Key takeaways 

Streptococcus suis continues to be a formidable yet manageable threat in pig production. It requires an integrated approach combining good husbandry, vigilant health monitoring, and medical interventions. Below are the key takeaways regarding S. suis infection: 

  • Widespread pathogen: Streptococcus suis is present on almost all pig farms worldwide and is a leading bacterial cause of illness in post-weaned piglets, manifesting as meningitis, arthritis, pneumonia, or sudden death. 
  • Carrier state and triggers: Most pigs carry S. suis without symptoms. Disease outbreaks tend to occur when young pigs are stressed (e.g. around weaning) or exposed to risk factors like overcrowding, poor ventilation, large temperature swings, or concurrent infections. Proper management and reducing these stressors are critical to prevent clinical disease. 
  • Zoonotic risk: S. suis can infect humans who have close contact with pigs or consume raw pork, causing serious infections (especially meningitis). Farm staff should take precautions (protective gear, good hygiene), and the general public should avoid eating raw or undercooked pork. Controlling S. suis in pigs also helps protect public health. 
  • Treatment: Early detection and treatment of S. suis infections in pigs are vital. Affected pigs are treated with antibiotics (typically penicillin or amoxicillin, for at least 5 days) and supportive care, which significantly improves survival. However, judicious use of antibiotics is necessary due to increasing antimicrobial resistance in S. suis. 
  • Control measures: There is no universal vaccine for S. suis yet. Control relies on high biosecurity, sanitation, and management practices to minimise exposure and stress. Autogenous vaccines can be used against farm-specific strains with mixed results. Ongoing research into better vaccines is promising, but in the meantime, farms must employ an array of preventive strategies to keep S. suis at bay. 

Streptococcus suis infection in pigs is a significant health issue in modern swine production worldwide. This bacterial disease primarily affects young pigs (especially post-weaning piglets) and can lead to serious conditions such as meningitis, arthritis, pneumonia, and even sudden death, resulting in economic losses and animal welfare concerns. S. suis is also an emerging zoonotic pathogen – it can spread from pigs to humans – underscoring the importance of effective control measures on farms. In this backgrounder, we provide a comprehensive overview of S. suis, including what it is, how to recognise and diagnose it, how it is transmitted, its global occurrence and serotype distribution, its zoonotic relevance, treatment options (and antibiotic resistance issues), and best practices for prevention and control in pig herds. 

What is Streptococcus suis? 

Streptococcus suis is a gram-positive bacterium (coccus) that is one of the most important pathogens of pigs, causing mainly septicaemia, meningitis (infection of the brain’s lining), arthritis, and endocarditis in young pigs. There are many strains of S. suis, distinguished by their polysaccharide capsules: 29 serotypes of Streptococcus suis are officially recognised as of 2023. (Earlier literature described 35 serotypes, but several were reclassified as different species, leaving 29 true S. suis serotypes.) Importantly, not all serotypes are equally pathogenic – only a subset cause most of the clinical disease in pigs. Serotype 2 is historically the most virulent and well-studied type, though other serotypes (like 1/2, 3, 7, 9, etc.) also contribute to disease outbreaks in various regions. 

In most pig herds, S. suis lives benignly in some animals as a commensal organism. However, under certain conditions, it can invade the body and cause severe illness. Streptococcus suis is ubiquitous in swine-producing areas around the world and very difficult to eliminate completely from farms. Once a herd has S. suis, it tends to persist because pigs can carry the bacterium without symptoms. There is currently no universal vaccine available that covers all serotypes of S. suis, and thus control relies on management practices and, when disease occurs, antibiotic treatment. Notably, S. suis can also infect humans (see zoonotic relevance below), making it both a veterinary and public health concern. 

Clinical signs and symptoms of Streptococcus suis in pigs 

Affected pigs often show non-specific early symptoms like high fever, loss of appetite, and depression. The infection typically becomes evident through one or more syndromes, most commonly meningitis or polyarthritis. In fact, the most well-known clinical manifestations of S. suis in pigs are nervous system signs (from meningitis) and lameness (from arthritis). Post-weaned piglets (around 4–8 weeks old) are the most susceptible age group. 

Meningitis due to S. suis is characterised by neurological abnormalities. Initially, pigs may appear listless, unsteady, or show a “star-gazing” posture. Often they hold their ears back and may squint their eyes in early stages. As meningitis progresses, pigs develop incoordination and may adopt a dog-sitting posture, followed by inability to stand. They commonly fall onto their side and exhibit paddling convulsions (running movements of the legs), along with seizures and opisthotonos (head thrown back). A pig with advanced meningitis may lie on its side with legs cycling and may have nystagmus (rapid eye movements). Without prompt treatment, meningitis is frequently fatal. 

