Ears, like all parts of the body, normally have a few bacteria and yeast cells present. Otitis occurs when bacteria or yeast organisms increase to an overwhelming number or are replaced by pathogenic (disease-causing) organisms. Otitis in the outer ear is common because that’s where your pet is most likely to be exposed to foreign bodies, bacteria and yeast, ear mites, and lake water. If your pet’s outer ear infection is untreated, over time your pet’s eardrum becomes porous and infection travels from the outer ear through the eardrum to the inner ear. Your pet’s eardrum can look intact, but bacteria can move through it to the inner ear

Which Bacteria and Yeast Cause an Ear Infection?

Dogs usually have yeast (Malassezia) and bacterial infections (Staphylococci, Pseudomonas). Dogs that swim typically develop Pseudomonas, and dogs with increased cerumen (earwax)  typically have Malassezia. Dogs with hypothyroidism typically have Staph ear infections. Less commonly, dogs have bacterial infections caused by Corynebacteria, Enterococci, E. Coli, Streptococci, and Proteus.

Cats usually have yeast (Malassezia) infections, but don’t usually have bacterial infections. Occasionally bacteria (Mycoplasma and Bordetella) are found in cats with middle ear infections, but it is unclear that these bacteria actually cause the infection.