I found this on a site,
http://www.pethealthcare.net, i hope it helps you!
CANINE EAR INFECTIONS
How common are ear infections in dogs?
Infections of the external ear canal (outer ear) by bacteria or yeast, are one of the most common types of infections seen in dogs. We call this otitis externa.
Some breeds, such as Cocker Spaniels and Miniature Poodles, seem more prone to ear infections, but they may occur in any breed.
What are the symptoms of an ear infection?
A dog with an ear infection is uncomfortable; its ear canals are sensitive. It shakes its head trying to get the debris and fluid out, and it scratches its ears. The ears often become red and inflamed and develop an offensive odor. A black or yellowish discharge commonly occurs.
Don't these symptoms usually suggest ear mites?
Ear mites can cause several of these symptoms, including a black discharge, scratching, and head shaking. However, ear mite infections generally occur most commonly in puppies. Ear mites in adult dogs occur most frequently after a puppy carrying mites is introduced into the household. Sometimes, ear mites will create an environment within the ear canal which leads to a secondary infection with bacteria and yeast (fungus). By the time the dog is presented to the veterinarian, the mites may be gone, but a significant ear infection remains.
Since these symptoms are similar and usually mean an infection, can I just come by and get some medication?
There are several kinds of bacteria and at least one type of fungus which might cause an ear infection. Without knowing the kind of infection present, we do not know which drug to use. In some cases, the ear infection may be caused by a foreign body or tumor in the ear canal. Treatment with medication alone will not resolve these problems. Also, the dog must be examined to be sure that the eardrum is intact. Administration of certain medications can result in loss of hearing if the eardrum is ruptured. This determination is made by the veterinarian and must be done in the office.