulin E (IgE) response from B-lymphocytes when exposed to individual allergens. IgE antibody binds to mast cells resulting in their degranulation and subsequent release of inflammatory medi­ators on subsequent exposure to that specific antigen. Often there will be a history of foot licking or chewing, face rubbing, and licking the groin area, in addition to scratching the ears. Many dogs with light-colored coats have the telltale red-orange saliva staining typically found with atopy. Cats may manifest their atopic dermatitis with miliary dermatitis, facial pruritus, or barbering of their hair on the belly and lower legs with their teeth in response to the pruritus.

Allergies can be seasonal or nonseasonal. In a seasonal allergy, the clinical signs are most intense during the period of high pollen counts and disappear with the reduction of the pollen. Atopic dogs and cats are not allergic to only one type of pollen, mold, or insect, so most allergic ears tend to be nonseasonal, with the severity extending for a long period from the spring to the fall, depending on which pollens, molds, or insects the patient is exposed to. In indoor-only dogs or cats with a nonseasonal history, or in a patient that flares up only during the winter, the atopic otitis may be due to indoor allergens such as house dust mites or molds. It is important to ask the owners about the times of the year in which the ear disease flares up. When clients move or relocate to a new geographical area, changes in pet environment may also cause flareups as the patient is exposed to new antigens.

Atopic dermatitis results from an inflammatory overreaction of the skin to anti­genic stimulation. The antigens that remain on the surface of the skin maintain the reaction. In dogs, areas like the feet, face, pinnae, and ventrum have increased mast cell density than other parts of the skin. In cats, the mast cells are concentrated behind the ears and over the dorsum. The areas that are constantly contacting envi­ronmental allergens are more often involved in the inflammatory process. Unlike dogs, atopic cats tend to have allergies that also affect the respiratory system (bron­chitis and asthma) and the eyes (conjunctivitis) as well as the skin.

Frequent bathing or rinsing of atopic patients with water helps to remove antigens physically from the surface of the skin and hair coat. Ear cleaners, flushes, and wet wipes help to remove antigens from the surface of the pinnae and ear canal.

Many patients get rapid relief from their ear disease when the atopic dermatitis is treated with corticosteroid therapy, systemically and/or topically. Most combination otic formulations contain corticosteroids to relieve the inflammation and pruritus in the ear. A new therapy for atopic dermatitis using oral cyclosporine-A-modified capsules (Atopica, Novartis) relieves the pruritic clinical signs without the side effects often seen with corticosteroids. Other dogs and cats get long-term benefit from successful immunotherapy to specific antigens identified through allergy testing. It is important to discuss atopic dermatitis as a cause of ear disease with clients so that they understand the value in pursuing a proper diagnosis. After the otic inflamma­tion resulting from atopic dermatitis is controlled, the ear canal epithelium is not as likely to support bacterial or yeast growth, and the patient can remain comfortable.