The U.S. Food and Drug Administration (FDA) said on Sunday, July 19, that a test that appeared to find Cyclospora in shredded iceberg lettuce was actually a “false positive.”

The FDA previously said that laboratory testing had identified Cyclospora, the parasite responsible for a multistate diarrhea outbreak that has sickened thousands, in a sample of shredded iceberg lettuce supplied by Taylor Farms de Mexico. But after reviewing the sample results again, the agency said it was revising its announcement. 

“Due to the complexity in detection of Cyclospora, FDA laboratory experts…have concluded that the finding does not represent true amplification and should be considered a false positive,” the agency wrote in a July 20 post on X. FDA officials stressed, however, that Taylor Farms locations in Mexico and iceberg lettuce remain linked to the outbreak despite the false positive result. 

Here’s what else you should know about the cyclosporiasis outbreak and what you can do to protect yourself.

What is causing the Cyclospora outbreak? 

Federal health officials previously linked shredded iceberg lettuce supplied by Taylor Farms to Taco Bell restaurants in five states to at least 1,600 cyclosporiasis infections. 

On Saturday, July 18, the FDA said (in a page that has since been deleted) that a sample of Taylor Farms shredded iceberg lettuce had tested positive for Cyclospora. The sample was collected as part of the agency’s outbreak investigation from a batch of imported products at the southern border, the FDA said. The following day, the agency said the test had been a false positive. It later stressed that Taylor Farms lettuce remained implicated as a potential source of the outbreak despite the testing error.

The FDA said in the post on X that its “traceback investigation and outbreak data,” including interviews with people sickened by Cyclospora, “continue to converge on shredded iceberg lettuce from Taylor Farms locations in central Mexico.”

“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,” the FDA post said. 

Taylor Farms, a major lettuce supplier to U.S. supermarkets and fast-food chains, said Sunday that it had voluntarily recalled its iceberg lettuce grown and processed in central Mexico “in an abundance of caution.” 

The company added that the FDA had “apologized” for its testing mistake. An FDA official said at a press conference Monday that the agency had not apologized to Taylor Farms, but had notified the company of the results of its testing process. 

Even if iceberg lettuce is confirmed as a source of some cyclosporiasis cases, it might not be the only culprit of infections across the country, says Don Schaffner, a food-science expert at Rutgers University. 

The FDA said at a press conference on Monday that it has investigated multiple clusters of cyclosporiasis cases since May, though the outbreak linked to Taco Bell and Taylor Farms is the largest. 

Past cyclosporiasis outbreaks in the U.S. have been linked to fresh produce such as raspberries, cilantro, snow peas, and leafy greens. Cyclosporiasis can also be acquired via contaminated water. “You can get it from a variety of different sources,” Schaffner says. “There’s this correlation with Taco Bell lettuce, but there are probably other outbreaks going on, making it hard to find just one cause.” 

What is a “false positive” result?

A false positive happens when a test says something is present, but it actually isn’t. Food-safety experts say a false positive result for Cyclospora in food is common. 

The way to test for Cyclospora in a food sample is to do a polymerase chain reaction, or PCR, test, Schaffner says. The test works by attempting to find the parasite’s DNA and then amplifying what it finds. “It will keep doubling the DNA until it reaches a detectable signal,” Schaffner says. 

But if the sample is compromised or too many cycles of the test are run, “you can also amplify garbage DNA in the sample” that is not from the parasite but is similar enough to fool the test, which could result in a false positive, Schaffner says. 

The FDA hasn’t explained how the false positive result came about or why it was reported publicly. The agency has said only that its laboratory experts “re-reviewed” the positive lettuce sample and concluded that the initial finding was incorrect. 

Cyclospora is difficult to detect in food, FDA officials have said, and detecting the parasite in iceberg lettuce is particularly tricky as the vegetable’s short shelf-life and the parasite’s long incubation period mean that by the time people are sickened, the contaminated lettuce is usually gone from store shelves and restaurant tables.

So, is it safe to eat lettuce? 

The FDA has urged consumers, restaurants, and retailers to discard and not consume all recalled iceberg lettuce distributed by Taylor Fresh Foods. The recalled lettuce was distributed from June 29 to July 16 in 27 states.

The FDA also says to avoid eating “recalled lettuce at any Taco Bell location” or other restaurants. “If you don’t know the source of the iceberg lettuce, ask the restaurant,” the agency advises.

To help reduce the risk of cyclosporiasis, the U.S. Centers for Disease Control and Prevention recommends that people wash all fruits and vegetables under running water, including produce labeled as pre-washed. Firm fruits and vegetables, such as cucumbers and melons, should be scrubbed with a produce brush, the agency says. 

Cooking vegetables and fruits to an internal temperature of at least 158°F can kill Cyclospora. But some food-safety experts say it is probably OK to eat that Caesar salad or burger with shredded greens. Schaffner, the Rutgers expert, says he hasn’t changed his own eating habits and continues to eat salads daily.

“Maybe don’t eat lettuce at Taco Bell in Michigan four weeks ago,” he jokes, referring to the state hard-hit by the Cyclospora outbreak. Michigan state health officials said on July 21 that more than 6,500 cases of cyclosporiasis have been reported in the state since June 22.  

“I know people are worried, but I don’t think you need to stop eating fresh produce,” Schaffner says.

Wade Syers, who lives in Michigan and is Michigan State University Extension’s food-safety specialist, says he too is eating lettuce and doesn’t think people need to shy away from fresh fruits and vegetables. 

“What I’ve been advising everyone to do is to follow food-safety guidelines, which is what we should be doing anyway: wash your hands before, during and after food prep, for example, and rinse all produce under running water,” he says.

With lettuce, he advises buying a whole head of it and removing the outer leaves before washing the rest thoroughly. Also keep an eye out for recall notices and outbreak updates, he says. 

If you do develop symptoms of cyclosporiasis, which include watery and sometimes explosive diarrhea, the CDC recommends contacting your doctor, getting tested, and staying hydrated.