First analysis of recent disease outbreak in China

First scientific analysis of Chinese outbreak of S.suis, spread from pigs to humans

Last year, there was major press coverage of an alarmingly large and deadly outbreak of Streptococcus suis disease in Sichuan province in China (see http://www.who.int/csr/don/2005_08_03/en/).

Now George Gao, Yu Wang, Jiaqi Tang, Xiaoning Wang and colleagues from the Chinese Academy of Sciences, the Chinese Center for Disease Control and Prevention, and other Chinese institutions publish the first scientific description of the outbreak in the international open-access journal PLoS Medicine.

S. suis is a pathogen with serious economic effects on the pig industry world-wide. The disease is endemic in adult pigs in most countries where pig farming is common. Infections in adult pigs are usually asymptomatic, but infant piglets that get infected through contact with colonized adult females can develop fatal sepsis.

Transmission to humans is rare and generally restricted to individuals with occupational exposure to live or dead pigs. The first human case of S. suis infection was reported in Denmark in 1968. Most of the 200 or so previously reported human cases were characterized by meningitis and septicemia; fewer than 1 in 10 infected humans died.

The recent Sichuan outbreak, in contrast, affected over 200 individuals and killed 38 of them. Besides the large number of infected individuals and the high mortality rate, it was the clinical symptoms associated with this outbreak that attracted interest and worry from scientists and health officials worldwide when the outbreak was first reported.

As Tang and colleagues detail in their article, a large proportion of the infected individuals (including all but one of the patients who died) showed symptoms of Streptococcal toxic shock syndrome (STSS), which had not previously been observed in patients infected with S. suis. However, as Tang and colleagues show, the pathogen in the recent outbreak (as well as in an earlier outbreak in Sichuan province in 1998 that killed 14 of 25 reported patients) was clearly a strain of S. suis. As they describe, both human outbreaks were closely linked to outbreaks in the local pig populations, and there is no reason to believe that any of the cases had been caused by human-to-human transmission.

One of the key questions that arose when the recent outbreak was first reported is whether a new and more virulent strain of S. suis has emerged in China. Tang and colleagues did a genetic study of the S. suis bacteria that they isolated from the Chinese outbreaks to look for unusual characteristics that could explain why these outbreaks were so severe. They did find some differences between the isolates from the two Chinese outbreaks (which appear very similar to each other) and other virulent strains of S. suis. However, more detailed studies are needed before it is clear whether any of these differences are important in explaining why some strains of S. suis are so lethal.

In an accompanying Perspective article, Shiranee Sriskandan and Joshua Slater suggest that S. suis infection “should now be in the list of differential diagnoses when clinicians encounter patients with unexplained sepsis who have a history of exposure to pigs.” They conclude that “the emergence of any new zoonotic disease [an animal disease than can be transmitted to humans] associated with high mortality is of global concern” and call for “international collaboration … to clarify differences between isolates circulating in different regions of the world.”