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German Scientists Finger Rare Serotype in Massive E. coli Outbreak
25 May 2011 11:18 am
BERLIN—German scientists are scrambling to find the source of an outbreak of enterohemorrhagic Escherichia coli (EHEC), a dangerous intestinal pathogen that has sent hundreds of people to the hospital with bloody diarrhea and killed at least one. The investigation is complicated by the fact that the outbreak has apparently been caused by a very rare EHEC serotype called O104.
"I hope that more patient samples will be identified today, but it certainly looks as if an E. coli O104 is the cause of these infections," says Klaus Stark, head of the division of gastrointestinal infections, zoonoses, and tropical infections at Robert Koch Institute (RKI) here, the German center for disease prevention and control.
RKI researchers first took notice last Thursday, when a clinic in Hamburg notified them of a host of patients suffering from hemolytic-uremic syndrome (HUS). The syndrome—characterized by a destruction of red blood cells and severe kidney problems—is the most severe complication of an EHEC infection.
According to RKI, nearly 140 patients in Germany have developed HUS since the second week of May, compared with only about 60 cases in an average year. "This is one of the biggest EHEC outbreaks in the world, certainly the biggest one we have ever had in Germany," says Stark.
Scientists have been baffled not only by the outbreak's size and rapid spread in northern Germany but also by the fact that it affects mostly adults and females, an odd pattern because EHEC usually sickens children. Of the 65 HUS cases in Germany in 2010, only six patients were more than 18 years old.
While E. coli bacteria are part of the human gut flora and usually not pathogenic, the strains classed together as EHEC produce a dangerous Shiga toxin that enters the cells in the gut and inhibits protein synthesis by cleaving ribosomal RNA. The destruction of cells lining the gut leads to abdominal cramping, watery diarrhea, and eventually bloody diarrhea. In about 10% of cases, the toxin also attacks the kidneys, leading to the potentially fatal HUS. EHEC has been confirmed as the cause of death in an 83-year-old woman who had been admitted to a hospital in Bremen on 15 May and who died on Saturday; another potential EHEC fatality is still under investigation.
Most EHEC infections are caused by a notorious serotype called O157:H7; researchers refer to that serotype and four others frequently found in Europe as the "gang of five." But the German reference laboratory for EHEC in Wernigerode has so far identified the serotype of EHEC in stool samples from five patients as O104.
That makes the outbreak highly unusual, says Helge Karch, head of the National Consulting Laboratory on Hemolytic Uremic Syndrome in Münster. Among Karch's E. coli isolates from 588 HUS patients collected over the past 20 years, only two are O104. In another unusual twist, Karch has found the strain to be eae-negative. The gene eae codes for the protein intimin, which the bacteria uses to attach to the intestinal wall. Most pathogenic EHEC serogroups are eae-positive.
It is still unclear whether the serotype might explain the strange pattern of infections. E. coli O104 first emerged as a pathogen in a small outbreak in Helena in early 1994. Four people developed abdominal cramps and bloody diarrhea. Experts at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta identified a serotype called O104:H21 as the culprit. A CDC investigation later found up to 18 patients; most of them were women and the median age was 36 years.
The source of infection in Germany is still unclear, and the rare serotype could make it harder for authorities to find it. "O104 is very hard to distinguish from normal, non-pathogenic E. coli," says Lothar Beutin, head of the National Reference Laboratory for E.coli at the Federal Institute for Risk Assessment here. He and other researchers are now trying to develop a more rapid and specific test for the rare serotype.
The natural reservoir of EHEC bacteria are ruminants such as cows and sheep, which carry the pathogens in their guts and spread them with their feces. It is transmitted to humans primarily through consumption of contaminated foods.
"When we first interviewed patients, a lot of them mentioned eating raw vegetables, but that does not mean that vegetables are the source," Stark says. RKI is using a detailed questionnaire to ask patients what they have eaten in the days before falling ill and comparing their answers with those of healthy controls who match the patients in characteristics such as age and sex, a common method in epidemiology.
In order to save time, however, the epidemiologists are not searching public records to find the matching controls. Instead, a team of 15 experts went door to door yesterday in the areas in Hamburg where some patients live, looking for roughly matching controls. "If we get a clear signal, something that 90% of the cases but only 10% of the controls ate, then we can be very sure that we have found the source," says Stark.
But a first statistical analysis this morning did not yield any clear results. Indeed, the cause of most of the smaller EHEC outbreaks in recent years has never been established definitely. Scientists are hoping that the current outbreak will turn out to be unusual in that respect as well.