The U.S. Food and Drug Administration (FDA) stated on Sunday that a prior laboratory test, which indicated a sample of shredded iceberg lettuce supplied by Taylor Farms de Mexico was contaminated with Cyclospora, was actually a false positive.
However, the FDA has not retracted its original conclusion. The agency's traceback investigation continues to indicate a link between the company and a multi-state Cyclospora outbreak.
"This does not mean they are taking back the conclusion," said Susan Mayne, former director of the FDA's Center for Food Safety and Applied Nutrition, which oversees outbreak investigations, in a text message on Monday. "Not detecting a positive result does not mean the result does not exist."
Many outbreak investigations are ultimately solved through basic epidemiological work—asking patients what they ate, tracing food sources, and finding commonalities among those infected. Laboratory testing is just one part of the overall investigative analysis.
The FDA stated that laboratory experts reanalyzed the sample and determined the initial test result "was not true amplification and should be considered a false positive." The agency also noted that no product samples have yet been confirmed as positive for Cyclospora.
Don Schaffner, a food scientist at Rutgers University, explained that Cyclospora testing works by chemically replicating DNA in a sample until a detectable signal is produced. He noted that if the parasite is detected after only a few amplification cycles, it indicates a high level of contamination; but if too many cycles are run, the reliability of the conclusion decreases—which is one way a false positive can occur.
"If a sample doesn't have Cyclospora DNA to begin with, but you run the amplification reaction enough times," he said, "eventually you will amplify background noise—signals that aren't really there—and mistake them for something that is."
The latest laboratory finding—that the FDA did not detect the parasite in a specific lettuce sample—does not affect the conclusions of the agency's other investigative work. This is because the sample the FDA tested was not part of the initial recall, and the recall decision was based primarily on epidemiological and traceback investigations that have linked the outbreak to iceberg lettuce supplied by Taylor Farms.
The title of the FDA's website announcement still reads: "Do not eat recalled iceberg lettuce from Taylor Farms de Mexico." It notes that the investigation is ongoing.
In a statement Sunday evening, Taylor Farms spokesperson Mindi Mayhew said the company "has completed a voluntary recall of iceberg lettuce from Central Mexico." She added that the recall is limited to iceberg lettuce grown and processed in Central Mexico and that all other Taylor Farms products, including all Taylor Farms brand products in the marketplace, are not part of the recall. "We will continue to cooperate with public health officials as the current outbreak investigation progresses."
Taylor Farms announced the recall on Friday, just hours after the company said it was voluntarily holding back iceberg lettuce from Central Mexico from the U.S. market. Unlike a voluntary hold or market withdrawal, a recall requires a firm to actively notify customers and make efforts to retrieve affected product from the distribution chain, making it broader in scope and more costly.
Late Friday, Taylor Farms de Mexico issued a detailed recall notice covering 27 states, listing all iceberg lettuce from Central Mexico distributed between June 29 and July 16. These products are mostly bulk foodservice packs with "use by" dates up to August 3.
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