Probably the most common mite infestation in felines, ear mites feed on a cat's ear wax. Formally known as otodectic mange, symptoms of ear mites include inflammation, a dark exudate coming from the ear, constant ear scratching or head shaking, and a bad smell emanating from the ear canal. That discharge coming out of the ear consists of a blend of wax, blood and actual mites.
While you can purchase ear mite remedies over the counter, it's always best to consult a vet if you suspect your cat has ear mites. It's quite possible that your cat's ear irritation is due to a bacterial or yeast infection, so treating for ear mites does no good and your cat continues to suffer. If ear mites are the culprit, your vet can give your cat an injection or use a single application medication that eradicates the mites and their eggs.