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.
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.
Eardrums can heal in 3-4 weeks when the cause of the rupture is addressed. In the meantime, do not put any medications or flushing solutions in the ear that were not prescribed by the veterinarian.
About 1 in every 5 dogs and 1 in every 15 cats that visit a veterinary clinic has ear disease–from mild inflammation to severe middle ear infection. In humid climates, the number of dogs with otitis is nearly 50%.
A middle ear infection (otitis media) is more common than once thought. In dogs, about 1 in 6 with an external ear infection develops a middle ear infection. Many cases of middle ear infection are unrecognized.
Dogs can be predisposed to otitis due to the pet's genetic tendency to have a particular type of skin or glands in the canal. Genetics can also influence the thickness of the hair in the canal, and pets with the most hair have the poorest air flow and the greatest incidence of infection. Other dogs are predisposed to develop otitis because they have allergies which make their ears itch and produce more cerumen. Their ears itch, the pet scratches, and infection sets in.
Approximately 80% of dogs with allergies develop otitis. In dogs and cats, symptoms caused by allergies to food, inhaled proteins, or contact materials—for example, corn, pollen, and grass—are different than allergic symptoms in people. People with allergies often have itchy, runny eyes or a runny nose. Dogs and cats are more likely to develop heat and inflammation of the ears, head, neck, and feet than of the eyes and nose. Heat and inflammation in the ears predisposes pets to increased yeast growth and chronic itchy, smelly otitis.