Streptococcus suis is a species of bacterium found in many parts of the world where pigs are raised. It is most adapted to domesticated pigs, but is also occasionally found from wild boars, horses, dogs, cats and birds. Human infection was first described in the 1960s and is increasingly recognized. It is unclear, however, whether this reflects a true rise in incidence or whether it is being diagnosed more frequently due to increased awareness by physicians.
Infection in pigs is usually asymptomatic, but can result in septicaemia (blood poisoning), meningitis, pneumonia and arthritis. Young pigs are most at risk, but the disease can occur in any age group. Predisposing factors are found in pigs reared in "suboptimal" conditions, for example poor housing with inadequate ventilation. This is compounded if pigs are raised under "intensive" conditions that can cause stress and subsequent immune suppression.
Asymptomatic pigs typically carry the bacteria in their tonsils and are probably responsible for the spread of infection between herds. Infected pigs respond to certain antibiotics. Vaccines are available, but their efficacy is not firmly established
Although rarely diagnosed in Europe, there is some evidence that Streptococcus suis is a not uncommon cause of bacterial meningitis in humans residing in some parts of South-east Asia. It usually does not cause outbreaks, but occurs in a sporadic manner. The most important risk factor in acquiring the infection is contact with pigs or uncooked pig products, typically farmers, veterinary personnel, abattoir workers and butchers. Individuals who are immunosuppressed, including those who have had their spleens removed, are also at increased risk.
Transmission to humans is most likely to occur through wounds on the skin, including minor abrasions. But infection via ingestion or through mucous membranes—such as the conjunctiva—has been suspected in some cases. The incubation period ranges from a few hours up to three days. Classically, infection in humans produces a fever and signs of meningitis (headache, vomiting, neck stiffness, intolerance of light and decreased level of consciousness). Hearing loss, which is generally permanent and affects around 50% of those infected, occurs due to involvement of the auditory nerve. Arthritis and pneumonia are also possible complications.
Diagnosis is made by the recovery of bacteria from the cerebrospinal fluid, blood or joint fluid, if arthritis occurs as a result of infection. The bacterium can be grown in culture. Polymerase Chain Reaction (PCR) can also be used to make a diagnosis. If diagnosis is early, prompt treatment with appropriate antibiotics will lead to recovery. Delay in treatment, however, will adversely affect survival.
Another possible consequence of infection is the occurrence of toxic shock syndrome (TSS). This syndrome is also associated with other streptococcal and staphylococcal infections. This may lead to severe damage of many vital organs, including the liver, kidneys and circulatory system. TSS is only partially responsive to treatment with antibiotics, and intensive supportive care is required. If TSS, occurs the outcome is likely to be worse.
Prevention of the disease in humans depends upon control in pig populations. This presents a difficult challenge to veterinary authorities and the pig industry. In serious outbreak situations, there should be strict controls on animal movements and slaughtering. The second line of defence relies on increasing awareness of the disease within those at highest risk. Awareness should extend to everyone who prepares and cooks pork, including those doing so in their homes. Those with open wounds should wear gloves when handling raw or uncooked pork, and all those who prepare pork should wash their hands and clean their utensils thoroughly after preparation. Adequate cooking is also essential. WHO recommends that pork should be cooked to reach an internal temperature of 70°C, or until the juices are clear rather than pink.