Otitis, or ear infection is an unfortunately common condition. There are three types of ear infection based on the location with the external ear canal being the most common. Other than ear mites, primary ear infections (those infections that have no underlying reason) are very infrequent, and rather rare. Secondary ear infections, however, are very common.
Ear infections typically occur secondary to allergies. Pets' allergies are not typically manifested as human allergies are. We tend to have upper respiratory signs. Pets tend to have skin oriented signs such as itching, scratching, and redness. The ear is a deep, dark, and potentially damp hole in the skin. It should be no surprise that, if the skin is irritated, the ears will often be one of the first things to become infected.
Your pet has millions of organisms such as bacteria and yeast on their skin including the ears. These organisms live there in a commensal fashion. However, with inflammation, these organisms replicate to the point of infection. Common clinical signs of ear infection are pain, redness, odor, debris (wax, pus, blood), and swelling.
Diagnosis is rather straightforward through a physical examination. An ear cytology will show the type of organism (bacteria, yeast, or both) as well as differentiate between common organisms and the more resistant bacteria, Pseudomonas. It is also important to discuss the common reasons (i.e. allergies) leading to ear infections.
The two main types of allergies leading to ear infections are seasonal/environmental allergies and food allergies. With seasonal/environmental allergies, we commonly see face rubbing, ear scratching, paw licking, and general itchiness. For this reason, we often say "Face and Feet". With food allergies, we often also see ear disease as well as rectal itching, scooting, and inflammation. For this reason, we often say "Ears and Rears". A detailed history can often clue us in as to the type. It is very common that pets have both types of allergies. In order to specifically diagnose the allergy, the pet would need an allergy test and/or undergo a long term food trial.
Treatment consists of thoroughly flushing the ear canal. Once the organism(s) is identified, a topical medication will be administered. Occasionally, an oral medication may be selected. All underlying allergies should be identified and avoided if possible. If your pet has chronic ear disease, the anatomy of the ear may have been changed which will prevent a cure. Therefore identification of the underlying problem should be sought after early.