Arthritis is another common presentation. Pigs develop swollen, warm joints (especially in the limbs), causing severe lameness or reluctance to move. Affected piglets may be visibly stiff or have difficulty standing up, and pain in the joints leads to recumbency. In suckling piglets, Streptococcus suis can cause a syndrome of polyarthritis (multiple joint inflammation) often accompanied by meningitis. 

In peracute cases, S. suis infection can lead to sudden death before obvious clinical signs are observed. Septicaemia (blood infection) may kill piglets very quickly, sometimes such that a well-grown piglet is simply found dead. Some growing pigs develop endocarditis (infection of heart valves) from S. suis; these pigs may die suddenly or show signs of heart failure (laboured breathing, cyanosis, wasting) prior to death. Respiratory disease (such as pneumonia) is a less common outcome of S. suis infection – respiratory signs might appear during outbreaks, but S. suis is not primarily a lung pathogen. Often, isolation of S. suis from the lungs at slaughter is considered incidental unless other signs of disease are present. 

It’s worth noting that many pigs carry S. suis without ever becoming ill, and only a small percentage of pigs in a herd typically develop clinical disease at any given time. When Streptococcus suis does cause disease, it is most often in the nursery phase, and the combination of high fever, nervous signs (such as tremors, paddling seizures) and lameness or swollen joints in post-weaned piglets is highly suggestive of an S. suis outbreak. 

Transmission and risk factors of Streptococcus suis

Within a herd: Streptococcus suis is primarily spread from pig to pig via oral and nasal contact (e.g. nose-to-nose touching, aerosol droplets) and through sow-to-piglet transmission at birth. Virtually all piglets are born as carriers of S. suis – they acquire the bacterium from the sow’s vaginal tract during farrowing and from the sow’s saliva or milk while nursing. Sows can carry S. suis in their tonsils or reproductive tract without showing illness, acting as a reservoir for piglets. 

After birth, piglets harbour S. suis in their upper respiratory tract, but maternal antibodies (colostrum-derived immunity) help protect them during the nursing period. When piglets are weaned (often around 3-4 weeks old) and maternal antibodies wane, the stress of weaning and mixing can allow the bacterium to invade and cause disease. Typically, clinical outbreaks occur 2-3 weeks post-weaning (around 6-9 weeks of age), when maternal immunity has declined and piglets are exposed to new penmates and possibly new S. suis strains. Subclinical carrier piglets can infect their penmates after mixing, especially in the nursery phase. 

S. suis can also spread through indirect routes, although pig-to-pig contact is the most important. The bacterium is fairly hardy in the pig environment – it can survive for days to weeks in moist, cool conditions or in organic material. Infection may occur via contaminated fomites (equipment, feeders, waterers, boots, etc.) and even via aerosolised dust in pens. Streptococcus suis has been isolated from insects (like flies) and other animals on farms, but these are thought to play a minor role compared to direct pig transmission. Skin wounds can be an entry point as well, though in pigs the respiratory route is more common. Importantly, once S. suis is present in a herd, it tends to persist, since even recovered pigs or unaffected carriers can continuously reintroduce the bacterium to naïve piglets. 

Between herds: The main mode of inter-farm transmission is the movement of live pigs. Healthy-looking pigs that carry S. suis (especially young breeding stock or growers) can introduce the bacterium into an unexposed herd. Introduction of a highly virulent strain into a naïve (unexposed) herd can result in severe disease outbreaks in weaned pigs shortly after. Fortunately, outright transmission via farm-to-farm aerosol or wild animals is not a significant concern; the disease typically moves with pigs. Using strict biosecurity and quarantine for incoming pigs can help prevent new S. suis variants from entering a farm. 

Certain risk factors make clinical Streptococcus suis disease more likely to occur. Even in herds where S. suis is present, farmers might observe little to no disease until these predisposing factors stress the pigs’ immune system or increase exposure. Key risk factors known to precipitate S. suis outbreaks include: 

  • Overcrowding – high stocking density increases direct contact and stress. 
  • Poor ventilation – inadequate airflow can raise ammonia and humidity, irritating piglets’ respiratory tracts and aiding bacterial spread. 
  • Temperature fluctuations or chilling – sudden changes from hot to cold, or chilling drafts on piglets, can stress pigs and impair their immunity. 
  • Mixing of pigs from different sources or ages – when pigs of different litters or with an age gap >2 weeks are mixed, naïve pigs may encounter carriers and become infected. 
  • Coinfections with other pathogens – concurrent infections, especially with viruses like PRRS (porcine reproductive and respiratory syndrome) or swine influenza, dramatically increase the likelihood and severity of S. suis outbreaks. These viruses weaken the pig’s immune defences and often precede S. suis meningitis or septicaemia cases. 

