Intranasally inoculated Mycoplasma hyorhinis causes eustachitis in pigs

Specific-pathogen-free pigs were experimentally inoculated with Mycoplasma hyorhinis, Pasteurella multocida, or both bacterial isolates to evaluate th e role of these bacteria in the pathogenesis of otitis media. Six pigs were inoculated intranasally with 4.3 x 10(8) colony-forming units (CFU) of M. hyorhinis. Twenty-one days later, three of these six pigs were inoculated i ntranasally with 5.0 x 10(8) CFU of P. multocida. Three additional pigs wer e also inoculated intranasally at the time with P. multocida alone. Two pig s served as uninoculated controls. Seven days later, all pigs were euthanat ized. Histologically, subacute inflammation was found in 10 auditory tubes of six pigs and two tympanic cavities of two pigs inoculated with M. hyorhi nis. Immunohistochemically, M. hyorhinis antigens were detected on the lumi nal surface of eight of 10 inflamed auditory tubes, and ultrastructural exa mination confirmed mycoplasmal organisms in two pigs. M. hyorhinis was isol ated from the inflamed tympanic cavities of two pigs. None of the pigs inoc ulated only with P. multocida had otitis, and P. multocida was not isolated from the tympanic cavity. These findings indicate that M. hyorhinis can ca use eustachitis but rarely otitis media in specific-pathogen-free pigs.