# Serum IgA and bactericidal immunity against Streptococcus suis serotype 2 is increasing between 2 and 6 weeks of age in a farm with autogenous bacterin vaccination pre-farrowing, while specific maternal IgG is decreasing

## TL;DR

This study shows that IgA antibodies in piglets increase over time and are linked to better protection against a bacterial infection, independent of IgG levels.

## Contribution

The study reveals that bactericidal immunity in piglets is associated with increasing IgA levels, not IgG, despite maternal vaccination.

## Key findings

- Serum IgA binding to S. suis increases between 2 and 6 weeks of age, while IgG decreases.
- Bactericidal immunity in piglets correlates with IgA levels, not IgG or colostrum uptake.
- An IgG-independent mechanism contributes to immunity after weaning in vaccinated herds.

## Abstract

Neonatal piglets take up maternal IgG and IgA antibodies via colostrum. Streptococcus suis (S. suis) is a major porcine pathogen that may cause invasive infections in the first ten weeks of life, leading to septicemia, polyarthritis, and meningitis. Preparturient dam vaccination with autogenous S. suis vaccines is common in the field. Vaccination with S. suis bacterins including water-in-oil adjuvants pre-farrowing elicits increased levels of S. suis specific serum IgG antibodies in suckling piglets. However, the influence of various factors associated with colostrum uptake on S. suis specific immunity in piglets has not been investigated thoroughly. This field study was designed to investigate the role of colostrum uptake on levels of IgG and IgA binding to S. suis and how these specific IgG and IgA levels are associated with bactericidal immunity during the nursery phase.

Levels of serum IgG and IgA antibodies binding to the homologous S. suis serotype (cps) 2 strain in 2-week-old piglets correlated significantly with respective levels in colostrum. The quantity of individual colostrum uptake in the first 24 h of life, ranging from 250 g to 733 g in the investigated piglets, showed a correlation with S. suis specific IgA but no correlation with IgG levels in 2-week-old piglets. Levels of specific serum IgG declined significantly in the following 8 weeks of life whereas levels of serum IgA binding to S. suis cps2 increased prominently. Whereas bactericidal immunity in porcine blood against S. suis cps2 was rising in most litters between the 2nd and 6th week of life, increased streptococcal survival factors were observed in single litters in the 6th week. Bactericidal immunity at 2, 6 and 10 weeks of age was not associated with colostrum uptake, body weight at birth, the level of specific IgG or IgA in colostrum or the level of specific IgG in serum of the piglets but with the level of serum IgA binding to S. suis cps2.

The levels of S. suis-specific serum IgG and IgA in 2-week-old suckling piglets in this herd with autogenous bacterin vaccination before farrowing are mainly determined by the respective levels in colostrum. In the following 8 weeks of life, specific serum IgG declines further, whereas specific serum IgA increases prominently and is associated with increased bactericidal immunity in the blood. Our results indicate that an IgG-independent mechanism plays a crucial role in bactericidal immunity after weaning in this herd.

The online version contains supplementary material available at 10.1186/s40813-025-00485-y.