Why does lettuce remain the top suspect in the cyclospora outbreak?

(NewsNation) — An iceberg lettuce sample initially linked to the cyclospora outbreak that has sickened thousands of people across the United States ultimately tested negative after further review, the Food and Drug Administration said.

The agency said it reexamined the sample after initial positive results and determined it was a false positive. The FDA notified supplier Taylor Farms of the finding and said it “continues working with the firm to ensure product implicated in this outbreak has been removed from the market.”

While the latest update means investigators still have not identified the exact source, shredded iceberg lettuce remains the leading suspect because the epidemiological and supply-chain evidence continues to point in that direction.

Patient interviews point to lettuce

Cyclospora is notably difficult to trace. Unlike other foodborne pathogens, such as Salmonella or E. coli, the parasite has an incubation period of up to two weeks, making it difficult for people to accurately remember what they ate before becoming ill.

Investigators have worked with epidemiologists to interview patients about what they ate, where they dined, and when they became sick before tracing those foods through the supply chain.

Those interviews linked a large outbreak in Michigan, Indiana, Kentucky, Ohio and West Virginia to shredded iceberg lettuce served at Taco Bell restaurants supplied by Taylor Farms’ operations in Mexico.

According to the FDA, about 90% of Michigan patients interviewed after eating at Taco Bell reported consuming shredded iceberg lettuce.

Taco Bell has since removed the affected Taylor Farms lettuce from its restaurants.

Why testing cyclospora is so difficult

Health officials say cyclospora presents unique challenges because it cannot be grown in laboratory cultures like many foodborne bacteria.

Scientists must instead extract tiny amounts of the parasite’s genetic material and amplify it by synthesizing DNA copies before testing, Reuters reported. The process is highly sensitive but can occasionally produce false-positive results, as happened with the lettuce sample tested by the FDA.

“It’s extremely difficult to detect in food, even with the highly sensitive methods that we use,” Donald Prater, the FDA’s acting deputy commissioner for food, said during a briefing.

Former FDA Commissioner Scott Gottlieb told Reuters investigators also lack one of their most valuable outbreak tools.

“Cyclosporiasis is very hard to test for. There’s no whole genome sequencing,” Gottlieb said. “FDA relies on whole genome sequencing to effectively fingerprint a pathogen to try to isolate a particular pathogen to a particular outbreak to a particular grower. You don’t have that in this case.”

False-positive test doesn’t change investigation

Although the border sample ultimately tested negative, the FDA said its broader investigation continues to point to shredded iceberg lettuce sourced from Taylor Farms’ operations in central Mexico.

The agency has continued working with the company to remove products linked to the outbreak from the marketplace.

Taylor Farms announced Friday that it was voluntarily removing iceberg lettuce sourced from central Mexico from the U.S. market and released a list of affected products.

The FDA later noted that the sample that produced the false positive came from a product outside the scope of that voluntary removal.

A voluntary product removal differs from a formal recall because companies aren’t required to notify customers or recover products already in commerce.

Investigation continues

No deaths have been reported from the outbreak, though at least 308 people have been hospitalized.

A U.S. Army veteran is suing Taco Bell, Taylor Farms and the franchise operator of the restaurant he visited after he says a cyclospora infection left him hospitalized for two days.

Even so, Taco Bell-associated illnesses account for only about 60% of the outbreak, according to Gottlieb, suggesting additional contaminated foods or unrelated outbreaks may also be contributing.

For now, investigators say shredded iceberg lettuce remains the strongest lead not because of a positive laboratory test, but because patient interviews and supply-chain tracing continue to point ot the same source consistently.

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