The short answer to this question is that the underlying problem is not controlled. If an underlying problem cannot be found, then long term therapy is likely needed or the infection will come back. There are 3 main types of underlying problems that cause ear infections to keep coming back (or not go away) when they are treated: allergies, an object in the canal, and a middle ear infection.
Ear infections can be very painful for the dog or cat and should be treated right away. Routine care of the ears and management of the underlying problem will help prevent the problem from coming back. Veterinary dermatologists specialize in the treatment of ear diseases and the underlying causes of most infections and can perform specialized procedures to help treat these problems.
Allergies are the most common underlying reason for ear infections in dogs. Allergies in dogs and cats cause the skin to be inflamed and itchy, and secondary infections are common. The ear is a typical location for allergies to manifest.
Objects in the Canal
“Objects” can include polyps, parasites, or plant material. In cats, ear mites are the most common cause of ear infections. If only one ear is infected, we must make sure there isn’t a mass or foreign material stuck in the canal.
Middle ear infection
With long term infections of the ear (from any cause), the bacteria can move past the ear drum and get into the middle ear. These infections are more difficult to treat and require long term medications. Often, a certain type of bacteria is involved called Pseudomonas. This bacteria is seen with a pus-like discharge, bright red skin, and bad odor from the ear.
When some dogs (especially cocker spaniels) have infections in the ear for a long time, the ear canal swells closed. If the canal cannot be opened with aggressive treatment, the infections can be very difficult to manage.
Sometimes ear infections need to be treated with a special procedure to clean out and infuse treatments directly into the middle ear while the pet is under anesthesia.
If left untreated, middle ear infections can cause a head tilt, paralysis of that side of the face, and pain while eating.