Impact of diversity of Mycoplasma hyopneumoniae strains on lung lesions in slaughter pigs.

The importance of diversity of Mycoplasma hyopneumoniae (M. hyopneumoniae) strains is not yet fully known. This study investigated the genetic diversity of M. hyopneumoniae strains in ten pig herds, and assessed associations between the presence of different strains of M. hyopneumoniae and lung lesions at slaughter. Within each herd, three batches of slaughter pigs were investigated. At slaughter, from each batch, 20 post mortem bronchoalveolar lavage fluid samples were collected for multiple locus variable-number tandem repeat analysis (MLVA), and lung lesions (Mycoplasma-like lesions, fissures) were examined. Multivariable analyses including potential risk factors for respiratory disease were performed to assess associations between the number of different strains per batch (three categories: one strain, two-six strains, ≥seven strains), and the lung lesions as outcome variables. In total, 135 different M. hyopneumoniae strains were found. The mean (min.-max.) number of different strains per batch were 7 (1-13). Batches with two-six strains or more than six strains had more severe Mycoplasma-like lesions (P = 0.064 and P = 0.012, respectively), a higher prevalence of pneumonia [odds ratio (OR): 1.30, P = 0.33 and OR: 2.08, P = 0.012, respectively], and fissures (OR = 1.35, P = 0.094 and OR = 1.70, P = 0.007, respectively) compared to batches with only one strain. In conclusion, many different M. hyopneumoniae strains were found, and batches of slaughter pigs with different M. hyopneumoniae strains had a higher prevalence and severity of Mycoplasma-like lung lesions at slaughter, implying that reducing the number of different strains may lead to less lung lesions at slaughter and better respiratory health of the pigs.