Evaluation of the efficacy of an autogenous whole bacterin Streptococcus suis serotype 9 vaccine in pigs

Introduction

 In intensively raised production animals vaccination is an important tool in on-farm disease management. Vaccination helps to limit the preventive and therapeutic usage of antimicrobials. However, for some bacterial animal diseases effective licensed vaccines are still not available. When no approved vaccine is available, it is allowed by law and under strict conditions, to prepare and apply autogenous whole bacterin vaccines. These vaccines are based on inactivated farm-specific isolates from clinical cases. For the control of Streptococcus suis (S. suis) infections in pigs, these autogenous vaccines are widely used in The Netherlands. S. suis is an important pathogen in pigs worldwide, causing arthritis, meningitis, endocarditis or sudden death in young pigs. S. suis can also cause severe infections in humans. Of the 33 known capsular serotypes of S. suis, serotypes 2 and 9 are worldwide most often associated with clinical signs in commercial pig farms. Serotype 9 is responsible for most cases in The Netherlands. Although widely used, the efficacy of autogenous S. suis vaccination against serotype 9 infections is, however, unknown. The aim of our study was to determine the effect of vaccination with an autogenous whole bacterin vaccine containing S. suis serotype 9 on mucosal colonization, on clinical signs, and on transmission of this serotype among pigs after homologous challenge.

Materials and methods

Animal experiments were performed with caesarean derived, colostrum deprived pigs (N = 50), housed pair-wise. Thirteen pairs were intramuscularly vaccinated at 3 and 5 weeks of age with 2–3×10⁹ colony forming units formalin inactivated S. suis serotype 9 and α-tocopherolactetaat as adjuvant. Twelve pairs served as non-vaccinated controls. At 7 weeks, one pig of each pair was intranasally inoculated with 1–2×10⁹ colony forming units (CFU) of the homologues strain; the other pig was contact exposed. Saliva swabs and tonsil brushings were collected for 4 weeks, and tested for the presence of S. suis by quantitative bacteriological culture. In the same period clinical scoring was performed daily. Blood samples taken at 3 and 7 weeks were tested for S. suis serotype 9 specific IgG in an indirect ELISA.

Results and discussion

Despite specific systemic IgG antibody responses in vaccinated pigs, no differences in quantity of S. suis in tonsilar or saliva samples were observed between vaccinated an control pigs. Mean values in vaccinated pigs were 2.71x10⁵ and 1.18x10⁶ CFU per saliva or tonsil sample, respectively. In control pigs this was 2.39 x10⁵ and 0.73 x10⁶ CFU for saliva (P=0.99) or tonsil (P=0.95), respectively. In all pairs, transmission between inoculated and contact exposed pigs occurred, and no difference was observed in transmission rate between the groups. The estimated transmission rate parameter β was 5.27/day among vaccinated pigs, and among non-vaccinated pigs 2.77/day (P=0.18). No differences in clinical signs were observed between vaccinated and control pigs, which is in contrast to published S. suis serotype 2 studies. Conclusion. Vaccination against S. suis serotype 9 under experimental conditions did not reduce transmission, nor colonization and there were no indications that protection against clinical signs was induced.