The source of the outbreak, which has killed at least 16 people — 15 in Germany and a Swede who visited there recently — remained unknown.
Public health officials are alarmed because a startlingly high proportion of those infected suffer from a potentially lethal complication attacking the kidneys, called hemolytic uremic syndrome, which can provoke comas, seizures and stroke. Dr. Robert Tauxe, deputy director of food-borne disease at the Centers for Disease Control and Prevention in Atlanta, said the rate of cases of acute kidney failure in the outbreak was unprecedented. “That makes this an extraordinarily large and severe event,” he said.
While most of the infections were among people who had traveled to northern Germany, the authorities acknowledged that the outbreak had spread to virtually every corner of the country.
Shoppers and vegetable sellers in Berlin expressed a blend of confusion, anger and stoicism; about 20 cases of E. coli infection have been reported in the capital city. “A lot of people are afraid or worried,” said Nursan Usta, 43, who runs a fruit and vegetable stall in Berlin’s blue-collar Neukölln district. “They aren’t even buying cherries” — even though the authorities have mentioned only cucumbers, lettuce and tomatoes as potential sources of infection. In Motril, a town in Spain’s agricultural heartland, greenhouses were empty of workers as demand for vegetables collapsed after the German authorities initially — and most likely mistakenly — pointed to Spain as a source of the outbreak.
“Working in a greenhouse can be tough, but I’ve never felt more exhausted and empty inside than now,” said Miguel Rodríguez Puentedura, who had been picking cucumbers until Monday, when the greenhouse that employed him shut down.
Health officials in Hamburg, the center of the outbreak, appealed Wednesday for donors to contribute blood.
Scientists are at a loss to explain why this little-known organism, identified as E. coli 0104:H4, has proved so virulent.
The European authorities have reported several differences from previous outbreaks, including that women make up more than two-thirds of those affected and that young and middle-aged adults account for a very high percentage of the most severe cases. Dr. J. Glenn Morris, director of the Emerging Pathogens Institute of the University of Florida, said that the German strain might have undergone genetic changes or mutations to make it more potent.
The high number of cases of acute kidney failure represents a much higher percentage of the total number of illnesses than in previous outbreaks associated with different strains of E. coli. Generally, 5 to 10 percent of E. coli illnesses result in this complication. Among the confirmed cases, according to the Robert Koch Institute, Germany’s disease control agency, 470 people had been diagnosed with the kidney syndrome.
That could be because German doctors are using a broader definition of kidney failure that captures more cases. Or it could mean that the total number of illnesses is much greater than has so far been revealed, which ultimately would lower the percentage of acute cases. Or it could be a signature of this form of E. coli.
There are many types of E. coli, most of which are harmless. But a small number have come under increasing scrutiny as dangerous pathogens. These all produce a poison known as shiga toxin and generally have the ability to cling to a person’s intestinal wall, allowing them to release the poison in large enough amounts to make people sick.
Dr. Phillip Tarr, a professor of microbiology at the Washington University School of Medicine in St. Louis, said that there were two main forms of shiga toxin found in E. coli, and that the strain detected in the outbreak in Germany appeared to have the more potent version. But he said the organism appeared to have other quirks that made it unusual, and potentially difficult to detect by conventional means.
“This outbreak is still evolving, and everyone is still in the fog of case definition,” Dr. Tarr said. With the source of the contagion unknown, the Robert Koch Institute on Wednesday warned against eating “raw tomatoes, cucumbers and lettuces to prevent further cases,” particularly in northern Germany.
European and German officials pledged to track down the cause of the outbreak and to pinpoint where in the food supply chain the contamination had taken place. “Hundreds of tests have been done,” the German agriculture minister, Ilse Aigner, said in a television interview. So far, those tests had determined that “most of the patients who have fallen ill ate cucumbers, tomatoes and leaf lettuce primarily in northern Germany.”
Alan Cowell reported from Berlin, and William Neuman from New York. Reporting was contributed by James Kanter from Brussels; Victor Homola, Stefan Pauly and Judy Dempsey from Berlin; and Raphael Minder from Motril, Spain.
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