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Some experts said it might be impossible to ever identify what caused the outbreak, as much of the tainted fresh produce already may have disappeared from markets.

“As in many food-borne disease outbreaks, the culprit may never be identified, and the epidemic just fades away,” said Brendan Wren, professor of pathogen molecular biology at the London School of Hygiene and Tropical Medicine.

To identify which E. coli strain is responsible, scientists must grow the suspect bacteria in a laboratory, which can take up to two days. Once that’s done, tests to characterize the strain may take another day or two, and those tests can be done only in specialized labs.

“These are complicated molecular tests, and it’s not something you can do in one day,” Mr. Hunter said.

In the Czech city of Brno, lab workers wearing white coats and latex gloves meticulously snipped hundreds of pieces off cucumbers, peppers and other vegetables from Spain, Germany, the Netherlands and Denmark as well as Eastern and Central Europe to be tested before allowing them into the Czech market for sale.

“None of the samples has tested for the bacteria so far,” lab chief Pavel Alexa said.

“It all takes up to four days to do the basic research to determine if a sample is suspicious,” Mr. Alexa said. If the sample is suspicious, it takes days more to isolate the strain of E. coli responsible, he said.

Spanish officials said, meantime, that they were considering legal action after Europeans swore off Spanish produce in droves after the initial report. And in Germany, farmers association President Gerd Sonnleitner said the call for people to avoid raw vegetables had cost local farmers an estimated 30 million euros ($43 million) so far.

Germany typically sees a maximum of 50 to 60 annual cases of HUS, which has up to a 5 percent fatality rate, according to the World Health Organization.

More than 60 percent of the EHEC cases in Germany have been women — 88 percent over the age of 20 — and nearly 90 percent of the HUS cases have been women over the age of 20.

Experts suspect the outbreak may be mainly striking women because they are the ones most likely to be eating fresh produce.

It remains unclear why most of those affected are adults and not children or the elderly, who are normally more susceptible to this illness, said Dr. Kruse, the food safety expert at the WHO.

“We should be open to whatever the investigation shows, but adult women are more likely to be exposed to vegetables than other populations,” she said.

Last week, Reinhard Burger, head of the Robert Koch Institute, said it was also possible more women were affected because they were predominantly the ones handling food in the kitchen.

The World Health Organization said cases of the E. coli illness have been reported in nine European countries: Austria, Denmark, Germany, the Netherlands, Norway, Spain, Sweden, Switzerland and the United Kingdom. All but two cases are either people in Germany or people who recently had traveled to northern Germany, the organization said.

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