Ear infections are usually secondary to inflammation of the external ear canals (the tube-shaped part of the ear visible under the ear flap). Inflammation of the canals leads to the reproduction of normal bacteria and yeast that live in the ear to the point where the body is unable to control their numbers (called overgrowth). Other bacteria can also take advantage of the inflammation and unhealthy environment inside the ear to establish infection. The overgrowth of these organisms causes more inflammation. Inflammation of the ear canal causes swelling, making the tube narrower than usual. Inflammation also causes an increase in the production of wax. The ears become very itchy and painful. Severe ear infections can lead to eardrum rupture and middle and inner ear infections. Deep infections can lead to deafness and neurologic signs.
Certain disorders or diseases may be the primary reason ear infections develop. These conditions include:
- Allergies (environmental and food)
- Ear mites
- Foreign bodies
- Skin disorders (like seborrhea)
- Thyroid disease (in dogs)
- Tumors or polyps in the ear
Ear infections may recur because of the inability to control the original infection or treat the underlying cause. Chronic changes lead to future infections, and scar tissue and permanent narrowing of the ear canals can make future infections difficult to treat.