Causes of Ear Disease

Ear disease is one of the most common ailments that we see in veterinary medicine.  Approximately 15-20% of all canine patients and 6-7% of all feline patients have some kind of ear disease.  Otitis externa refers to an inflammation of the external ear canals, whereas otitis media refers to middle ear disease, and otitis interna is a disease of the internal ear.  In order to adequately treat these problems, it is essential to understand the mechanisms of disease and to aim treatment at all of the contributing factors.  Because there are so many components to ear disease, it is easier to categorize them into primary, predisposing and perpetuating causes.

Primary Causes of External Ear Disease

The external ear canal is lined with skin, so any disease that affects skin can affect the ear canal.  Primary causes of ear disease are those conditions that we traditionally think of as causing “skin” disease, but which we now know also affect the skin of the ear canals as well.  Some of these include parasites (including fleas), foreign material (including old medications), endocrine diseases (such as low thyroid), allergy (to food or inhaled substances) and other inherited or acquired skin disorders.

Predisposing Causes of External Ear Disease

Although the skin lining the ear canals is very similar to that covering the rest of the body, it is much less adaptable to changes within its environment.  Predisposing factors of ear disease are those that change the climate within the ear and allow for inflammation or infection to take hold.

Anatomical variations between breeds are common predisposing factors.  Narrowed ear canals or excessive ear folds (Shar Peis, Bulldogs and others) can trap moisture, wax and debris, which prevents the body’s ability to clean the ears.  Other breeds (Cockers, Springers, Retrievers and others) have an over abundance of cerumen (wax) glands, which make material that promote yeast growth.  Humid environments, such as those encountered here in the northwest, and excessive moisture in the ears from swimming or bathing promote bacterial growth.  Trauma to the ear canal, even from something as well-intentioned as ear cleaning, may disrupt the skin and provide cracks in the skin for bacteria and yeast to take hold.

Perpetuating Factors of External Ear Disease

Perpetuating factors include anything that prevents normal and prompt resolution of ear disease.  These things are not the reason for the initial onset of disease, but need to be addressed before healing is possible.  A common misconception is that bacterial or yeast infections are a cause of ear disease.  In fact, these infections are only possible because of primary or predisposing factors.  It is essential to keep this in mind since our job does not end with ridding the ear of infection. 

Another set of perpetuating factors are caused, ironically, by the treatments themselves.  Some animals will develop contact reactions to topical medications.  Overmedicating can also be problematic since that keeps the skin of the ear canals wet and ulcerated.  Undermedicating may not rid the ear of organisms or adequately treat inflammation.  Inappropriate use of antimicrobial drugs is also detrimental.  For example, treating a yeast infection with an antibiotic (meant for bacteria) will not cure the infection and may do more harm than good. 

Another common, but unfortunately underdiagnosed perpetuator of external ear disease is otitis media, or middle ear disease.  Otitis media is present in up to half of all dogs with chronic otitis externa and needs to be resolved before external ear disease can be cured.

A Case Example

To illustrate the interaction of these factors, consider a dog with an allergy, either to dietary proteins or inhaled allergens.  This causes the skin of the ear to become inflamed, which results in redness, swelling, heat and pain.  Allergy is our primary cause of ear disease.  In a cylindrical tube like the ear canal, inflammation and swelling reduces the diameter or space in the tube.  This decreases ventilation of the ear and normal drying.  The narrowing of the ear canals and the excessive humidity are predisposing factors.  Humidity and alteration of normal anatomy allow for infection by yeast and/or bacteria.  Infection becomes a perpetuating factor and must be eliminated.  Because we often don’t see signs of ear disease until there is a significant infection, it is easy to focus on only that one part of the whole process.  For complete resolution, however, it is necessary to identify and manage the underlying cause of ear disease, allergy.