FDA retracts test result but still links Taylor Farms lettuce to cyclosporiasis

The Food and Drug Administration said Sunday that a laboratory finding showing cyclospora contamination in a sample of shredded iceberg lettuce supplied by Taylor Farms de Mexico was a false positive.

But the agency did not back away from its conclusion that the company remains linked to a multistate outbreak of cyclosporiasis through its traceback investigation.

“FDA’s traceback investigation and outbreak data continue to converge on shredded iceberg lettuce from Taylor Farms locations in central Mexico,” the agency said on social media Monday afternoon. It said there was “overwhelming epidemiological data” to support the voluntary recall.

“It does not mean they are walking it back,” Susan Mayne, a former head of the FDA’s center on food safety that oversees outbreak investigations, said in a text message Monday. “Absence of evidence (of a positive test result) is never evidence of absence.”

Numerous outbreaks are solved on the basis of basic disease detective work, talking to people who have fallen ill, tracing back what they ate and finding connections among others who were infected. Laboratory tests are one part of a wider investigative analysis.

The FDA said laboratory experts reanalyzed the sample and determined that the initial result “does not represent true amplification and should be considered a false positive.” The agency said there are currently no confirmed positive product samples for cyclospora.

Cyclospora is detected by running chemical reactions that double the amount of DNA in a sample until there is a noticeable signal, said Don Schaffner, a food scientist at Rutgers. Detecting a parasite after just a few tests points to a strong presence, but that conclusion weakens as more and more tests are run — one potential source of false positives, he said.

“If you have a sample that, let’s say, has no cyclospora DNA, but you run enough reactions,” he said, “you’ll eventually amplify noise — stuff that’s not real — and it’ll turn into something that’s real.”

The updated laboratory finding — that the FDA did not find the parasite in a particular lettuce sample — does not change the rest of the agency’s investigation. The sample that the FDA tested was not part of the original recall; the evidence around the recall is based on epidemiological and traceback determination linking the outbreak to Taylor Farms-supplied iceberg lettuce.

The top line of the notice on the FDA website continues to say: “Do not eat recalled iceberg lettuce from Taylor Farms de Mexico.” It says the FDA’s investigation is ongoing.

In a statement late Sunday, Taylor Farms spokesperson Mindi Mayhew said the company had “completed a voluntary recall of iceberg from central Mexico.” She said the recalled product was limited to iceberg lettuce grown and processed in central Mexico.

All other Taylor Farms products, including all Taylor Farms-brand products available for purchase, are not involved in the recall, she said. “We are committed to working with public health authorities as the ongoing outbreak investigation continues.”

Taylor Farms announced Friday that it was initiating a recall hours after it said it was voluntarily removing iceberg lettuce sourced from central Mexico from the U.S. market.

Unlike a voluntary product hold or removal, a recall requires a company to actively notify customers and work to recover affected products from commerce, making it a broader and more costly response.

Late Friday, Taylor Farms de Mexico published a detailed 27-state recall list covering all iceberg lettuce sourced from central Mexico and distributed from June 29 through July 16. The products — many of them bulk food service packages — had “best if used by dates” extending to Aug. 3.