In the present study, all bacterins evoked a serological response in the vaccinated animals. However, after challenge with a highly virulent M. hyopneumoniae strain, a clear protection against macroscopic lung lesions was not demonstrated in these animals. This confirms a lack of correlation between serum antibodies and protection [20–22]. Studies aiming to elucidate immune mechanisms playing a role in protection against infection with this very fastidious micro-organism are urgently needed.
It is remarkable that vaccination with a bacterin based on the homologous M. hyopneumoniae strain did not result in an increased protection in comparison with bacterins containing other strains, including the J-strain which has been shown to be antigenically quite different from the challenge strain [7, 11]. In a previous study, it was demonstrated that the effect of vaccination with a whole cell bacterin on the course of an experimental M. hyopneumoniae infection may vary, depending on the strain used for challenge . Further studies are required to elucidate the importance of the M. hyopneumoniae strain included in the vaccine for induction of protection. Such studies may include vaccines with different amounts of antigen and different adjuvants.
The challenge infection was successful because in all infected pigs, lung lesions were induced and M. hyopneumoniae was demonstrated by nPCR testing in BAL fluid at the end of the study. The values for RDS and lung lesions in the positive control group E (non-vaccinated, infected) were similar to those found in previous experimental studies using the same challenge strain [4, 5, 16]. Based on nPCR testing, serology and histopathology, the negative control group F remained negative for M. hyopneumoniae throughout the study. This confirms that the pigs used in this study were free of M. hyopneumoniae. It is not clear which pathogen may have caused the very mild pneumonia lesions in two pigs of group F, and neither what may be the reason for the positive IF testing found in one of these pigs (only one of the three sampled lung lobes).
Vaccination with commercial M. hyopneumoniae bacterins generally provides significant improvement of clinical symptoms and lung lesions [1, 2]. In the present study, clinical symptoms (average 0.38 in vaccinated groups versus 0.45 in group E) and histopathological lesions (average 3.20 in vaccinated groups versus 3.45 in group E) were lower in the vaccinated groups, but the difference was only moderate, and smaller than in previous experimental infection studies [5, 16]. In addition, the macroscopic lung lesions in the vaccinated groups (4.34) were on average similar to those in the positive control group (4.03). The reasons for this are not clear. The number of M. hyopneumoniae organisms was kept similar in the different bacterins to avoid that concentration of M. hyopneumoniae cells in the different bacterins would have biased the results. It is possible that the concentration in the bacterins of the present study (7.7 log10 CCU/ml, before inactivation) was lower than in commercial bacterins, and that this can (partially) explain the poor or moderate efficacy. The antigen concentration in commercial bacterins is expressed in different units and therefore not directly comparable with the concentration of the present bacterins.
The bacterins in the present study were not able to prevent colonization nor to eliminate M. hyopeumoniae from the lung tissue, as the percentage of pigs with positive nPCR in nasal swabs and BAL fluid were the same for groups A-E. Previous studies had shown that also commercial vaccines cannot prevent colonization with M. hyopneumoniae and are not able to significantly reduce the transmission of the organism. All bacterins did induce serum antibodies rather quickly, as in all vaccinated groups except for group A, serum antibodies were already detected at D28 i.e. 3 weeks after the first vaccination. At the time of challenge (D56), all vaccinated pigs had detectable serum antibodies.
Performance parameters are generally included in experimental studies with M. hyopneumoniae [5, 8] mainly because it is not that cumbersome to weigh a small number of pigs and to measure feed consumption. In general, no significant results are obtained because of the high standard deviation for these parameters and the rather small number of pigs included in experimental studies. This was also the case in the present study. The rather high number of pigs that died during the study, mainly due to E. coli infection before challenge, may also have influenced the performance of the pigs. Although performance parameters are very important from a financial point of view, these parameters are better investigated under practical circumstances rather than under experimental conditions.