Streptococcus Suis Disease in Pigs
The bacteria, Streptococcus suis (S. suis) causes significant disease in swine operations worldwide. In fact, it is now the most common cause of systemic disease in nursery piglets. Furthermore, there has been a marked increase in S. suis cases recently, mainly in herds who have reduced and/or eliminated the use of preventive medications (Poeta Silva, 2021). S. suis colonizes the tonsils of most pigs and is capable of causing disease in the brain via meningitis (brain barrier inflammation) and through septicemia (bloodstream infection), thus reaching other organs. While S. suis is most often associated with meningitis, other manifestations include arthritis, polyserositis (widespread membrane inflammation), endocarditis (heart valve inflammation) and, as a secondary cause, pneumonia (lung inflammation). S. suis is also a zoonotic agent able to cause serious disease (mostly meningitis) in people working with pigs or pork-derived products.
- S. suis may be a primary or secondary cause of disease. There are many strains of S. suis, some that are considered commensal or opportunistic while others have the potential to cause severe disease.
- Diagnostic testing and bacterial subtyping are critical to determine which strains are present in a herd and to differentiate from Glaeserella parasuis infection
- Describe various factors that influence disease development including co-infections, environmental stress, management variables and immune status.
- S. suis disease is not always associated with meningitis. The development of septicemia can affect many organs and can result in sudden death.
- Discuss S. suis presence, transmission, treatment, control and prevention strategies
Cause and Distribution
There are many different S. suis serotypes: a good way to identify who is who…
S. suis is a highly varied species, containing 35 variations of the bacteria, called serotypes. Serotypes are determined by the isolated bacteria’s capsule (protective shield that surrounds the bacteria). In North America, most strains recovered from diseased pigs belong to serotypes 1 through 8 (including serotype 1/2, which is a mix of serotypes 1 and 2) and 14. Internationally, serotype 2 is described as the most virulent (disease causing) serotype affecting pigs (and rarely, humans). Serotype 9 is also frequently recovered from ill pigs in several countries, although strains from Europe have more virulent potential. Serotyping is an essential procedure done in experienced laboratories to reveal if one or more serotypes are affecting pigs in a herd, which is very common in Canada and the U.S.
Almost 100% of farms (and almost 100% of pigs within farms) are colonized by S. suis, mainly in the tonsils (“carrier state” – pigs are healthy, but carry the bacteria). It has been shown that pigs can carry S. suis for more than 500 days and medication does not eliminate the carrier state. Most of these strains are not disease causing since they are a part of the normal bacteria in the host, although potentially virulent strains may also be present. Invasion of the host by virulent (disease causing) strains usually occurs through the respiratory tract. Some reports indicate that the intestine may also be a reservoir and, from there, S. suis may invade the bloodstream of animals. However, this has not yet been confirmed and, so far, attempts to reproduce disease orally in a research setting have been unsuccessful.
Transmission of S. suis between herds occurs mainly through the introduction of healthy carrier animals. The main route of spread is by direct contact from pig to pig or by air droplets transmitted over short distances. S. suis has been isolated from many other animal species, although their role in the carry and spread of the bacteria has not been proven. Carcasses of dead pigs has been suggested to be a likely source of infection for other pen mates. Thus, proper disposal of infected carcasses by burning, burial or removal from the premises is recommended. S. suis survives 1-2 weeks in water at 39 °F (4 ºC), but only 10 minutes at 140 °F (60 ºC). At typical nursery environmental temperatures of 72–77 °F (22–25 ºC), the organism could survive about 8 days in feces, but less than 24 hours in dust. The bacteria survives in pig carcasses for 6 weeks at 39 °F (4 ºC) and for 12 days at 72–77 °F (22–25 ºC), providing a potential source for spread by birds, rats, mice, dogs or other species (still not confirmed). Commonly used disinfectants can kill S. suis in less than 1 minute, even at concentrations less than those recommended by the manufacturers.
The cycle of infection usually starts with the bacteria being naturally present in sows leading to piglet transmission during the (nose-to-nose contact) lactation phase. Maternal antibodies (immune response component) are normally present since most sows possess high amounts of antibodies against S. suis. Antibodies provided in the dam’s colostrum usually protect piglets against disease during the lactation phase, but do not prevent bacterial colonization. Transmission between piglets also occurs pre and post weaning. The persistence of antibodies from the damn may protect piglets until 3-5 weeks of age, depending on the sow’s immunity. Usually a low proportion of pigs are affected by this pathogen pending control measures, including PRRSV co-infections, all in and all out management, and appropriate cleaning and disinfection protocols. Still, outbreaks are common when these measures are not considered, novel strains enter a herd, or due to environmental or management stressors.
