Studies on mycoplasmas in relation to porcine respiratory disease.

The isolation of M. hyopneumoniae from field material of cases of Enzootic Pneumonia is difficult since the pneumonic lungs also contain M. hyorhinis. Originally, only 3% of the infected lungs yielded M. hyopneumoniae on standard media. The difficulties were chiefly due to overgrowth of the cultures by M. hyorhinis. This, however, was largely overcome when kanamycin was added to the isolation medium; since 13% of pneumonia lungs yielded M. hyopneumoniae when this modified medium was used. Among the range of substances tested, this antibiotic proved to be the most successful in selectively inhibiting M. hyorhinis. The present studies have shown that these two organisms are indistinguishable from each other, as well as from most other porcine mycoplasmas, on the basis of their colonial appearance or cellular morphology. However, the two organisms were found to have a few characteristics which could be helpful in their differentiation. These included differences in heat-sensitivity, behaviour in tissue cultures and organ explants, and the Methylene Blue Staining technique which was useful in recognising colonies of M. hyopneumoniae when they were mixed with those of M. hyorhinis. The serological diagnosis of M. hyopneumoniae infections was also studied. It was demonstrated that in natural infections, this organism does not stimulate the production of macroglobulin antibodies, and therefore, neither agglutination or precipitation tests could be used as a means of diagnosing Enzootic Pneumonia. However, positive results were obtained with the complement-fixation test in all artificially infected animals. Immunofluorescence of lung smears was also useful, as it detected as many as 87.5% of recent cases of Enzootic Pneumonia. It was concluded therefore, that these two tests, together with the improved isolation procedures, could detect most field cases of M. hyopneumoniae infections. The action of various species of porcine mycoplasmas both in pig kidney primary cultures, and in pig tracheal explants, appeared to relate directly to their production of hydrogen peroxide. M. hyopneumoniae did not produce hydrogen peroxide, and therefore, the pathogenicity of this organism must involve a different mechanism. Alternative mechanisms for the production of pneumonia by M. hyopneumoniae are discussed