Virulence, transmission, and heterologous protection of four isolates of Haemophilus parasuis

Abstract

Haemophilus parasuis causes Glässer's disease, a syndrome of polyserositis, meningitis, and arthritis in swine. Previous studies with H. parasuis have revealed virulence disparity among isolates and inconsistent heterologous protection. In this study, virulence, direct transmission, and heterologous protection of 4 isolates of H. parasuis (SW114, 12939, MN-H, and 29755) were evaluated using a highly susceptible pig model. In an initial experiment, isolates 12939, MN-H, and 29755 caused Glässer's disease, while strain SW114 failed to cause any clinical signs of disease. One pig from each group challenged with MN-H or 29755 failed to develop clinical disease but was able to transmit H. parasuis to noninfected pigs, which subsequently developed Glässer's disease. Pigs colonized with SW114, 29755, or MN-H that were free of clinical disease were protected from a subsequent challenge with isolate 12939. In a following experiment, pigs vaccinated with strain SW114 given as either a bacterin intramuscularly or a live intranasal vaccine were protected from subsequent challenge with isolate 12939; however, some pigs given live SW114 developed arthritis. Overall these studies demonstrated that pigs infected with virulent isolates of H. parasuis can remain healthy and serve as reservoirs for transmission to naive pigs and that heterologous protection among H. parasuis isolates is possible. In addition, further attenuation of strain SW114 is necessary if it is to be used as a live vaccine.

Fig 1

Complement-mediated killing of H. parasuis strains SW114 and 12939. (A) Log₁₀ CFU of H. parasuis SW114 and 12939 after incubation for 1 h in PBS without added serum, with heat-inactivated (HI) caesarian-derived, colostrum-deprived (CDCD) pig serum, or with non-HI CDCD pig serum (bars represent the means ± standard errors of 5 replicates). (B and C) Log₁₀ CFU of H. parasuis SW114 (B) and 12939 (C) after a 1-h incubation with non-HI CDCD pig serum as a complement source and HI prevaccination (Pre-vax) or postvaccination (Post-vax) sera collected from pigs vaccinated with SW114 bacterin intramuscularly (bacterin) or live SW114 intranasally (Live IN) or not vaccinated (No vax) in experiment 2 (bars represent the means ± standard errors). For data in panel C, the group receiving SW114 intranasally was separated into 2 groups depending on ceftiofur treatment as described in Results. Capped lines indicate statistical significance.