Streptococcus suis is a bacterium that causes substantial disease in pigs globally, leading to considerable economic losses. It contributes to decreased performance and increased mortality within swine herds. Almost all pig farms have carrier animals, making its control a concern due to new regulations on antimicrobial use and the lack of widely effective vaccines.
How Pigs Get Infected and What Happens
Streptococcus suis commonly transmits among pigs through direct contact, aerosols, or contaminated environments. The bacteria primarily colonize the tonsils and upper respiratory tract, serving as a natural niche. From these sites, S. suis can invade the bloodstream, leading to systemic infection.
Once in the pig’s system, S. suis can cause various clinical signs. Meningitis, an inflammation of the brain’s membranes, is common, often leading to neurological signs like loss of balance, lameness, paralysis, paddling, shaking, and convulsions. Affected pigs may also exhibit fever, loss of appetite, and reddening of the skin.
Beyond meningitis, S. suis can cause septicemia, a bloodstream infection that may result in sudden death, especially in post-weaned piglets. Other presentations include arthritis, characterized by swollen joints and lameness, and polyserositis, involving widespread inflammation of body membranes. Endocarditis, an inflammation of the heart valves, can also occur, sometimes leading to sudden death in older pigs without prior signs. Pneumonia may also develop as a secondary condition.
Identifying and Treating the Infection
Diagnosis of Streptococcus suis in pigs often begins with observing clinical signs, considering the animal’s history, and noting gross lesions during post-mortem examination. These initial observations guide further investigation to confirm the pathogen’s presence.
Laboratory tests are then used for definitive diagnosis. Bacterial culture from sterile samples, such as brain swabs or joint fluid, is a primary method for isolating the organism. Polymerase Chain Reaction (PCR) assays also confirm S. suis from tonsil samples. Histopathological evaluation of affected tissues, like the brain or synovium, can confirm tissue damage associated with the infection.
Treatment involves antibiotics, with beta-lactam antimicrobials such as penicillin and amoxicillin being effective. Early diagnosis and intervention are important for better outcomes. A challenge in treatment is increasing antibiotic resistance, which complicates therapeutic choices and underscores the need for susceptibility testing to guide antibiotic selection.
Preventing and Managing Outbreaks
Prevention of Streptococcus suis outbreaks in pig herds involves reducing exposure and enhancing pig resilience. Biosecurity measures are important, including strict hygiene protocols and controlled access to facilities to minimize bacterial spread. Maintaining proper ventilation and managing stocking density prevent overcrowding, which can increase stress and disease transmission.
Reducing stress factors for pigs, such as minimizing commingling and ensuring stable environmental conditions, supports their immune systems and lowers susceptibility. While no universally effective commercial vaccine exists, autogenous vaccines are used to stimulate immunity. Vaccination of sows before farrowing may also help transfer passive protection to piglets.
During an outbreak, immediate management practices limit spread and impact. This includes isolating affected animals to prevent further transmission and promptly initiating antimicrobial treatment based on diagnostic findings and susceptibility testing. Culling severely affected animals may also be considered to reduce the bacterial load and protect the rest of the herd. Continuous monitoring for new cases and adjusting management practices based on disease patterns are also important for effective outbreak control.
Risk to Human Health
Streptococcus suis is a zoonotic pathogen, meaning it can transmit from pigs to humans, though human infections are relatively uncommon. Most human cases are reported in Southeast Asia, where the disease is endemic, but sporadic cases occur worldwide. Primary routes of human infection involve direct contact with infected pigs or contaminated pork, often through cuts or abrasions on the skin. Consuming raw or undercooked pork or blood products is another identified risk factor.
When humans become infected, symptoms include meningitis, which can lead to permanent hearing loss. Septicemia, a severe bloodstream infection, and septic shock are also possible outcomes, sometimes resulting in high mortality rates. Other manifestations can include arthritis and endocarditis.
Individuals working closely with pigs, such as farmers, veterinarians, and meat industry workers, face a higher risk of exposure. Preventive measures for these individuals include wearing protective equipment like gloves and practicing thorough hand hygiene, especially after handling pigs or raw pork. Ensuring pork is thoroughly cooked before consumption is also an important preventive step for the general public.