Ear infections are one of the most frequent reasons dogs are seen by veterinarians. These infections are often recurrent, making them uncomfortable for the pet and frustrating for pet parents. Yeast, particularly Malassezia pachydermatis, can often be isolated from “early” cases of ear infections. As the disease becomes more chronic, bacteria tend to take over. But these organisms are able to thrive in some ears and not others because of what are known as primary causes of ear infections.
It is estimated that approximately 75% of all canine ear infections have atopic dermatitis as the primary cause. Several factors may be at play. When the ear canal becomes inflamed in a dog with atopic dermatitis, it produces more waxy cerumen. Excessive cerumen allows yeast organisms to thrive, leading to further inflammation. The skin, and presumably ear canal epithelium, of atopic dogs is also more prone to infection with bacteria. This may result from defects in the barrier function of the skin or immunological differences in local bacterial defenses.
Food hypersensitivity is likely the second most common primary cause leading to ear infections in dogs in most veterinary practices. Some of the same factors described above for atopic dermatitis come into play, with inflammation leading to excessive cerumen production, then yeast and bacterial overgrowth. Diet trials are necessary to diagnose or rule out food hypersensitivity, but any current yeast or bacterial infections must be addressed as well.
In some canine populations, parasites can be a significant primary cause of ear infections. The ear mite, Otodectes, is more common in young dogs and cats. Their feeding behavior on the ear canal epithelium leads to significant inflammation and discomfort. Dogs tend to have fewer mites than cats in a typical infestation but may still have intense discomfort and inflammation. Once again, the inflammation may lead to bacterial and yeast overgrowth.
Ticks, mange mites, and chiggers are some of the other parasites that may occasionally be primary causes of ear infections in dogs.