Yeast (Malassezia pachydermatis) naturally inhabits certain areas on dogs and cats, like the ear canal, interdigital area (between the toes) etc.. But given a hospitable environment, like humid weather, environmental or food allergies, or a bacterial infection, Malassezia can reproduce and grow abnormally, and requires treatment.
In the ear canal, a Malassezia infection often occurs at the same tiime as a bacterial infection, which means that both the yeast and the bacteria need to be treated concurrently.
A challenge to treating chronic yeast infections is identifying the underlying cause which is responsible for starting the inflammatory process which leads to infection by bacteria and yeast. Environmental allergies (atopic dermatitis) are believed to be the most common primary triggers for otitis externa.
(In this RightPet section, we're focusing on treatments for the actual yeast infection itself - and for supplements which boost the immune system. But if your dog is diagnosed as having environmental or food allergies, we have great sections for these conditions too.)