Field Study on the Immunological Response and Protective Effect of a Licensed Autogenous Vaccine to Control Streptococcus suis Infections in Post-Weaned Piglets

Abstract

Streptococcus suis is one of the most important bacterial pathogens in weaned piglets and responsible for serious economic losses to the swine industry. Currently, mostly autogenous vaccines composed of killed bacteria (bacterins) are available. However, immunological and protective data from field studies are missing. We report for the first time a comparative field study on the immunological response induced by an autogenous vaccine applied to either piglets or sows in a farm with recurrent S. suis problems. (I) Piglets from non-vaccinated sows received an autogenous bacterin during the first week and at three weeks of age. (II) Sows received the vaccine at five and three weeks pre-farrowing and piglets were non-vaccinated. Levels, isotype profile and opsonophagocytosis capacity of the serum antibodies induced by vaccination were evaluated. Vaccination of piglets failed to induce an active immune response. Vaccination of sows induced a significant increase in anti-S. suis antibodies, mainly composed of IgG1. However, isotype switching was modulated by the S. suis serotype included in the vaccine formulation. Despite this antibody increase in vaccinated sows, transfer of maternal immunity to piglets was not different from the control group (i.e., piglets from non-vaccinated sows). Notably, levels of maternal antibodies in piglets were already very high with marked opsonophagocytosis capacity at one week of age, independently of the vaccination program. However, their levels decreased by three weeks of age, indicating possible absence of antibodies in the post-weaning high-risk period. These observations correlated with lack of clinical protection in the farm. Overall, a piglet or a sow vaccination program herein mostly failed to induce lasting protection in nursery piglets. An improvement of vaccine formulation or an optimized program may be required.

Keywords: Streptococcus suis; autogenous bacterin; field; infection; pigs; vaccine.

Figure 1

Experimental design of the field study. Experiment 1: Piglets from non-vaccinated sows were vaccinated during the first week at 4 days ± 2 and at weaning with the autogenous vaccine (containing S. suis serotype 7). Blood samples were collected from randomly chosen piglets at 1, 3, 5 and 8 weeks of age. Control (including placebo animals) were also included. Experiment 2: (A) Sows were at their parity 0 and received two doses of autogenous vaccine (containing S. suis serotypes 7 and 9) intramuscularly at 5 and 3 weeks before farrowing. Blood samples were taken from all enrolled sows prior to vaccination at 5 weeks before farrowing and at 1 week before farrowing (2 weeks after the 2nd vaccine dose). (B) Randomly selected piglets from vaccinated and non-vaccinated sows were sampled at 1, 3 and 5 weeks of age.

Figure 2

Experiment 1: Kinetics of total Ig against S. suis serotype 7 in piglets. Blood samples were collected from randomly chosen (and tagged) piglets at one, three, five and eight weeks of age from 50 vaccinated and 70 non-vaccinated (including the 20 placebos) animals to follow the immune response. The vaccination protocol is shown in Figure 1. Total Ig [IgG + IgM] titers were determined by ELISA. Antibody titers for individual piglets are shown with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value. Arrows indicate first and second vaccination doses.

Figure 3

Experiment 1: Isotype profile of antibodies against S. suis serotype 7 in piglets. Blood samples were collected from randomly chosen (and tagged) piglets at one and five weeks of age from 50 vaccinated and 50 non-vaccinated (including the 20 placebos) animals to evaluate: (A) IgM titers; (B) IgG1 titers; and (C) IgG2 titers by ELISA. The vaccination protocol is shown in Figure 1. Antibody titers for individual piglets are shown with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value.

Figure 4

Experiment 1: Opsonophagocytosis killing of S. suis serotype 7 induced by serum antibodies from piglets. Blood samples were collected from randomly chosen piglets at one, three and five weeks of age. Blood samples from 50 vaccinated and 70 non-vaccinated (including the 20 placebos) at one and three weeks of age and from 25 vaccinated and 25 non-vaccinated (including the 20 placebos) at five weeks of age were tested in an opsonophagocytosis assay (OPA) to evaluate their functionality. For OPA, blood leukocytes were mixed with S. suis serotype 7 (vaccine strain) at a multiplicity of infection of 0.01. Control sera or sample sera from vaccinated piglets or non-vaccinated piglets were added to a final concentration of 40% v/v in microtubes which were incubated for 4 h. After incubation, viable bacterial counts were performed and the percentage of bacterial killing was determined. The results are expressed as percent of bacterial killing for individual sera, with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value. Arrows indicate first and second vaccination doses.

