Streptococcus suis is a bacterium that causes systemic disease in swine, particularly in intensive production settings. The disease, often referred to as “Strep suis,” is one of the most common causes of bacterial mortality in weaned pigs, leading to substantial economic losses for the swine industry globally. While pigs of any age can be affected, the most severe outbreaks consistently occur in the nursery phase, shortly after weaning. This bacterium is also classified as a zoonotic agent, meaning it can be transmitted from pigs to humans, causing serious illness, especially in individuals with occupational contact with swine or pork products.
The Causative Agent and Transmission Routes
The bacterium Streptococcus suis is a Gram-positive organism that is a common resident in the pig population. Most healthy pigs carry the organism asymptomatically, primarily colonizing the tonsils and the upper respiratory tract. This asymptomatic carriage means the bacteria is widespread on almost all swine farms.
Transmission occurs through several primary routes, including direct nose-to-nose contact between pigs and from sow to piglet during farrowing or nursing. Environmental contamination also plays a role, as the bacterium can be spread through feces, dust, and fomites like boots or equipment. The presence of the bacterium requires a stressor to transition from a harmless resident to a pathogen.
Stressful events, such as weaning, mixing pigs from different litters, moving animals, or experiencing sudden changes in barn temperature or ventilation, often trigger the onset of clinical disease. These stressors compromise the pig’s immune system, allowing the S. suis to invade the bloodstream and spread to other organs. Co-infections with other swine pathogens, such as Porcine Reproductive and Respiratory Syndrome virus (PRRSv), can also significantly increase the incidence and severity of Strep suis cases.
Recognizing the Signs of Infection
The signs of S. suis infection are diverse because the bacteria can target multiple organ systems, though the symptoms are most frequently neurological. Meningitis is the most common manifestation, often displaying a sudden onset of symptoms including a high fever, depression, and loss of appetite.
As the infection progresses and inflammation affects the central nervous system, pigs show severe neurological signs. These include a lack of coordination, head tilting, tremors, and an inability to maintain balance. Affected pigs may be found lying on their side with characteristic paddling movements of their legs, potentially progressing to paralysis and convulsions. The inflammation can sometimes result in permanent blindness or deafness.
A second, rapidly fatal form of the disease is septicemia, or blood poisoning, which may lead to sudden death. Pigs with septicemia might exhibit a reddening or purple discoloration of the skin, severe depression, and reluctance to move. When the bacterium localizes in joints, it causes acute arthritis, characterized by painful, swollen joints and lameness.
Other manifestations include polyserositis (inflammation of the membranes lining the body cavities) and endocarditis (inflammation of the heart valves). Endocarditis typically results in chronic illness and eventual sudden death in older pigs. S. suis is also considered a secondary contributor to pneumonia, complicating respiratory infections.
Confirmation and Immediate Response Protocols
Observing neurological signs or sudden deaths necessitates an immediate response to control disease spread. The first step involves isolating any pig showing symptoms, such as paddling or lameness, away from the rest of the herd to limit transmission. Supportive care, including anti-inflammatory drugs and ensuring access to water and electrolytes, is important for affected animals.
A definitive diagnosis requires submitting samples from affected or recently deceased pigs to a veterinary diagnostic laboratory. Samples typically include joint fluid, brain tissue, spleen, or cerebrospinal fluid. The laboratory will culture and isolate the S. suis organism.
Following isolation, the bacteria are identified and serotyped to determine the specific strain causing the outbreak. Advanced diagnostic techniques, such as Polymerase Chain Reaction (PCR), are often used to rapidly detect the bacterial DNA and identify virulence genes. Rapid confirmation allows the veterinarian to select the most effective, targeted antimicrobial treatment and implement control measures before the infection spreads widely.
Long-Term Management and Prevention Strategies
Preventing S. suis outbreaks relies heavily on proactive management aimed at minimizing stress and reducing bacterial load in the environment. Controlling environmental factors is a primary focus, which includes ensuring proper ventilation to reduce humidity and maintaining consistent temperatures within the pig housing. Overcrowding should be avoided, and management strategies should aim to reduce the mixing of different age groups, which increases stress and transmission opportunities.
Biosecurity measures, such as strict hygiene, regular cleaning, and disinfection protocols, are implemented to control the circulation of the bacteria. Since the bacteria can be transmitted from the sow, improving colostrum intake in newborn piglets provides passive immunity and protects them during their susceptible early life phase. The use of all-in/all-out management systems, where barns are completely emptied, cleaned, and disinfected between groups of pigs, helps break the cycle of infection.
Vaccination is a strategy used in some herds, often involving autogenous vaccines created from the specific S. suis strain isolated on the farm. The effectiveness of vaccination can be variable due to the wide diversity of bacterial serotypes, meaning a vaccine effective against one strain may not protect against others. Strategic use of antimicrobials, guided by sensitivity testing from the laboratory, may be employed during high-risk periods like weaning, but this must be balanced against the growing concern of antimicrobial resistance.