Immunogenicity of an autogenous Streptococcus suis bacterin in preparturient sows and their piglets in relation to protection after weaning

Abstract

Streptococcus suis is an important porcine pathogen causing meningitis and other invasive diseases in piglets of different ages. Application of S. suis serotype 2 bacterins to specific-pathogen-free (SPF) weaning piglets has been demonstrated to protect against the homologous serotype. However, autogenous S. suis bacterins are also applied to sows and suckling piglets in the field. Therefore, comparative evaluation of different bacterin immunization regimes, including sow vaccination, was performed in this study. The main objectives were to determine the immunogenicity of an S. suis bacterin in sows prepartum and its influence on active immunization of piglets. Experimental infection of 6- and 8-week-old weaning piglets was performed to elucidate protective efficacies. Humoral immune responses were investigated by an enzyme-linked immunosorbent assay (ELISA) measuring muramidase-released protein (MRP)-specific IgG titers and by opsonophagocytosis assays. Bacterin application elicited high MRP-specific IgG titers in the serum and colostrum of sows, as well as opsonizing antibodies. Piglets from vaccinated sows had significantly higher MRP-specific titers than respective piglets from nonvaccinated sows until 6 weeks postpartum. Vaccination of suckling piglets did not result in high MRP-specific titers nor in induction of opsonizing antibodies. Furthermore, neither vaccination of suckling nor of weaning piglets from immunized sows was associated with a prominent active immune response and protection at 8 weeks postpartum. However, protection was observed in respective 6-week-old weaning piglets, most likely because of protective maternal immunity. In conclusion, this study provides the first results suggesting protective passive maternal immunity for S. suis serotype 2 after bacterin vaccination of sows and a strong inhibitory effect on active immunization of suckling and weaning piglets, leading to highly susceptible growers.

FIG. 1.

Experimental design of this study for comparative evaluation of autogenous S. suis serotype 2 bacterin vaccination regimes.

FIG. 2.

Detection of MRP- and CPS2-specific antibodies in sows of the 1st experiment vaccinated 5 and 3 weeks antepartum with a bacterin of S. suis Br3/6 (mrp⁺ epf* sly⁺ cps2). Serum samples were collected 5 weeks (pre) and 1 week (post) antepartum. (A) Serum anti-MRP IgG1 and IgG2 responses (see Materials and Methods). (B) Comparison of MRP-specific titers in colostrum of vaccinated (vac) and nonvaccinated (non-vac) sows. (C) Serum total IgG responses against purified serotype 2 capsular polysaccharides.

FIG. 3.

Opsonophagocytic killing by porcine neutrophils in the presence of serum of sows (data from all three experiments). The y axis depicts the survival factor of the S. suis strain in the opsonophagocytosis assay with porcine neutrophils and sow serum, which is reciprocal to the titer of opsonizing antibody. Bacterin application was performed 5 and 3 weeks antepartum.

FIG. 4.

Kaplan-Meyer morbidity diagrams of piglets challenged intravenously with 10⁸ CFU of S. suis Br3/6 (mrp⁺ epf* sly⁺ cps2) after application of different vaccination regimes, including sow (5 and 3 weeks antepartum) as well as suckling and weaning piglet vaccination with an autogenous bacterin. (A) Challenge in the 6th week postpartum after suckling piglet vaccination (2nd and 4th weeks postpartum) combined in one group with sow immunization (1st experiment). (B) Challenge in the 8th week postpartum after different vaccination regimes, including sow and suckling piglet immunization (2nd and 4th weeks postpartum, 2nd experiment). (C) Challenge in the 8th week postpartum after vaccination of sows combined in one group with weaning piglet vaccination (4th and 6th weeks postpartum, 3rd experiment).

FIG. 5.

MRP-specific IgG1 and IgG2 (A) or opsonizing antibody (B) titers in serum of piglets with or without sow vaccination and their course after suckling piglet immunization in the 2nd and 4th weeks postpartum (data of the 1st experiment). The y axis in panel B depicts the survival factor of the S. suis strain in the opsonophagocytosis assay with porcine neutrophils and piglet serum, which is reciprocal to the titer of opsonizing antibody.