Figure 5

Experiment 1: Number of S. suis-related injectable treatments and S. suis-induced number of deaths in piglets over time. Piglets from non-vaccinated sows were selected (n = 1494). During the first week, at 4 ± 2 days, each litter was randomly divided into two groups. Group 1 (“vaccinated group”; n = 583) received the autogenous vaccine (as shown in Figure 1) and Group 2 was processed normally (“non-vaccinated group”; n = 911). Piglets were randomly weaned into three nursery rooms, with pigs from Groups 1 and 2 mixed in each pen. S. suis-related mortality and injectable treatments were recorded from all enrolled piglets by farm staff daily by room. Pigs were identified as ear notched or not, and there were no individual pig identifiers. As such, an individual pig could be represented more than once in the dataset for treatments. Pigs were followed until the end of the nursery period at approximately 10 weeks of age. In this graph, only S. suis-related mortality and injectable treatments are displayed.

Figure 6

Experiment 2: Total Ig levels against S. suis serotype 7 (A) or serotype 9 (B) in sows. Blood samples were collected one and five weeks before farrowing from 20 vaccinated and 20 non-vaccinated sows to follow the immune response. The vaccine was administered at five and three weeks before farrowing. Total Ig [IgG + IgM] titers were determined by ELISA. Antibody titers for individual sows are shown with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value. Arrows indicate first and second vaccination doses.

Figure 7

Experiment 2: Isotype profile of antibodies against S. suis serotype 7 (A) or serotype 9 (B) in sows. Blood samples were collected one and five weeks before farrowing from 20 vaccinated and 20 non-vaccinated sows to follow the immune response. The vaccination protocol is shown in Figure 1. IgM, IgG1 and IgG2 titers were determined by ELISA. Antibody titers for individual sows are shown with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value.

Figure 8

Experiment 2: Kinetics of total Ig and isotype profile of antibodies against S. suis serotype 7 or serotype 9 in piglets from either vaccinated or non-vaccinated sows. Randomly selected 120 piglets (3 piglets/sow, for a total of 60 piglets/group) from vaccinated and non-vaccinated sows were originally included in the serological study. Nevertheless, nine piglets from three different non-vaccinated sows could not be identified, for a total of 51 piglets finally enrolled for serology in this group. Piglets were sampled at one, three and five weeks of age and total Ig [IgG + IgM] titers were determined by ELISA against S. suis serotype 7 (A) or serotype 9 (B). Piglets were sampled at one week to evaluate titers of IgG1 against S. suis serotype 7 (C) or serotype 9 (D) and titers of IgG2 against S. suis serotype 9 (E). Antibody titers for individual piglets are shown with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value.

Figure 9

Experiment 2: Opsonophagocytosis killing of S. suis serotype 7 induced by serum antibodies from sows and from their piglets. Blood samples were collected one week before farrowing from seven vaccinated and seven non-vaccinated sows and from randomly chosen piglets (1 per sow) at one and five weeks of age (n = 20 per group) to evaluate their functionality in an opsonophagocytosis assay (OPA). For OPA, blood leukocytes were mixed with S. suis serotype 7 (vaccine strain) at a multiplicity of infection of 0.01. Control sera or sample sera from vaccinated or non-vaccinated animals were added to a final concentration of 40% v/v in microtubes, which were incubated for 4 h. After incubation, viable bacterial counts were performed and the percentage of bacterial killing determined. The results are expressed as percent of bacterial killing for individual sera, with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value.

Figure 10

Experiment 2: Number of S. suis-related injectable treatments and S. suis-related deaths in piglets over time. In total, 207 piglets from vaccinated sows and 183 piglets from non-vaccinated sows were enrolled in the trial. Piglets were weaned into one nursery room, with mixed vaccinated and non-vaccinated groups in each pen. Total and S. suis-related mortality and injectable treatments were recorded from all enrolled piglets by farm staff daily by room. Pigs were identified as notched or not notched, and there were no individual pig identifiers. As such, an individual pig could be represented more than once in the dataset for treatments. Pigs were followed until the end of the nursery period. In this graph, only S. suis-related mortality and injectable treatments are displayed.

Figure 11

Experiment 1 and 2: Total Ig and IgM profile of antibodies against S. suis serotype 7 purified capsular polysaccharide (CPS) from either vaccinated or non-vaccinated piglets from Experiment 1 (A,B) and vaccinated sows from Experiment 2 (C,D). Randomly selected 20 piglets (10 vaccinated and 10 non-vaccinated) from Experiment 1 and 20 randomly selected vaccinated sows from Experiment 2 were used for the quantification of antibodies against S. suis serotype 7 CPS. Total Ig [IgG + IgM] titers (A,C) and IgM titers (B,D) were determined by ELISA against purified S. suis serotype 7 CPS. Piglets were sampled at five weeks to evaluate titers of antibodies against serotype 7 CPS and sows were sampled at one and five weeks before parturition. Antibody titers for individual piglets or sows are shown with horizontal bars representing mean ± SEM. Values significantly different are shown in the graph with corresponding p value.