The statistical evidence pointed to lettuce, cheese and ground beef, Braden said yesterday in a conference call. However, the cheese is pasteurized and the beef is cooked, leaving raw lettuce as the most likely vehicle.
If lettuce is confirmed as the source of the outbreak, it would be the 21st time since 1995 that leafy green vegetables had caused an outbreak of food-borne illness, according to the federal Food and Drug Administration.
Taco Bell Corp. said yesterday that it had no plans to stop serving lettuce at its outlets, because it switched suppliers for its restaurants in New York, New Jersey, Pennsylvania and Delaware.
Taco Bell president Greg Creed said lettuce was served with about 70 percent of the items on its menu. But he also said the company's restaurants in the affected region used only 20 percent of its former supplier's lettuce, implying that contaminated lettuce could have been distributed widely.
"We've been informed by the CDC and FDA that a handful of the people who became ill did not eat at Taco Bell," Creed said.
The FDA said it had no indication that lettuce served at any other restaurant or sold in any other venue was connected with the outbreak.
"We expect to see some cases of illness from people who deny eating at Taco Bell, even if infected with the outbreak strain," FDA spokeswoman Julie Zawisza said. "One explanation is that these people may have been infected by other ill persons."
Creed said the possibly contaminated lettuce was grown in the West, but he would not be specific. David Acheson, chief medical officer at the FDA's Center for Food Safety and Applied Nutrition, also declined to give any information about where the lettuce was grown.
Creed said Taco Bell, a subsidiary of Yum Brands Inc., had no plans to bring green onions back into its restaurants, although investigators have found no evidence of a link between green onions and the outbreak.
The chain pulled green onions from its restaurants a week ago after preliminary tests for Escherichia coli O157:H7 were positive. The FDA said Monday that it had been unable to confirm those results in a definitive test.
Taco Bell temporarily closed 90 of its restaurants as a precaution. It has reopened most of them.
While the CDC and local health officials have been gathering information from victims and a control group, the FDA has been testing food samples and studying Taco Bell records to pin down the sources of various food items.
"The fact that the testing is negative does not put me off," Acheson said. "Finding a positive in produce is very difficult" because it has rapid turnover and is very perishable.
Braden has been saying this week that the outbreak appeared to be near an end, but the number of cases has continued to climb. There were 64 confirmed cases Monday and 67 yesterday.
The latest start of an illness was Dec. 6, Braden said.
New Jersey has the largest number of cases: 33. State health officials said they were investigating 84 cases altogether.
Pennsylvania has 13 cases; Delaware, 2; and New York, 22, Braden said.
Contact staff writer Harold Brubaker at 215-854-4651 or firstname.lastname@example.org.