Wanted: A targeted approach to tackling S. suis infections

Streptococcus suis infections have long been causing significant challenges on sow farms. The bacteria lead to losses among weaned piglets and increased use of antibiotics, and still, they remain difficult to eliminate. A large study project in the Netherlands is in the middle of mapping the infection route from piglets to sows.

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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 the disease occurs in piglets. On Dutch pig farms, 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. Given that a large proportion of antibiotics use is being applied in controlling S. suis-related infections, it was time for a major research project.

Virtually all piglets are natural carriers of S. suis. Under certain circumstances, e.g. when immunity is reduced, S. suis can invade a piglet’s body and cause disease. That typically occurs between four and ten 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 more resilient to the bacterium. Nor is there an effective commercially available vaccine, whereas farms using autovaccines have reported a wide range of results.

Dr Manouk Vrieling is a molecular microbiologist at Wageningen Bioveterinary Research (WBVR). She is the project leader of a project established to target S. suis. She commented, “We are working together with a broad range 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.

The third project started in 2025

Two previous projects have already helped increase knowledge about S. suis, serotype 9. See also the box Tackling S. suis together. Still, many questions remain about how this can lead to an on-farm management strategy. This is why a third project started in 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 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 use

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. The use of antibiotics is currently the only effective way to treat piglets. Improved control or management of this bacterium could therefore lead to a significant reduction in antibiotic use 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.

Manouk Vrieling, researcher at Wageningen Bioveterinary Research in the Netherlands. Photo: Koos Groenewold.

Streptococcus suis in a nutshell

Streptococcus suis (S. suis), which is a bacterium, is a major cause of disease in weaned piglets. Most pigs are carriers of S. suis, and the bacterium is found mainly in the tonsils. It can cause various clinical indications in the body and in different organs. The best-known symptoms in pigs are lameness and meningitis. In addition, S. suis can cause pneumonia, endocarditis and sudden death. S. suis is present on all farms, but its impact varies between farms.

At present, 29 S. suis serotypes have been identified. In the Netherlands, where the project is running, serotypes 1, 2, 7 and 9 have been identified in diseased piglets. Which serotypes occur most frequently differs between countries and farms. S. suis can only be detected by bacteriological examination. A PCR test can determine the specific serotype and whether it is a pathogenic variant. Worldwide, serotype 2 is the main cause of disease; in the Netherlands, most piglets are currently affected by serotype 9.

Tackling S. suis together

A broad range of partners from pig farming have been keen to cooperate to jointly develop a new approach through joint research into the most common and pathogenic S. suis types. In public-private partnership research projects, led by WBVR, animal health companies, breeding companies, feeding companies, veterinarians as well as academia are involved. Chronologically, this is what has been studied so far:

2016-2020: Project “Integral approach to S. suis”

In the first study project, the aim was to increase knowledge about serotype 9, as not as much research had been conducted on this serotype in comparison to serotype 2. The outcome was that both pathogenic and non-pathogenic variants of serotype 9 exist. In an animal model, the difference was very clear; the “pathogenic serotype 9” made the piglets ill, whereas the “non-pathogenic serotype 9” did not, or would only do so to a minimal degree. A PCR test was developed that can distinguish between pathogenic and non-pathogenic S. suis serotype 9. In addition, an animal model was established to investigate serotype 9 infections.

2021-2025: Project “S. suis in focus”

A second project – to be concluded mid-2025 – was started up in April 2021. Its goal was to better map the S. suis problem and to develop targeted control strategies. In this project, the amount of antibiotics used against S. suis in the Netherlands was determined. The newly developed PCR test was put into practice. Investigations were carried out to determine whether serotype 9 occurs in weaned piglets on farms in the Netherlands, and how frequently the pathogenic variant occurs.

The result was that about one-third of the antibiotics used for treating weaned piglets are administered for disease caused by S. suis, estimated in 2021 at about 6.5 animal daily dosages (DDDAF). Almost all piglets are carriers of S. suis serotype 9, but the carrier status of the pathogenic serotype 9 varies considerably between farms and even between batches. There is possibly a hereditary component to the carrier status of serotype 9 and the pathogenic S. suis serotype 9, suggesting that genetic selection might contribute to preventing diseases caused by S. suis.

2025-2028: Project “S. suis in the upper respiratory tract of the pig”

The new project focuses primarily on the dynamics of S. suis carrier status. In the second project, piglets were sampled only once; in the new set-up, researcher teams will follow the pigs from the piglet through to the replacement sow. The aim is to determine how and when animals become positive for pathogenic S. suis and whether, through an intervention, negative animals can be maintained as negative. They will also examine in the laboratory the interaction between S. suis and cells of the pig’s upper respiratory tract, to see whether bacterial adhesion can be prevented with antibodies or phytogenic plant extracts.

Managing S. suis with a future vaccine

There is currently no vaccine available that provides effective protection against all pathogenic S. suis infections. The absence of a vaccine is largely due to the large number of serotypes. Autovaccines based on inactivated S. suis bacteria appear to offer reasonable protection against an infection from the same serotype, but these vaccines do not work against other serotypes. Independent of the project, MSD Animal Health has been working on vaccine development. That is not based on the classification via 29 serotypes, but on blocking a specific antigen (protease) that degrades the immune system.

Nico Wertenbroek, an MSD veterinarian, said, “It’s a completely different way of looking at the bacterium. If we compare all streptococcal strains to a set of building blocks with 29 colours and three shapes, the method used in the new vaccine does not focus on the colour of the blocks, but rather on their shape.” He added, “The vaccine is effective against about 85% of the pathogenic S. suis bacteria currently found in the autovaccines used by Dutch farms, regardless of the detected serotype.” A specialised PCR test can be used beforehand to determine whether the vaccine offers protection against the farm-specific pathogenic S. suis strain. Pre-vaccination research is necessary, as the same serotype on different farms may not necessarily be susceptible to the vaccine. The vaccine is still in the registration process and is expected to come to market in about 1.5 to 2 years.

The third project started in 2025

S. suis: The main reason for antibiotic use

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