S. suis may be a primary or secondary cause of disease
The presence of a disease causing S. suis strain in tonsils does not guarantee appearance of clinical signs and, sometimes, clinical signs can be observed in the absence of such strains on the tonsils. On the other hand, some strains are highly virulent and may cause disease when present in a herd. This is still one of the major challenges we face with the diagnosis of S. suis infections: how to determine if a given strain that was isolated from a diseased animal is, in fact, really responsible for the health problems observed on a farm.
Many factors can contribute to the development of disease by S. suis (Table 1). Among the predisposing factors, Porcine reproductive and respiratory syndrome virus (PRRSV) infections play a major role: S. suis is one the most common secondary complications of this virus. Clinical disease caused by S. suis significantly increases in PRRSV affected herds. However, the disease is sometimes present in PRRSV-free, high health status farms that have both good hygiene and management practices.
S. suis disease outbreaks occur most frequently in 2-6 week old pigs. These pigs are affected because the level of maternal antibodies drastically decreases at weaning. In general, levels of antibodies generated by the piglet will not increase until around 8 weeks of age and will be optimized as an adult. This explains why clinical cases in late nursery, grow finish pigs are not frequent, normally only found as sudden death cases due to endocarditis. Sometimes pigs are found dead without previous clinical signs of disease. However, pigs with S. suis meningitis usually go through a progression of loss of appetite, reddening of the skin, fever, depression, loss of balance, lameness, paralysis, paddling (Image 1), shaking and convulsions. Blindness and deafness may also occur. Septicemia and arthritis without meningitis is less noticeable and sometimes will not be recognized. The role of S. suis as a cause of pneumonia is not clear: it may contribute to lung disease in the presence of viral co-infections (PRRSV, swine influenza virus), but is rarely the primary cause of lung disease.
Diagnosis of S. suis infections cannot be determined by clinical signs and necropsy (comparable to autopsy) findings alone, since it is difficult to differentiate S. suis from similar disease processes (Glaeserella parasuis, Shiga toxin-producing Escherichia coli, salt intoxication, etc.). Samples from animals that were not treated with antibiotics must be taken and sent to a diagnostic laboratory for complete diagnosis. To prevent the spread of non-virulent S. suis strains to organs, when possible, diseased animals should be euthanized just before taking samples. Type of samples to be sent will depend on the clinical signs observed (brain, cerebrospinal fluid, joints, spleen). Lungs should not be sent unless respiratory clinical signs only are present. In cases of septicemia, it is not uncommon to find one serotype that causes the disease (recovered from most tissues) and another non-related one in lungs.
Serotyping is essential to clearly identify if S. suis is a primary problem or a secondary problem due to cofactors (Table 1). It is recommended to send samples from at least three pigs at three different time points to the laboratory. After bacterial growth, isolation and serotyping, usually one of the following situations will occur:
- Repeated identification of a few serotypes, which may indicate that mostly virulent strains are present;
- Isolation of several different serotypes (most of them once), which may indicate that cofactors are making normally present strains in the tonsils more virulent, leading to disease. Approaches to control theses situations may vary.
More recently, the use of novel sequencing technologies such as whole genome sequencing has been applied to North American strains. Although there are no universal and accepted virulence markers that clearly identify virulent strains, data from genome sequencing may help to improving pig and gilt sourcing and tracking antimicrobial resistance.
There are no recognized methods to detect carrier animals who may have virulent strains. First, there is no clear definition of a “highly virulent” strain. Secondly, since S. suis is normally found in the tonsils, it is extremely hard to diagnose which type of strains are present in a given animal/herd. No validated blood test exists.
Treatment and control
Prior to learning the antimicrobial susceptibility of the S. suis organism affecting the animals, sick piglets may be treated individually with injectable antibiotics and given supportive care. Most strains are susceptible to penicillin, amoxicillin, ceftiofur, enrofloxacin, florfenicol and ampicillin. In general, most S. suis strains are resistant to tetracyclines. Antimicrobial use should be discussed and determined by working with a veterinarian, utilizing available susceptibility data and accounting for local regulations.
Early treatment prevents death and may result in complete recovery. If a pig is down or convulsing, it should be removed from the pen. These pigs are disabled and may be shedding high amounts of bacteria that could infect other pigs in the same pen. Additional treatment approaches may include anti-inflammatory drugs, supportive fluids or electrolytes. Affected pigs should be kept comfortable, warm and propped up on their sternum when possible. Treatments aimed at the rest of the group must also be considered, especially when other pigs become affected or flow history reveals an increased likelihood of S. suis outbreak. Alteration of management approaches to help minimize stress is a key factor in the control of this disease. In PRRSV unstable farms, control measures should be considered to reduce secondary clinical disease by S. suis.