In practice, outbreaks of Streptococcus suis are frequently reported in association with PRRS virus circulation in a herd, and to a lesser extent with flu or other immunosuppressive diseases. Stressors such as moving, mixing, or aggressive handling of pigs can also trigger latent S. suis infections to flare up. Farmers and veterinarians work to minimise these risk factors – for example, by providing good housing conditions, stable group structures, and controlling other diseases – to keep S. suis cases to a minimum. 

Global occurrence of Streptococcus suis and serotype distribution 

Geographical occurrence: Streptococcus suis is found wherever pigs are raised. The bacterium has a worldwide distribution, particularly in swine-intensive regions. In fact, most pig farms globally are likely to have S. suis present in some pigs. Almost every pig is a potential carrier of S. suis (often with multiple serotypes in one animal), although only a small fraction may develop disease at any given time. Surveys have shown near 100% of healthy pigs can carry S. suis in their tonsils, yet the incidence of clinical disease in a herd is usually low (often <5% of pigs showing illness). Sporadic, low-level disease or subclinical infections might go unnoticed.  

Serotype distribution: While there are 29 known serotypes of S. suis, the majority of pig disease is caused by a subset of these. Globally, serotypes 1/2, 2, and 3 are reported to be the most common causes of clinical infections in pigs.  

Diagnosis of Streptococcus suis

Diagnosing Streptococcus suis infection in a pig herd involves a combination of clinical observation and laboratory testing. In the face of an outbreak, clinical signs and history provide the first clues. When post-weaned piglets present with classical signs of meningitis (paddling, convulsions) or sudden deaths with signs of septicaemia, a presumptive diagnosis of S. suis is often made on-site. However, definitive confirmation requires identifying the bacterium in sick or dead pigs. Typically, a veterinarian will collect samples from affected animals – such as cerebrospinal fluid, brain tissue, joint fluid, or spleen – and submit them for bacterial culture. Streptococcus suis is relatively straightforward to isolate in the lab on blood agar.  

Once S. suis is isolated, serotyping is usually performed to determine which capsular type is involved. This can be done by conventional methods or by PCR assays that detect genes unique to each serotype.  

Treatment and antibiotic resistance in Streptococcus suis infection

Treatment of Streptococcus suis infection in pigs must be fast and aggressive, because the disease can progress rapidly. The cornerstone of therapy is antimicrobial treatment to eliminate the bacteria. In practice, as soon as early signs of S. suis (such as fever or slight incoordination in a weaner pig) are noticed, the pig should be given an appropriate antibiotic. Penicillins are the traditional first-line treatment: S. suis is usually susceptible to beta-lactam antibiotics, so injections of penicillin G, ampicillin, or amoxicillin are commonly used and are effective in most cases. In many outbreaks, affected pigs are treated individually with penicillin/ampicillin injections once or twice daily for several days. If many piglets are showing signs, herd treatment via medicated drinking water or feed can be implemented (e.g. amoxicillin in water) – this is particularly done in countries where metaphylactic antibiotic use is permitted. Whichever route is used, therapy should continue for at least 5 days, because shorter courses may not fully clear the bacteria and pigs can relapse. 

Along with antibiotics, supportive care is important. Anti-inflammatory drugs (NSAIDs or corticosteroids) are often administered to reduce the inflammation and swelling in the brain during meningitis.  

Prevention and control of Streptococcus suis

Completely eradicating Streptococcus suis from a farm is generally not feasible under conventional management – the organism is too widespread and piglets acquire it as soon as they are born. Instead, prevention and control efforts aim to minimise the occurrence and severity of S. suis disease. Many farms manage to coexist with S. suis with only infrequent problems by implementing good husbandry and health practices. Here are key strategies for preventing and controlling S. suis in pig herds: 

  • Optimise hygiene and biosecurity: Maintain a clean environment to reduce the bacterial load. Use an all-in/all-out production system in farrowing and nursery units if possible – this means piglets move through in groups, and between groups the facilities are emptied and thoroughly cleaned. Streptococcus suis is susceptible to common disinfectants (e.g. aldehydes, chlorine, iodine, quaternary ammonium compounds), so disinfect pens and equipment between batches to break the cycle of infection. Prevent cross-contamination via tools, and enforce biosecurity protocols for farm staff and visitors (changing boots, using footbaths, etc.). 
  • Improve housing and reduce stress: Avoid overcrowding and provide adequate space per pig. Good ventilation is crucial – eliminate ammonia buildup and keep air fresh to support pigs’ respiratory health.  
  • Strengthen piglet immunity: Ensure all piglets receive sufficient colostrum from the sow after birth, as this is their primary source of antibodies against S. suis and other microbes. In endemic herds, sows may be vaccinated (with autogenous vaccines) to boost specific antibodies in colostrum. Provide excellent nutrition to young pigs – a quality creep feed and later a well-balanced nursery diet help the piglets’ immune system develop.  
  • Control concurrent infections: Because coinfections greatly exacerbate S. suis, it’s critical to keep other diseases in check. PRRS virus in particular should be controlled through vaccination and biosecurity. 
  • Implement prudent antibiotic use: While not a true “prevention” in the long term, strategic use of antimicrobials can help manage S. suis. For example, some high-risk herds administer a dose of long-acting penicillin to piglets at weaning (“weaning medication”) to suppress S. suis during the stress period. Others put a medicated feed or water course (e.g. amoxicillin in water) for a week post-weaning if the farm has a history of Strep problems.  
  • Management and monitoring: Train farm personnel to recognize early signs of S. suis and respond quickly. Segregate and treat any sick piglets immediately to prevent them from becoming a source of infection to pen-mates.  

