Outbreak associated with Streptococcus suis in pigs in China: Update
16 August 2005
As China moves towards concluding its investigation into the recent outbreak in Sichuan Province associated with Streptococcus suis in pigs, the Ministry of Health of China has shared more details with WHO about the outbreak. A group of international specialists on Streptococcus suis convened by WHO has assessed, based on the information provided by the Ministry of Health, that the outbreak in humans is compatible with Streptococcus suis etiology.
Further details of the investigation
To date, the Ministry of Health of China has reported 215 cases of human disease associated with the outbreak. Of these human cases, 39 have been fatal. No new cases have been reported since 5 August. The data provided by China depict an outbreak that peaked from the second through the fourth week of July, and dwindled rapidly thereafter. Authorities say several human cases were discovered retrospectively, once the epidemiological investigation was underway.
The initial cases were suspected of having haemorrhagic fever with renal syndrome, but laboratory tests ruled that out. More cases were reported, with a range of clinical presentations -- high fever, malaise, nausea and vomiting, followed by meningitis, subcutaneous haemorrhage, toxic shock and coma in severe cases. Nearly all of the patients were reported to be local farmers and butchers by profession, about 80 percent of them men, who had been killing sick pigs or processing and selling the meat. More than 40 percent of the cases were aged between 50 to 60 years.
Subsequently, laboratory tests on several of the human samples confirmed infection with Streptococcus suis serotype 2. A concurrent investigation by the Ministry of Agriculture of China discovered the presence of Streptococcus suis serotype 2 in pigs in the area. Authorities say tests on human samples have not shown the presence of any other bacterial agent. A viral etiology was also considered in pigs, and both influenza and Nipah virus infections were reported to have been ruled out.
Authorities add that based on their investigation so far there is no evidence of human-to-human transmission, and no healthcare workers tending to the patients have been infected. The Ministry of Health says further study is still needed, however, to try to determine why this outbreak was so large, including so many fatalities, especially compared to previous outbreaks in recent years.
Current assessment by international specialists
A group of international specialists on Streptococcus suis, including WHO technical staff, held a teleconference on 9 August to discuss this outbreak. Overall, the specialists expressed no concern related to the validity of the laboratory identification of Streptococcus suis serotype 2. The clinical picture, they felt, could be explained by a strain or strains of Streptococcus suis with a higher virulence in humans.
The specialists reiterated that Streptococcus suis is a relatively rare disease in humans. It was first identified in humans in the 1960s, and few outbreaks in humans have been reported around the world since then. Even though the Streptococcus suis genome has been sequenced globally, the specialists noted, the function of 20 percent to 30 percent of the genome is still unclear.
The specialists suggested that further testing on samples from the Sichuan outbreak might be helpful, as would comparisons between the strain associated with this outbreak and strains linked to other occurrences in the past in China and other countries. Chinese authorities have indicated that they are keen for such studies to be conducted.
The specialists also noted the lack of epidemiology to suggest person-to-person spread in the present outbreak – including the absence of cases among children -- and considered this reassuring. They said person-to-person transmission was unlikely to occur unless there was very close contact with infected material such as blood (something that would almost only occur in hospital settings).
The group reiterated that although consumption of raw or undercooked pork may lead to disease, eating properly cooked pork is unlikely to represent an increased risk, even if the strain of Streptococcus suis involved is more virulent. This outbreak once again raises the wider global issue of the links between – on the one hand – food safety, animal husbandry and slaughtering practices (especially in poor, backyard farms and rural areas), and, on the other hand, an ever-growing range of zoonotic diseases.
Regarding a possible national and international spread of the outbreak, the specialists concurred with the Chinese authorities that the movements of live pigs, and the trade of pig carcasses and meat within and from the outbreak area, had to be carefully regulated and monitored. China says it has put strict measures in place to ensure this.
The Ministry of Health has provided WHO with regular updates through the course of this outbreak. WHO will continue to monitor the situation, and is pleased to offer the Ministry of Health of China its support.