It is probable that the first report of transmission of a mycoplasmal infection in swine is that of McNutt et al. (1945). Unfortunately, the active agent causing the transmissible arthritis was not preserved. Switzer (1953) attempted to isolate filter passing agents from swine turbinate atrophy cases by inoculation of filtrates into embryonating hens' eggs. He was successful in a high percentage of attempts in growing an agent that produced embryo mortality and lesions very similar to those produced by the organism now called Mycoplasma gallisepticum. This swine nasal mycoplasma was named M. hyorhinis (Switzer, 1955). It was subsequently demonstrated that this mycoplasma was present in about 60% of the swine nasal cavities regardless of whether turbinate atrophy was present or absent. Inoculation of cultures of this organism into the nasal cavities of normal swine failed to produce any gross lesions. There was some microscopic evidence of mild lymphoid hyperplasia.So far, this organism is only known to infect swine. Attempts to infect mice, chickens, turkeys, calves, guinea pigs, hamsters, rabbits or sheep have been unsuccessful. Attempts to produce pneumonia in swine with this organism have been unsuccessful, although it has been found that M. hyorhinis may be recovered from 50% (L'Ecuyer et al., 1961 ) of the chronic pneumonia lesions in swine.In spite of the inability of M. hyorhinis to produce either turbinate atrophy or pneumonia in swine, it causes a fibrile reaction followed by polyserositis and arthritis in some pigs when it gains entrance to the bloodstream. The age of the inoculated experimental pig is of importance. A marked reduction in severity of disease occurs after the pigs are eight weeks of age. The intraperitoneal route of inoculation has proven satisfactory for production of these lesions. A comparable polyserositis and arthritis occurs naturally in swine. There is usually a good correlation between damage to the respiratory tract, either from turbinate atrophy or chronic pneumonia, and the natural occurrence of the polyserositis. Not all isolates of M. hyorhinis produce identical lesions in pigs. It has been observed that some isolates produce more severe pericarditis lesions, some produce more generalized polyserositis, and some produce a higher incidence of arthritis.Most isolates of M . hyorhinis produce a clearly defined cytopathogenic effect on primary swine kidney cell cultures. This effect first appears in the portions of the cell culture covered the deepest by medium. The eEect can be intensified by increasing the depth of the medium. In one series (Roberts, 1962 andRoberts et al., 1963), the isolate producing the most intense cytopathic effect of several examined was selected for inoculation into a series of experimental pigs. Every pig in this series developed arthritis even though few of them developed polyserositis. The possible correlation between speed of swine kidney cell destruction and ability to cause arthritis has not been studied further.The sequence of events that precede...

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