With Two False Starts, Why Officials Can’t Finger the Source of the E. Coli Outbreak

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German officials were forced for the second time to retract a statement about the source of a deadly E. coli outbreak that has killed 22 and sickened hundreds.

On Sunday, local authorities pointed the finger at bean sprouts from an organic farm in the northern the part of the country after many restaurants where sickened people had eaten said they had received sprouts from the farm. But testing of sample produce showed no signs of the relevant strain of E. coli O104:H4.

(More on TIME.com: 10 Pivotal Food-Safety Scares)

Last week, local German officials fueled international tension when they cited cucumbers grown in Spain as the culprit. When those vegetables didn’t show signs of the bacteria in question, Spanish exporters demanded a formal apology and compensation of at least $584 million for losses that growers suffered when demand for their products plummeted.

Why so many false starts? Part of the uncertainty has to do with the nature of the investigation. Advanced technology allows for fast isolation and identification of pathogens like viruses and bacteria, but tracing the source of such an outbreak involves old-fashioned investigative techniques. Scientists must start by conducting detailed interviews with both affected and unaffected people in order to determine what factors differentiate the two groups — whether it was something they ate, something they were exposed to or some place that they visited.

(More on TIME.com: Three Possible Cases of E. Coli Illness in U.S.)

From there, investigators need to find common threads that connect those who are sick. If it’s a food source, then where did the victims eat? Where did the food come from? What farms or growers provided food to the restaurant or grocery store from which victims got their food?

In the German case, the agriculture ministry of lower Saxony, where many of the patients first became ill, focused its attention on bean sprouts when interviews revealed that most of the victims had eaten salads with sprouts and that many of the restaurants had received their sprouts from a farm in Bienenbuettel, 50 miles south of Hamburg. Two of the farm’s employees also reported having diarrhea, one of the symptoms of infection with O104, and officials thought that water used to seed the sprouts, which was kept at a bacteria-friendly 100 degrees F, was the source of the E. coli strain.

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Based on that information, the local agriculture minister updated the public on the new potential culprit — the bean sprouts — even before genetic testing confirmed the link. Some German researchers involved in the investigation, however, voiced concerns that such information was premature, and warned that any conclusions drawn before the final results of testing were available would simply be speculation.

That indeed turned out to be the case, when 23 of 40 samples taken from the sprout farm were negative for O104.

Making matters more complicated, say some experts, is that Germany’s public health system is so decentralized, with individual states holding responsibility for the handling of outbreaks like E. coli, which leads to confusion over the interpretation of data.

(More on TIME.com: Q&A: A Food-Safety Expert Explains Germany’s E. Coli Outbreak)

“The questionnaires filled out by patients suffering from [E. coli O104:H4] infections and the timely analysis of these data will produce several hints as to where the bacterium may have spread from,” says Holger Rohde, a medical microbiologist heading up the investigative team at the Institute for Medical Microbiology at the University Medical Center in Hamburg-Eppendorf. “However, bearing in mind the disaster related to accusing Spanish cucumbers as a source, I feel that we should really wait until there is clear indication that the vegetables under suspicion are indeed related to the outbreak before drawing any conclusions.”

But that may take time, which for now leaves consumers, farmers and government health officials more than a little uneasy.

—With reporting by Tristana Moore / Hamburg