ATLANTA (AP) – For more than two weeks, industry and health officials have tried to untangle how E. coli bacteria sickened scores of Taco Bell customers. The mystery still isn’t solved, an indication of the trickiness of tracking illness through a mass-produced food system.

It’s becoming a terrible routine. And despite the media attention this outbreak has drawn, it isn’t even a particularly remarkable case.

Overall, foodborne illnesses are not increasing. But federal health officials have dealt with five multi-state outbreaks of produce-related illnesses in the last four months, including a spinach-caused E. coli outbreak that killed three and sickened more than 200 in September, and two tomato-caused salmonella outbreaks that made about 400 people ill in October and November.

“We’ve been in intense investigation mode since September,” said Dr. Christopher Braden, a U.S. Centers for Disease Control and Prevention epidemiologist leading the investigation.

The outbreaks cast media klieg lights on the issue of food safety, and the focus grew more intense this month, when at least 71 customers at Taco Bell restaurants in New York, New Jersey, Pennsylvania and Delaware were reported ill.

Indeed, the CDC has taken the unusual step of using its Atlanta emergency operations center – a “war room” facility designed for disaster situations – for coordination of the Taco Bell investigation.

Still, this latest probe is not particularly unusual. Through a process of elimination, health officials have named lettuce as the culprit, and they feel confident about it despite a lack of scientific evidence.

“I think the chances are very good” that investigators will track the contagion back to its source, Braden said.

That’s not to say the probe has been easy.

At issue is E. coli, or Escherichia coli, a common and ordinarily harmless bacteria found in the gut of cattle and other animals. The E. coli O157:H7 strain can cause abdominal cramps, fever, bloody diarrhea, kidney failure, blindness, paralysis, even death.

Earlier this year, the CDC reported that rates of most forms of foodborne illness had dropped since the mid-1990s. That included the dangerous O157 strain of E. coli, which fell 29 percent.

There’s been a clear decline in E. coli outbreaks involving ground beef, thanks to changes in the beef industry, Braden said. But E. coli illnesses involving leafy green vegetables have continued, at a rate of one to five outbreaks a year, he said.

Taco Bell uses a variety of ingredients in its Mexican menu items. Hundreds of food samples taken from restaurants and their suppliers failed to find the E. coli that sickened restaurant patrons.

Briefly, there was a false lead: A laboratory hired by Taco Bell detected E. coli in green onions, and the chain removed the vegetables from its restaurants. But subsequent tests found those results to be incorrect.

Also complicating the investigation were a couple of timing issues: First, high-volume restaurants like Taco Bell tend to use up and throw out food items in only a matter of days. Second, E. coli victims usually don’t began feeling ill until one to 10 days after eating a contaminated meal.

By the time victims got sick, went to the doctor and had tests done to diagnose the cause, the contaminated food was probably gone, noted Jean Halloran, a food policy expert with Consumers Union.

Sometimes, health investigators get a break. In the recent investigation into tainted spinach, sick people still had bags of the bacteria-tainted greens sitting in their refrigerators.

Those bags of spinach were “the smoking gun” that helped the U.S. Food and Drug Administration trace the outbreak all the way back to the California fields that grew the infected produce, said Michael Doyle, director of the University of Georgia’s Center for Food Safety who was hired by Taco Bell to help it test its food.

“The spinach outbreak was an unusual circumstance. Everything kind of fell into place,” Doyle said.

Health officials settled on spinach as the cause just one day into that investigation, Braden said.

But in the Taco Bell probe, health officials had to rely on what 58 patients said they ate, and on food distribution records that would explain why those restaurants had outbreaks but not others.

Ground beef, cheddar cheese and lettuce were the common ingredients eaten by all the people who became ill. The first two were considered unlikely, because of cooking, pasteurization and the fact that they were delivered to restaurants where people did not get sick.

On Wednesday, health officials announced lettuce was the likely cause, although patient interviews and other work will continue.

On Thursday, they said case reports have peaked and the danger has apparently ended. “The outbreak is over,” said CDC spokesman Dave Daigle.

Taco Bell officials noted that they changed produce suppliers last week, and said the food in the restaurants is safe.

“We have replaced all the lettuce,” said Rob Poetsch, a company spokesman.

Of course, not everyone is happy with how the investigation is going. Tom Russell, president of Salinas, Calif.-based grower Pacific International Marketing, said farmers were doing everything they could to ensure the safety of their crops.

Federal health officials were too quick to link lettuce to the E. coli outbreak, he said.

“They should do their research before they start smearing products,” Russell said. “Each time they do it, they drive prices down.”

Halloran, of Consumers Union, said she believe investigators have done the best job they could, given the complexities involved. She called for better infection control at farms and better inspections at processing plants to prevent such illnesses from recurring.

“We have to do a better job dealing with this problem at the source,” she said.

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