Pathogenicity study of Mycoplasma hyorhinis and M. flocculare in specific-pathogen-free pigs pre-infected with M. hyopneumoniae.

Mycoplasma (M.) hyopneumoniae is the initiator agent of the porcine respiratory disease complex (PRDC) and the etiological agent of enzootic pneumonia. M. hyorhinis and M. flocculare are also found in extensive gross pneumonia-like lesions, but their role is not known. We investigated the pathogenicity of M. hyorhinis and M. flocculare in specific-pathogen-free pigs pre-infected or not with M. hyopneumoniae. Mono-inoculated pigs with M. flocculare showed no clinical signs, hematological changes or macroscopic lesions upon necropsy. Mono-inoculated pigs with M. hyorhinis showed, overall seven days after inoculation, an increase in mean temperature with increases in white blood cell (monocyte) counts and in concentrations of pig major acute phase protein, whereas the average daily weight gain (ADWG) decreased compared with non-infected animals. M. hyorhinis was detected in serous membranes (polyserositis) but not in bronchi. Co-infected pigs with M. hyopneumoniae and M. hyorhinis or M. flocculare showed lower ADWG during the third week of the experiment and higher haptoglobin concentrations in contrast to pigs only mono-infected with M. hyopneumoniae. In pigs co-infected with M. hyopneumoniae and M. hyorhinis, it was interesting to observe that (i) M. hyorhinis was detected in bronchi of six pigs, (ii) M. hyopneumoniae was detected in polyserositis and (iii) there was a slight delay in the production of anti-M. hyopneumoniae IgG. The extent of pneumonia was not statistically different between groups. These results suggest that mycoplasmal associations appear to induce an additive effect and increase the inflammatory status in pigs, probably involving in the impairment of the immune system.