Shredded Lettuce Prime Suspect in E. coli Outbreak

ROCKVILLE, Md., Dec. 14 -- Discarding green onions as a red herring, the FDA and the CDC have focused on shredded lettuce as the probable source of an outbreak of Escherichia coli infections traced to Taco Bell restaurants in the northeast.

The outbreak appears to have tapered off, and the current task for investigators is to follow the distribution channels of the lettuce and identify the farms where the lettuce was grown, as well as the facilities that handled the lettuce between harvest and delivery.

Although the focus is on shredded lettuce, a statistical analysis by the CDC also implicated cheddar cheese and cooked ground beef. The USDA is looking at beef, the investigators said, and the cheese connection, though weak, still waits in the wings.

"Multiple foods make it hard to dissect out the source of the infection, so that this investigation is more complex than the earlier spinach study," David Acheson, M.D., of the FDA's Center for Food Safety and Applied Nutrition said at a press conference.

As of yesterday, 71 persons with illness associated with the Taco Bell E. coli 0157:H7 outbreak have been reported from five states. They are New Jersey (33), New York (22), Pennsylvania (13), Delaware (2), and South Carolina (1). Among these ill persons, 53 (75%) were hospitalized, and eight developed hemolytic-uremic syndrome. The single South Carolina patient ate at a restaurant in Pennsylvania.

The number of new cases has declined substantially, Dr. Acheson said. The peak time for illness was in late November.

Information about items likely to have transmitted the pathogen came from a CDC case-control study in which ill and well Taco Bell restaurant patrons were interviewed about the foods they ate.

The now-discarded green-onion connection arose because quick screening to protect consumers and to rule out negative samples at the start of the outbreak came back positive for the green onions. However, later analysis found the onions free of the bacteria, Dr. Acheson said.

No other samples beyond the shredded lettuce have come up positive, he said, leaving an ongoing "traceback" analysis of distribution, packaging, and farm records to find the source of the implicated product.

In addition to looking at individual ingredients, the investigators are studying food processing and how the product is handled. They must also ask whether the food is one on which bacteria can survive, said CDC medical epidemiologist Christopher Braden, M.D., speaking at the FDA press conference.

The farm, he said, is the most likely source of the contamination. However, he added, the study will take time. Epidemiological studies of outbreaks are much harder than testing a single food for contamination.

The FDA has no indication that lettuce or even cheese served at any other restaurant or sold in any other venue is connected with this outbreak. The E. coli at Taco John restaurants in Iowa and Minnesota do not appear to be related to the Taco Bell outbreak, according to the agency.

Summing up, the investigators said the evidence points to shredded lettuce as the "most likely" candidate for the outbreak, though it is not conclusively linked.

Asked about whether cheese could yet turn out to be implicated in the outbreak, Dr. Braden said, "It would be folly at this point to drop cheese, although the emphasis is on lettuce. It is important to keep all sources open."