Healthy weaners in a pen. S. suis tend to emerge between three to four weeks post-weaning. Photo: Ronald Hissink
Streptococcus (S.) suis infections have long been causing significant challenges on sow farms. The bacteria lead to losses among weaned piglets and an increased use of antibiotics, and they remain difficult to eliminate. A large study project in the Netherlands is busy mapping the infection route from piglets to sows.
Streptococcus suis (S. suis) is a major causative agent of disease in piglets. To date, no solution has been found to eradicate this bacterium, and it appears nearly impossible to completely eliminate it. There are 29 different known S. suis serotypes, which do not all have the same pathogenic potential. Even within these serotypes there are differences in how frequently disease occurs in piglets. On pig farms in the Netherlands, the most frequently observed serotype is S. suis serotype 9. This serotype exists in several variants, some of which are pathogenic, frequently causing problems in weaned piglets. Seeing that a large proportion of antibiotics use in the Netherlands is being applied in controlling S. suis-related infections, it was time for a big research project.
Natural carriers
Virtually all piglets are natural carriers of S. suis. Under certain circumstances, e.g. when immunity is reduced, S. suis can invade the a piglet’s body and cause disease. That would typically occur between 4 and 10 weeks of age. Sows can also be carriers, but they usually do not get ill. Currently, there are no concrete solutions to tackle pathogenic S. suis serotype 9 or to help make piglets become more resilient against the bacterium. Nor is there an effective commercially available vaccine, whereas farms using autovaccines have reported a wide range of results.
Third consecutive S. suis project
Dr Manouk Vrieling is molecular microbiologist at Wageningen Bioveterinary Research (WBVR). She is project leader of a project to target S. suis. She commented, “We are working together with a broad group of farms and partners within pig farming in a public-private partnership to gain insight into the pathogenic S. suis serotypes. We have launched the third consecutive S. suis project this year. This in itself demonstrates that there is wide interest within the sector in obtaining further insight.”
The ultimate goal is to develop a new, targeted approach against S. suis to improve animal health and welfare while further reducing the use of antibiotics where possible. As S. suis is a zoonosis, this targeted approach will also improve the health and safety of people working in pig farming and slaughterhouses.
Manouk Vrieling, researcher at Wageningen Bioveterinary Research in the Netherlands. Photo: Koos Groenewold.
Creating an on-farm management strategy
2 previous projects already helped increase knowledge about S. suis, serotype 9 (see the box Tackling S. suis together). Still, many questions remain about how this can lead to an on-farm management strategy. A third project therefore started early 2025.
Dr Vrieling said, “We want to gain more insight into the transmission from sow to piglet and the dynamics of carrier status during rearing. We are not focusing on a single serotype, but on a broader group of pathogenic S. suis variants. If we know when piglets become carriers of a pathogenic variant, and whether they acquire it from the sow or from pen mates, we may be able to intervene. In addition, we will investigate in the laboratory how S. suis attaches to cells in the upper respiratory tract and whether there are ways to influence that interaction. This could also offer leads to reduce carrier status.”
S. suis: The main reason for antibiotic usage
The second project showed that in weaned piglets, S. suis accounts for a large proportion of antibiotic usage. In 2021, of the average 20 animal daily dosages used in weaned piglets, one-third was administered for the treatment of disease caused by S. suis. About 90% of the total amount of amoxicillin and ampicillin prescribed for weaned piglets was used to treat diseases caused by S. suis. At present, the use of antibiotics is the only effective method to treat piglets. Improved control or management of this bacterium could therefore lead to a significant reduction in antibiotic usage in weaned piglets.
Approximately one-third of the antibiotics used in weaned piglets are administered for the treatment of diseases caused by S. suis, estimated in 2021 at approximately 6.5 animal daily dosages.
