确认大肠杆菌来源仍是一挑战

Frustration is building in Europe as the source of Germany's E. coli outbreak remains unsolved. U.S. health experts say tracking down a deadly germ can be a complicated task.

When they notice an unusual number of people coming down with food poisoning, the first thing public health officials do is try to find out what they all ate recently. E. coli usually takes three or four days to make a person sick, but it may take a week or more. And that presents the first problem.

"Can you tell me what you ate for dinner on May 30?" asks David Weber, professor of epidemiology at the University of North Carolina at Chapel Hill. He says people often do not remember everything they ate over the past week. And even if they do, they may not know all the ingredients, or where they came from. For example, he says, a tomato condiment called salsa was one of the suspects in a 2008 Salmonella outbreak in the United States.

"It's not just the salsa, but in the salsa are tomatoes, onions, peppers, spices. And then, which manufacturer made the salsa? Where did they get the tomatoes from? Who was the wholesaler? How many different growers provided tomatoes during that week to that wholesaler?"

Tracing the germ all the way back to the grower may help investigators figure out what went wrong and how to keep it from happening again. But whatever crop is causing the outbreak was picked some time ago. And by the time investigators get to the farm,

"That crop may be gone. It may even be plowed under," says Ben Chapman, a food safety expert at North Carolina State University. He notes that investigators have been focusing on fresh produce, but at this point, weeks after the outbreak started, they may not even be able to trace the offending item as far as the patient's refrigerator.

"It's pretty unlikely that you have a cucumber or a tomato or lettuce sticking around in your fridge six or seven weeks after being exposed," says Chapman.

Given all the complications, it may be surprising that investigators ever figure out the cause of an outbreak. But the science of epidemiology has gotten very good in recent years, and most of the time they do. 

The U.S. state of Oregon's public health department is recognized as one of the nation's best. But sometimes officials there are stumped.

"There have been a couple where there's been, really, at the end of the day, no idea what the product was. It's clearly gone. The outbreak ends. It's very frustrating. But, whatever it was, it disappeared," says William Keene, senior epidemiologist for the Oregon public health department.

Meanwhile, European officials say the number of new E. coli cases appears to be slowing down. That may mean the outbreak will soon come to an end, with or without identifying the cause.