Common signs of ear infection include head shaking, frequent scratching, foul odors, abnormal discharge, redness, and pain. The ear is actually a complex organ that can become irritated because of a whole host of primary causes and contributing factors. In cats and dogs, the external part of the ear consists of the pinna, the ear canal, and the eardrum. The ear canal consists of a vertical part (the part you can see when you look inside the ear) and a horizontal part, which extends deeper to the eardrum. This abrupt angle in the ear canal that can make cleaning difficult and can help trap debris and wax inside the ear.

Many factors can make ear infection more likely. Things that increase moisture and decrease ventilation can contribute to infection. Animals that like to swim or that have a naturally narrow ear canal can be at risk.

A large amount of hair or floppy ears can also trap moisture inside the ear. Owners sometimes make infections worse by using water or irritating substances such as rubbing alcohol or vinegar to clean the ear, or by causing trauma to the ear canal by over-aggressive cleaning. Water increases moisture within the ear and does not evaporate adequately deep in the canal.

Irritating substances can damage the fragile lining of the ear canal and make it more susceptible to bacterial infections. Only gentle cleansers designed for use in the ear canal should be used. Prescription ear cleansers that you can obtain from a veterinarian may be used to treat specific disorders or infections.

Use of cotton swabs is not good because they tend to pack the earwax into the ear canal, making it more difficult to remove. Bacteria and yeast are not considered a cause of ear disease. They are considered the result of inflammation of the ear canal.

An infection of the external ear is almost always a sign of an underlying disease. Changes in the ear canal resulting from the cause allow normal bacteria and yeast to overproduce. Eventually the overgrowth becomes infection and results in ongoing disease even after the cause has been resolved. The most common cause of ear problems in dogs is allergic reactions. The pollens and molds that cause hay fever symptoms in people are the main causes of skin and ear disease in dogs. Dogs can also become allergic to an ingredient in their food, even after they have been eating that food for years.

Other causes for ear infections include foreign objects that become stuck in the ear, ear mites, polyps, or tumors within the ear canal. Successful management of ear disease requires both identification of the primary cause and management of secondary infection. Addressing just one or the other is a common reason for treatment failure. If the ear canal continues to be inflamed and infected for a long time, infection may move across the eardrum to the middle ear.

Over time or in certain breeds of dogs, such as cocker spaniels, the ear canal becomes thickened or even mineralized, causing it to be inflexible. Once the ear canal is mineralized, changes may be irreversible and require surgery to relieve chronic pain.