Taylor Farms lettuce is still focus of cyclospora investigation despite false positive test, FDA says

A sample of lettuce from Taylor Farms that had tested positive for cyclospora was a false positive, the Food and Drug Administration said Sunday. But the FDA said Monday that its investigation and data "continue to converge on shredded iceberg lettuce from Taylor Farms locations in central Mexico." 

In the statement announcing the false positive, the FDA said that as of Sunday it had not found any "confirmed positive sample results for product testing for Cyclospora."  

In a subsequent statement Monday, it said, "To clarify, this false-positive lab sample DOES NOT change the basis for FDA's ongoing outbreak investigation or the overwhelming epidemiological data supporting the current voluntary recall by Taylor Farms." 

Taylor Farms on Friday said it was "voluntarily removing" iceberg lettuce sourced from central Mexico after the lettuce had been linked to one of the largest outbreaks of cyclosporiasis in the U.S. in recent years.   

"While the FDA traceback is indicating a specific independent farm, which represents less than 1% of the U.S.'s iceberg lettuce supply, as the potential source of the outbreak, we have removed all iceberg lettuce from the region indefinitely," the company said Friday in a statement.

Testing for the cyclospora parasite is more complicated than for other pathogens, such as E. coli or Salmonella, Jennifer McEntire, founder of food safety consulting firm Food Safety Strategy, told CBS News.

"With things like cyclospora, it's much more difficult to recreate it in the labs, which means that it's harder to run tests," former associate FDA commissioner Peter Pitts said.

CBS News medical correspondent Dr. Céline Gounder said that a false positive test result was "ordinary as a laboratory event."

"The unusual thing isn't the false positive. It's that an unconfirmed single-sample result went public on a Saturday and was withdrawn on a Sunday," Gounder said. "The question I'd ask FDA is not 'how did the lab get it wrong' but 'what is your policy for announcing a presumptive positive before confirmatory testing closes, and did you follow it?'"

On Thursday, the U.S. Centers for Disease Control and Prevention identified lettuce supplied to select Taco Bell restaurants as a source of the multistate outbreak, more than a month after cases first began to appear.

The iceberg lettuce was distributed to Taco Bell restaurants in Indiana, Kentucky, Michigan, Ohio and West Virginia, according to the CDC. Taco Bell said on Friday it had "completed removal of affected Taylor Farms lettuce from our restaurants."

In a new statement on Monday, Taco Bell CEO Sean Tresvant addressed "fans" of the fast food chain, promising transparency and putting customer safety first.

"This means bringing the right people together, asking the hard questions and doing the work to help build a stronger food system," Tresvant said in a post on LinkedIn. "My thoughts are with everyone affected. There is a better way forward, and we are going to lead the way. We owe that to you."

Walmart also announced over the weekend it had removed four Marketside bagged iceberg lettuce salad products from its stores in more than two dozen states, but added there was "no indication that products sold in our stores are affected by the current Cyclospora investigations."

Taylor Farms said Sunday, "Our thoughts remain with everyone who has fallen ill in this outbreak. We are committed to working with public health authorities as the ongoing outbreak investigation continues."

The outbreak, first reported in May, has spread to 34 states and sickened more than 1,600 people, according to the CDC. However, the CDC's tally lags behind state health departments, which have reported much larger numbers of cases. For instance, the Michigan Department of Health and Human Services reported more than 5,000 confirmed cases as of Friday.

"It may be the case, as in the case in most investigations, we will never find out definitively where the source was from," said Timothy Lytton, a professor of law at Georgia State University College of Law who researches health and safety regulation.

No deaths have been reported.

Cyclosporiasis is a food- and waterborne illness caused by the cyclospora parasite, which causes long-lasting, watery and sometimes explosive diarrhea, according to public health experts. 

Outbreaks typically occur in late spring and summer and have historically been linked to bagged salads, cilantro, basil, raspberries, snow peas and green onions.