Infection with a low virulent Mycoplasma hyopneumoniae isolate does not protect piglets against subsequent infection with a highly virulent M. hyopneumoniae isolate.

The study aimed to evaluate the effect of an infection with low virulent isolates of M. hyopneumoniae (LV1 and LV2) on the subsequent infection with a highly virulent isolate (HV). Fifty-five, 3-week-old piglets free of M. hyopneumoniae were randomly allocated to 6 different groups. At 4 weeks of age (D0), groups LV1-HV and LV1 were intratracheally inoculated with LV1, groups LV2-HV and LV2 with LV2, and group HV with sterile culture medium. Four weeks later (D28), the pigs of these different groups were either intratracheally inoculated with the highly virulent isolate (groups LV1-HV, LV2-HV, HV) or with sterile culture medium (groups LV1 and LV2). A negative control group consisted of pigs inoculated with sterile culture medium on D0 and D28. All animals were necropsied at 28 days after the second inoculation (D56). Clinical symptoms were evaluated daily using a respiratory disease score (RDS). After necropsy, macroscopic and histopathological lung lesions were quantified and immunofluorescence (IF) testing on lung tissue and nested PCR on BAL fluid were performed for the detection of M. hyopneumoniae. Disease signs and lung lesions were not observed in pigs of the negative control group. In the other groups, there were no or only very mild clinical symptoms from D0 until D28. A significant increase in the average RDS values was, however, observed during D28-D56, especially in groups LV1-HV (1.48) and LV2-HV (1.49), in group HV (0.79), and to a lesser extent in groups LV1 (0.50) and LV2 (0.65) (P<0.05). The clinical symptoms during D28-D56, the lung lesions and intensity of IF staining were more pronounced in groups LV1-HV, LV2-HV and HV compared to groups LV1 and LV2. All pigs, except those from the negative control group, were positive on IF testing and PCR at D56. The present study demonstrates that pigs inoculated with low virulent isolates of M. hyopneumoniae are not protected against a subsequent infection with a highly virulent isolate 4 weeks later and may even develop more severe disease signs. This indicates that subsequent infections with different M. hyopneumoniae isolates may lead to more severe clinical disease in a pig herd.