Several factors can cause dog ear infections. Often, an underlying problem causes the normal protective barrier in a dogs ear to stop working properly. Once the ear becomes moist or inflamed, it's easy for bacteria or yeast to overgrow and cause an infection.
Ear Infections in Dogs (Otitis Externa) | VCA Animal Hospitals.
Ear infections caused by yeast and/or bacteria are much more likely in adult dogs. However, microscopic examination of the debris is the best way to differentiate the cause of infection. This is a test your veterinary clinic can perform in the clinic.

Dog Ear Infection Dog Ear Infections occur within the ear, although they can start on other parts of the dogs body. The deeper the infection lies, the more difficult to reach for treatment. Ear infections are most commonly caused by bacteria and yeast. Ear mites, excessive hair, moisture...
Ear infections in dogs are most often caused by bacteria or yeast. Additional testsx-rays, CT scan, MRI testing, or electronic hearing testsmight be needed to confirm certain conditions, including deafness. Diagnosing a tumor usually requires a biopsy.

If a dogs ear is hairy, it can cause a significant buildup of wax in the ear canal. This wax is the perfect sticking point for bacteria, often clumping together and making the environment excellent for bacterial growth and infection.
Learn about the causes of ear infections in dogs and how to treat them effectively.Learn about dog ear infections, their causes, and key prevention tips to keep your pet healthy.

Ear infections are a common condition in dogs, especially dogs with demodectic mange. This is because the type of yeast that causes most ear infections just happens to be the same one that causes fungal skin infections in the majority of dogs with demodicosis.
The most common cause of ear infections in dogs is an imbalance of bacteria or yeast in otherwise healthy ears. A holistic approach that takes your dogs diet and lifestyle into account can help naturally prevent ear infections in the long term.