Homologous whole bacterin vaccination is not able to reduce Streptococcus suis serotype 9 strain 7997 transmission among pigs or colonization

Streptococcus suis (S. suis) is an important porcine pathogen worldwide, and antibiotics are often applied to treat or prevent clinical signs. Vaccination could be an alternative measure to reduce the abundant use of antimicrobials. The aim of this study was to determine the effect of vaccination with homologous whole bacterin vaccine containing S. suis serotype 9 strain 7997 on transmission of this serotype among pigs and on mucosal colonization. Caesarean derived, colostrum deprived pigs (N = 50) were housed pair wise. Thirteen pairs were vaccinated intramuscularly with 2-3 x 109 colony forming units (CFU) inactivated S. suis serotype 9 per dose and α-tocopherolactetaat as adjuvant at 3 and 5 weeks of age; twelve pairs served as non-vaccinated controls. At 7 weeks of age, one pig of each pair was intranasally inoculated with 1-2 x 10⁹ CFU of the homologous strain, whereas the other pig of each pair was contact-exposed. Tonsil brushings and saliva swabs were collected for 4 weeks, and tested for the presence of S. suis by bacteriological culture. No differences in number of S. suis in the tonsils or saliva samples or in clinical signs were observed between vaccinated and control pigs. In all pairs, transmission between inoculated and contact exposed pigs occurred, and no difference was observed in rate at which this occurred. The estimated transmission rate parameter β between vaccinated pigs was βv = 5.27/day, and for non-vaccinated pigs βnv = 2.77/day (P = 0.18). It was concluded that vaccination against S. suis serotype 9 did not reduce transmission, nor colonization and that there were no indications that protection against clinical signs was induced.