(HealthDay News) -- Investigators are no closer to determining the source of an outbreak of E. coli than they were when the first of 64 people in the Northeast became ill in early November, federal health officials said Monday.
Tests on green onions, believed to have been a possible cause, were negative, they said.
But the outbreak, linked to Taco Bell restaurants, may be winding down. No new cases have been added to those reported in five states since late last week, the U.S. Centers for Disease Control and Prevention reported.
Dr. Christopher Braden, an epidemiologist with the U.S. Centers for Disease Control and Prevention, told a news teleconference there may yet still be unconfirmed cases of people sickened by E. coli.
"We are not ready to say it's absolutely over, but we haven't had any new cases in the last few days," he said.
Last week, officials from the CDC and U.S. Food and Drug Administration were focusing on green onions as the likely source of the bacterial outbreak linked to patrons who ate at Taco Bell restaurants. But laboratory tests haven't been able to prove such a link.
But Taco Bell isn't taking any chances. The food chain Saturday announced it had removed all green onions, also called scallions, from its 5,800 restaurants nationwide. "We're focused on working with the authorities to find the root cause," said Rob Poetsch, a spokesman for Yum! Foods, which owns Taco Bell.
"We have obtained samples from the laboratory that Taco Bell used to test samples of green onions," Dr. David Acheson, chief medical officer at the FDA's Center for Food Safety and Applied Nutrition, said during the teleconference. "Testing of these samples was negative for E. coli.
"We have been unable to confirm that green onions are the source of the outbreak. We have not ruled out any food items, including green onions," Acheson added.
Acheson said testing of a sample of white onions from a Taco Bell on New York's Long Island by county health officials found the produce was contaminated with E. coli, but not the same strain as the one identified in the outbreak. "It doesn't match the outbreak strain or any strain associated with the illnesses," Acheson said.
As of Monday afternoon, the CDC reported 64 confirmed cases of E. coli infection in five states. New Jersey has 28 confirmed cases; New York has 22; Pennsylvania has 11, Delaware has two, and South Carolina has one. The South Carolina patient ate at a Taco Bell in Pennsylvania, according to the CDC.
Of the confirmed cases on the CDC list, 82 percent of the victims required hospitalization and 13 percent developed a form of kidney failure called hemolytic-uremic syndrome, the agency said.
Besides testing vegetables, the CDC and the FDA are examining cheese used at Taco Bell restaurants, Acheson said last week.
The E. coli outbreak linked to Taco Bell was the third food-borne illness to plague U.S. consumers in recent months. In September, an outbreak of E. coli-contaminated spinach sickened 199 people in 26 states and Canada and left three dead.
Also in September, an outbreak of salmonella was traced to tomatoes served in restaurants. The outbreak sickened 183 people in 21 states, as well as two people in Canada.
One expert said the recent spate of food-borne infections is a sign of new dangers in the U.S. food production and distribution system, which has become increasingly mechanized.
"This [the latest E. coli outbreak] is one of a series of outbreaks, which represent a change in the pattern of food-borne outbreaks," said Dr. Pascal James Imperato, chairman of the Department of Preventive Medicine and Community Health and director of the Master of Public Health Program at SUNY Downstate Medical Center in Brooklyn, N.Y.
Previously, E. coli contamination occurred at the place where food was served as opposed to the source of growing and production, Imperato said. "This outbreak and the spinach outbreak are really a newer development. We are now seeing contamination at the source of production," said Imperato, a former New York City health commissioner.
Since growing and distributing vegetables has become an "agribusiness," with fewer but larger growers, processors and distributors, there's more chance of contamination, Imperato added.
Contamination can occur from irrigation, which can spread E. coli from neighboring animal grazing lots, and during the packaging in large plants. And that packaging increasingly relies on plastic bags, which create an ideal environment for bacteria such as E. coli to grow, he explained.
Imperato said he thought the only solution to the problem is increased government oversight and regulation.
Currently, the FDA is responsible for monitoring produce and seafood, while the U.S. Department of Agriculture has oversight for meat and poultry. But while the FDA has published sanitary standards for produce farmers, the agency has no regulatory authority to enforce those standards. Also, the FDA has few inspectors to even observe the level of voluntary compliance to those standards, Imperato said.
"It's going to require more rigorous oversight and the implantation and adherence to standards from the time the crop is grown in the field through the entire processing of the product and its distribution," he added.
Acheson agreed that new farming and distribution practices have increased the risk for contamination. Given the latest outbreak and the spinach problem in September, "it's fairly clear that something needs to changed," he said.
New regulation may be a part of the solution, Acheson said. But, he added, more may have to be done, including changing some farming and processing practices.
E. coli O157:H7 is one of hundreds of strains of the bacterium Escherichia coli. Although most strains are harmless, this strain produces a powerful toxin that can cause severe illness, such as bloody diarrhea and abdominal cramps. The symptoms usually clear up within five to 10 days, according to the CDC.
For more on the latest E. coli outbreak, visit the U.S. Centers for Disease Control and Prevention.