Conclusion  

By understanding the epidemiology of S. suis and adhering to sound veterinary practices, pig producers and veterinarians can significantly reduce the impact of this disease. Continuous monitoring, rapid response to clinical signs, and a focus on prevention will collectively ensure healthier pigs and safer working conditions, despite the ever-present nature of S. suis in global swine herds. 

FAQ on Streptococcus suis infection

Q1: What is Streptococcus suis and why is it important? 
A1: Streptococcus suis is a gram-positive bacterium that lives in pigs. It’s important because it’s one of the leading causes of disease in young pigs – it can cause meningitis (brain infection), septicaemia (blood infection), pneumonia, and other serious conditions. S. suis infections lead to economic losses on farms due to piglet deaths and treatments. It’s also zoonotic, meaning it can occasionally infect humans, so it has public health significance in regions where people have close contact with pigs or eat raw pork. 

Q2: What symptoms does Streptococcus suis cause in pigs? 
A2: In piglets, S. suis most often causes neurological signs due to meningitis. Affected pigs may show fever, loss of balance, reluctance to rise, paddling seizures, and head tilts, sometimes along with nystagmus (eye twitching). Some pigs develop arthritis (swollen joints causing lameness) or sudden death from septicaemia. These signs are especially observed in weaned pigs around 4-10 weeks old. If you see piglets “sitting and paddling” or found dead with no prior signs, S. suis is a prime suspect. 

Q3: How do pigs get infected with Streptococcus suis? 
A3: S. suis is often present in the tonsils or nasal passages of healthy pigs. Infection can spread from sow to piglets (during or soon after birth) and between pigs through oral or respiratory routes. Outbreaks tend to occur when piglets are stressed – for example, around weaning time the stress and mixing of pigs allows the bacteria to invade. Poor environmental conditions (chilling, overcrowding, poor ventilation) or co-infections can predispose pigs to S. suis disease. Essentially, anything that lowers a piglet’s immune defence can trigger the bacterium (already carried or in the barn environment) to cause an infection. 

Q4: How is Streptococcus suis infection treated and prevented? 
A4: Treatment relies on antibiotics and supportive care. Penicillin or cephalosporin-class antibiotics are usually effective if given early, though antibiotic resistance is emerging (so vets may do lab sensitivity testing). Affected pigs often need anti-inflammatory drugs and fluid support as well. For prevention, the focus is on management: improve hygiene and ventilation, reduce stress during weaning, and segregate sick or carrier animals. Some farms utilise autogenous vaccines (farm-specific vaccines made from local S. suis strains) to help control recurrent problems, but there is no commercially available universal vaccine due to the many serotypes of S. suis. 

Q5: Can humans get Streptococcus suis from pigs? 
A5: Yes – Streptococcus suis is zoonotic, meaning it can infect humans. However, human cases are relatively rare and usually associated with certain exposures. People at risk include pig farmers, abattoir (slaughterhouse) workers, butchers, or veterinarians who have cuts or wounds exposed to infected pigs or raw pork. In such cases, S. suis can cause severe illness like bacterial meningitis in humans, leading to headaches, fever, neck stiffness, and often hearing loss as a lasting complication. In parts of Southeast Asia, a number of S. suis infections in humans have occurred from eating raw or undercooked pork dishes. While infection in people is uncommon, it is serious – so those handling pigs or pork should wear protective gear and avoid consuming uncooked pork. Proper cooking of pork kills the bacteria. 

Q6: Is there a vaccine for Streptococcus suis in pigs? 
A6: Currently, no universal vaccine exists for S. suis. The main challenge is that there are many different serotypes of S. suis, each with distinct surface antigens, so a vaccine against one serotype might not protect against others. Some pig farms, especially those with recurring S. suis problems, use custom-made vaccines (autogenous vaccines) created from the specific strain present on their farm – these can help reduce outbreaks for that farm. Researchers are actively exploring vaccines, including targeting common proteins or using new technologies, but as of now, management practices are the key prevention strategy. This means maintaining good farm hygiene, reducing stress in pigs, and possibly strategic antibiotic use or probiotics to keep S. suis in check until a broadly effective vaccine becomes available.