Otitis media, inflammation of the middle ear structures, is seen in small and large domestic animals, including dogs, cats, rabbits, ruminants, horses, pigs, and camelids. It can be unilateral or bilateral and can affect animals of all ages. Although typically sporadic, outbreaks are possible in herds. Otitis media usually results from extension of infection from the external ear canal through the tympanic membrane or from migration of pharyngeal microorganisms through the auditory tube. Occasionally, infection extends from the inner ear to the middle ear, or reaches the middle ear by the hematogenous route. Primary otitis media has been reported in certain breeds of dogs, particularly Cavalier King Charles Spaniels. Untreated otitis media can lead to otitis interna (inflammation of the inner ear structures) or to rupture of an intact tympanic membrane with subsequent otorrhea or otitis externa.