In seriously affected farms, strategic use of antibiotics in the feed, prior to known periods of heightened risk (during the nursery period) has been beneficial, but sometimes results in the shift of clinical cases to occur later in the production cycle. Restrictions in the use of antibiotics brings, among other consequences, an increase of clinical disease due to S. suis infections in post-weaned piglets. One of the most important challenges for the swine industry is to control such disease in the absence of medication. Good hygiene, environmental and management practices may help to prevent and reduce clinical cases caused by S. suis.
Since the hypothesis of a possible infection through the intestinal route (not yet proven), the use of non-antibiotic feed additives (lauric acid, cinnamon, oregano, rosemary, peppermint, etc.) has become popular. However, there is currently no scientific evidence supporting additive use for the control and prevent S. suis infections.
S. suis vaccine use has been a controversial topic. Currently, there is no commercially available vaccine that confers protection against all strains of S. suis. Thus, production systems and swine veterinarians have explored the use of autogenous vaccines. Autogenous vaccines are created from cultures of organisms collected from a herd and then used in the same herd. Although S. suis is easy to culture, the complexity of strains from diseased pigs within the same farm can difficult selection of the adequate strain(s) to be included in the vaccine. Therefore, isolation and further strain characterization of many strains from diseased animals should be carried out to make the correct choice. Strains isolated from lungs should usually not be included in autogenous vaccines. The efficacy of autogenous vaccines is difficult to evaluate and evidence of vaccine protection under controlled conditions and in the field is mostly unknown. The choice of adjuvant (an agent added to vaccines to improve the body’s immune response) is highly important for the success of a S. suis vaccine.
Furthermore, there is no clear data on the optimal timing of autogenous vaccine application, and there are minimal scientific studies available. In the field, autogenous vaccines are used mostly in sows, sometimes in piglets or in both. Although levels of antibodies against S. suis are already very high in sows, vaccination before farrowing might increase that level. Being less costly than piglet vaccination, it represents an attractive economical alternative. Available results indicate pre-farrowing vaccination of sows results in increased sow antibodies, which are passed on to the piglets through milk and start to wane at 3-5 weeks of age, thus only protecting piglets at the beginning of the high-risk nursery period. Vaccination of young animals (with or without sow vaccination), such as suckling piglets, has the concern of possible interference with maternal antibodies. Vaccination of older piglets (for example, at 3 and 5 weeks of age) may not induce a booster of antibody production early enough to protect piglets in the nursery – there is clearly a problem due to window of vaccination. Oral reports of field autogenous vaccine use are contradictory, being highly effective for some practitioners but not for others. More scientific data is needed before determining the real value of this intervention.
Eradication of S. suis so far is not feasible for commercial farms, and medicated early weaning (MEW) is not effective since this bacteria is a early colonizer. Cesarean section (C-section) can be used to obtain pigs free of S. suis. However, this option is cost prohibitive. Since there are no validated diagnostic tools to detect carrier animals with virulent strains, and to avoid introduction of S. suis in uninfected herds, it is recommended to ask the source of new stock or replacement gilts whether there is a clinical problem in the herd and to understand the disease status of the herd.
Zoonotic S. suis (Infection in Humans)
Some types of S. suis can also affect humans (mainly serotypes 2 and 14), producing septicemia, meningitis and septic shock. It is an uncommon occurrence in North America with less than 20 cases described to date, all related to swine production. Significantly higher numbers of S. suis cases have been described in Europe and it is an endemic disease in humans in some Asian countries. Meat industry workers are at greatest risk. In addition, farmers, veterinarians, food preparers and anyone else who handles uncooked pork or is in contact with live and mainly ill pigs may be exposed. Butchers and others become infected through cuts and abrasions while handling and cutting pig carcasses. The disease may manifest itself as a “flu-like” disease followed by meningitis. Approximately, 60% of the human cases that recover have permanent hearing loss. The use of gloves and correct disinfection after handling ill animals (with clinical signs that may be associated to S. suis) should be implemented.
S. suis is one of the most important pig diseases, commonly affecting nursery aged pigs. There is a high assortment of serotypes and strains, which makes diagnosis difficult. It can be a primary problem, causing disease alone. However, in many farms, clinical disease is mainly observed in the presence of cofactors such as coinfections with other pathogens (mainly PRRSV), as well as environmental and management problems. Disease can be well treated and even prevented with the use of antibiotics. However, those antibiotics that have high success rates are regulated in the animal industry, given their importance to both human and veterinary medicine. Restrictions in the use of antibiotics brings, among other consequences, an increase in S. suis-related cases. As the complexity of S. suis distribution increases in swine (multiple strains, multiple serotypes), field reports describing difficulties in disease control and management are common. Autogenous vaccines are largely used, but their impact on the control of the disease should be further evaluated.
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