Why Streptococcus suis remains one of the toughest pig diseases to control

For decades, Streptococcus suis (Strep. suis) sat in the background of swine health conversations – present in most herds, often overlooked, and largely controlled through antibiotics. Today, that approach no longer works. As antibiotic use tightens globally and co-infections continue to complicate herd health, Strep. suis has emerged as one of the most challenging bacterial diseases facing pig producers worldwide.

Few researchers understand that challenge better than Dr. Marcelo Gottschalk, full professor in the Department of Pathology and Microbiology at the Faculty of Veterinary Medicine at the University of Montreal. Gottschalk also serves as director of the International Reference Laboratory for Streptococcus suis and advises the World Health Organization on human disease caused by the pathogen.

From vaccine development to management strategies and the growing role of gut health, Gottschalk’s work highlights why Strep. suis is so difficult to control – and why simple solutions remain elusive.

Why vaccines have proven so difficult

Despite the disease’s importance, commercial vaccines against Strep. suis remain largely unavailable. According to Gottschalk, that gap reflects both biological and practical challenges.

“There are very few commercial vaccines for Strep. suis,” he said. “That is one of the most important problems, because it has been treated and prevented using antibiotics for many, many years.”

In the absence of licensed vaccines, producers and veterinarians have turned to autogenous vaccines – farm-specific products made from strains isolated on individual farms. But these vaccines come with significant limitations.

“There are no standardized protocols,” Gottschalk said. “Each company will keep the secret of the production of these autogenous vaccines, so an autogenous vaccine produced in one lab and another produced in another lab may be completely different products.”

Autogenous vaccines are also not required to demonstrate protective efficacy, adding another layer of uncertainty.

“The future is not necessarily autogenous vaccines, it's a more universal vaccine because these are produced under very strong regulations and are tested for protection,” he explained.

Recognizing those shortcomings, Gottschalk’s research group has focused both on improving autogenous vaccine formulation by looking, among other factors, at bacterial concentrations, the simultaneous use of different strains in the same vaccine as well as different adjuvants.

In parallel, Gottschalk and his team are researching and developing a synthetic vaccine that targets the bacterial capsule.

“The bacteria have a capsule –it’s kind of a shield that protects bacteria that is composed of sugars which are not very immunogenic. So, there are not a lot of antibodies,” he explained. “If you get antibodies against sugar, they're very protected. In humans, they will use the complete capsule to produce a vaccine. But the problem is that is extremely expensive, so you can do it in humans, but you cannot do it in pigs.”

His solution is to synthetize a small part of the capsule which his linked with another protein. The protein will then make the sugar visible for the immune system, and the immune system will produce antibodies against it.

That strategy has already shown protection in experimental trials and represents a potential breakthrough not only for swine health, but for veterinary vaccines more broadly.

The immunology problem: low response, high diversity

Even with promising vaccine candidates, Strep. suis presents fundamental immunological hurdles. The bacterium is poorly immunogenic, meaning pigs do not naturally mount strong antibody responses following exposure.

Compounding that challenge is the pathogen’s genetic and serotype diversity. Strains differ widely between regions, and a vaccine effective in one part of the world may offer little protection elsewhere.

“The strains present in Europe are different from those present in North America,” he noted. “And that’s another problem for the vaccine, trying to cover everybody, because strains are very diverse.”

While Strep. suis does not evolve as rapidly as some viral pathogens, regional variation remains a persistent obstacle to universal vaccine coverage.

Who to vaccinate – and when?

Even if an ideal vaccine existed, deployment would remain complicated. Strep. suis disease typically strikes piglets between 4 to 10 weeks of age, overlapping with the post-weaning period.

Vaccinating sows seems logical, but maternal immunity introduces variability.

“Colostrum intake is not homogeneous,” Gottschalk said. “We have many, many studies where we demonstrate that even siblings vary – one will have very high level of maternal antibodies, the other one very low.”

That variability makes it difficult to ensure consistent protection throughout the entire risk window.

Vaccinating piglets directly raises its own concerns. Because Strep. suis is poorly immunogenic, piglets often require multiple doses, and maternal antibodies may interfere, in theory, with vaccine efficacy.

“One of the problems is when you have maternal antibodies and you need a vaccine many times there is an interference because the antibody from the mother will neutralize the vaccine that you are giving to the piglet,” Gottschalk explained. His team is presently studying if this interference really occurs.

These competing constraints – cost, timing, immune interference and stress – help explain why no single vaccination strategy has emerged as clearly superior.

The role of co-infections

Strep. suis rarely acts alone. Co-infections play a critical role in determining whether colonization progresses to disease.

Porcine reproductive and respiratory syndrome (PRRS) remains the most significant driver.

“When you have a herd which is unstable, you will have problems with Strep. suis,” Gottschalk said. “You have countries like Spain where they have a very high virulent strain of PRRS, and they’ve had a lot of problems with Strep. suis in the last couple of years.”

Influenza also acts as a major predisposing factor. Other pathogens, however, appear less consistently linked than often assumed.

“We have tested co-infection with Mycoplasma, but we didn’t find anything,” he said. “We didn’t find anything with Glaesserella (Haemophilus) parasuis.”

Those findings underscore the importance of prioritizing control of primary viral pathogens like PRRS and influenza rather than assuming all respiratory agents contribute equally to Strep. suis outbreaks.

Managing stress over medicine

From Gottschalk’s perspective, Strep. suis control is less about chasing individual strains and more about addressing underlying stressors.

“Strep. suis is not the origin of the problem,” he said. “It’s the consequence of the problem.”

The bacterium is a normal inhabitant of the tonsils in most pigs. Disease emerges when environmental, nutritional or health stressors create the right conditions for invasion.

“If you try to attack each time that a strain of Strep. suis appears, it will not work,” Gottschalk said.

Instead, effective control focuses on stabilizing PRRS and influenza, reducing environmental stress, managing post-weaning nutrition and minimizing intestinal disruption.

“This is still one of the most difficult diseases to be controlled today,” he added.

Looking ahead

As the industry explores disease-resistant genetics and gene editing, Gottschalk remains cautious. Because Strep. suis is part of the normal flora, eliminating colonization could have unintended consequences.

“You would need the pig with the modification in the genes that will become resistant to disease, but not to colonized bacteria,” he said.

While nothing is impossible, he emphasized humility in the face of biological complexity.

“Biology is biology; the technology is changing extremely fast,” Gottschalk said. “You never know.”

For now, Strep. suis remains a reminder that progress in pig health often depends less on silver bullets and more on understanding and managing the intricate interactions between pathogens, immunity, nutrition and environment.