The U.S. Food and Drug Administration’s announcement that a lettuce sample previously believed to contain the Cyclospora parasite was a false positive has left many consumers wondering whether investigators are back at square one.
According to Dr. David Weber, an infectious disease specialist with UNC Health, the answer is no.
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“It’s a balance,” Weber said. “You don’t want people eating something that’s contaminated while you’re trying to do multiple validation steps. On the other hand, you don’t want to incorrectly label something too early. It’s a fine line between being upfront with the public, but the faster you do it, the more likely there will be an error.”
Over the weekend, the FDA said an earlier laboratory finding that detected Cyclospora in a shredded lettuce sample from Taylor Farms de Mexico was a false positive.
Taylor Fresh Foods later said in a statement, "Today, FDA apologized to us." The statement also said the company was voluntarily recalling iceberg lettuce shipped from central Mexico.
The FDA later clarified that the false positive has not ended its investigation into the supplier as officials continue working to identify the source of a multistate outbreak.
Weber said one revised laboratory result does not automatically eliminate a food supplier from consideration because investigators rely on much more than a single test result.
In fact, he explained that investigators may still be able to link a supplier to illnesses through epidemiological evidence — including what people ate, where they purchased food and whether illness patterns point to a common source — even if another contaminated food sample is never identified.
Foodborne illness investigations combine laboratory testing with patient interviews, statistical analysis, and detailed traceback investigations to determine whether a food item is linked to illnesses.
Weber explained that a situation like this, where a supplier may get produce from multiple farms, can only add to the complexity of the investigation.
“Even if you can find an importer, they may be importing from 20 or 30 different farms, and they may be mixing the food together when they chop it up and put it into a lettuce bag,” he said. “So, it is complex trying to figure out exactly where the source was.”
Those supply-chain challenges are only part of the problem. Science itself presents another hurdle.
Weber said another reason Cyclospora investigations can take longer than outbreaks caused by bacteria such as Salmonella is that the parasite is more difficult to analyze at the molecular level.
While scientists routinely use whole genome sequencing to connect bacterial illnesses to contaminated food, similar testing for Cyclospora is far more technically challenging.
Investigators also face another unique obstacle with cyclosporiasis: time.
“The incubation period — the time from when you eat to when you get sick — averages about seven days, but it can be as long as 14 or more days,” Weber explained. “It’s easy for people to remember what they ate yesterday, but if you ask people, ‘What did you eat on July 7th,’ they’re going to have trouble remembering.”
Weber noted that foods often implicated in outbreaks, such as fresh leafy greens, herbs, and berries, are rarely eaten on their own.
“Nobody eats cilantro by itself,” Weber said. “It’s part of a salad and you have lettuce, carrots, onions and peppers. If you can incriminate the salad, figuring out which part of the salad is the source is difficult.”
Wake County added another 16 confirmed cyclosporiasis cases to its dashboard Wednesday, bringing the total countywide count to 366 as of July 22. The North Carolina Department of Health and Human Services reported 561 confirmed cases statewide as of July 21.
NCDHHS announced there are likely multiple food sources contributing to a rise in cyclosporiasis infections in multiple counties this summer.
For consumers, Weber recommends rinsing fresh fruits and vegetables before eating them and removing the outer leaves of lettuce. Cooking produce is the most effective way to kill the parasite. Weber added that refrigeration or freezing should not be relied upon to eliminate the risk.
Weber said he is personally choosing locally grown produce while the investigation continues.
“Virtually all of the contaminated foods come from outside of the U.S. For our consumers here, if you go to the local farmers markets during the week, you’re going to be safe,” Weber said. “For me, I’ll just play it safe and eat local fruits and vegetables until I know that others are safe, and because I like supporting local industry.”
He also emphasized that Cyclospora is not spread from person to person. Anyone who experiences diarrhea lasting several days should seek medical care because the infection can be diagnosed with a stool test and treated.
