Characterization of a Pasteurella multocida type a strain associated with a severe bronchopneumonia outbreak in gilts

Abstract Background In pigs, Pasteurella multocida (P. multocida) serotypes A, B and D are causative agents of pneumonia, hemorrhagic septicemia and progressive rhinitis atrophicans, respectively. In the case of bronchopneumonia in pigs, P. multocida is generally regarded as an opportunistic pathogen following predisposing infectious agents. This study was initiated by an outbreak of bronchopneumonia in gilts associated with high mortality during lactation. Objectives of this study were to characterize the P. multocida isolate and systemic humoral immunity in gilts against this pathogen. Results Bacteriological analyses revealed a high load of P. multocida and Trueperella pyogenes (T. pyogenes) in the lungs of diseased gilts, whereas other infectious agents such as viruses or Mycoplasma species were not found. The P. multocida isolate belonged to serotype A, sequence type 3, lipopolysaccharide genotype L3 and carried virulence-associated genes such as ompA, hgbA, tonB, ptfA, nanB and nanH. The lung isolate proliferated in porcine blood drawn from piglets and gilts of a non-affected farm, but showed reduced survival in blood drawn from gilts of the affected breeding farm, whereas the strain proliferated in blood of 6- and 8-week-old piglets. Significantly higher levels of IgG binding to the P. multocida isolate were detected in gilts of the affected farm in comparison to gilts of the unaffected herd. Gilts of both farms showed elevated levels of serum IgM binding to the P. multocida isolate in comparison to weaning piglets. Specific degradation of IgM in serum of gilts resulted in increased survival of this strain in reconstituted blood. Conclusion The P. multocida serotype A isolate described in this manuscript is associated with severe bronchopneumonia in gilts as part of a mixed infection with Trueperella pyogenes, but without detection of predisposing viral or mycoplasmal infections. Our data suggests that affected gilts undergo a systemic immune response leading to increased specific IgG levels and bactericidal immunity. Furthermore, our in vitro results indicate that IgM plays an important role in mediating killing of this P. multocida strain in blood of gilts.