Streptococcus suis

Streptococcus suis is spherical, Gram-positive bacteria, and an important pathogen of pigs. Endemic in nearly all countries with an extensive pig industry, S. suis is also a zoonotic disease, capable of transmission to humans from pigs. Humans can be infected with S. suis when they handle infected pig carcasses or meat, especially with exposed cuts and abrasions on their hands. Human infection can be severe, with meningitis, septicaemia, endocarditis, and deafness as possible outcomes of infection. Fatal cases of S. suis are uncommon, but not unknown.

Penicillin is the most common antibiotic used in treatment of S. suis infection; in cases with cardiac involvement (endocarditis), gentamicin should also be given for synergistic effect.

Zoonotic outbreaks
In July 2005, an outbreak of the disease in humans was reported in Sichuan, China, with higher than usual human morbidity and mortality; over 100 cases and more than 20 deaths were initially reported. Prior to this outbreak, less than 200 total human cases had been reported, and mortality was assumed to be less than 10%. Details of this outbreak and a similar earlier outbreak, also in Sichuan province, have recently been published.

A total of 204 human cases were documented during the Sichuan outbreak, with 38 fatalities. The human outbreak co-incided with one in the local pig populations. There was no evidence of human-to-human transmission; all of the patients had been in direct contact with pigs. Many of the patients, and almost all of the fatal cases, had typical symptoms of Streptococcal toxic shock syndrome (STSS). To date, STSS has only been documented in patients infected with S. pyogenes, another member of the Streptococcus family but very different from S. suis. However, the bacteria isolates from the human and pig samples were clearly S. suis, and those isolates were able cause typical S. suis disease in piglets. The genome of S. suis isolates from the outbreak were examined to see whether its DNA sequence could explain why these particular bacteria were able to cause STSS. However, none of the genes that are present in S. pyogenes and thought to cause STSS were detected in the S. suis isolates. Comparison with other S. suis isolates from around the world, including one from an earlier smaller S. suis outbreak in Sichuan province that killed 14 out of 25 reported human cases, revealed that the two Chinese isolates were more similar to each other than to any other strains.

Additional experiments are necessary to determine whether the size and high mortality of the recent outbreak is because the Chinese S. suis version is more virulent than other strains or due to the circumstances under which the Chinese patients got infected and treated. Studies are under way to characterize the bacterial isolates from the outbreak in detail. Physicians around the world should be aware of the possibility of S. suis-associated STSS when they see patients with unexplained sepsis who had been in contact with pigs.