FDA Says False-Positive Test Does Not Change Cyclospora Outbreak Investigation

Federal officials sought to reassure the public that a recent false-positive laboratory result has not altered the evidence supporting the ongoing Cyclospora outbreak investigation, which continues to point to shredded iceberg lettuce supplied by Taylor Farms in central Mexico.

During a media briefing, FDA Acting Commissioner Kyle Diamantas said the agency, working alongside the CDC and state partners, is currently investigating multiple Cyclospora illness clusters nationwide. Since May, investigators have examined 5 subclusters, with 3 investigations now closed. One of the largest active outbreaks includes more than 1,600 illnesses reported across multiple states.

"For this particular cluster, we have also seen cases reporting illness as recent as last week, which means it's imperative for us to take action to ensure contaminated products are not making their way to the market," Diamantas said.

Last week, the FDA and CDC advised consumers not to eat shredded iceberg lettuce sourced from Mexico and served at Taco Bell restaurants in Indiana, Kentucky, Michigan, Ohio, and West Virginia. Diamantas also noted that Taylor Farms Mexico voluntarily removed all iceberg lettuce sourced from central Mexico from the US market, with recalled products listed on the FDA website.

The agency emphasized that the investigation extends beyond this single cluster.

"I want to stress that our investigation continues, including into other potential sources and commodities for other clusters of illnesses," Diamantas said. He added that Cyclospora remains a seasonal foodborne hazard commonly associated with fresh produce during the summer months, with more than 4,500 US cases reported in 2023 and more than 3,500 in 2024.

Much of the briefing focused on confusion surrounding a laboratory test that initially suggested Cyclospora contamination in imported lettuce outside the scope of the voluntary recall. FDA Acting Deputy Commissioner for Food Don Prater explained that subsequent laboratory quality checks determined the result was a false positive.

"It's important to note that the reevaluation of the sample does not alter the data supporting the outbreak advisory that prompted the current voluntary recall," Prater said. "FDA's traceback investigation and the outbreak data continue to converge on shredded iceberg lettuce from Taylor Farms' location in central Mexico."

Prater said detecting Cyclospora in food remains technically challenging because the parasite cannot be cultured using conventional laboratory techniques and often occurs at very low levels in contaminated produce.

"False positives are not common. In fact, it's very difficult to detect this parasite in food," he said. "Unlike bacteria, this parasite cannot be cultured using conventional laboratory technique, so it's extremely difficult to detect even in food, even with the highly sensitive methods that we use."

FDA officials explained that epidemiologic evidence and traceback investigations often provide stronger evidence than laboratory confirmation alone during foodborne outbreak investigations. By the time illnesses are recognized and reported, implicated produce has frequently been consumed or discarded.

"Ultimately, the protective public health action results, in this case, from strong epidemiologic information and traceback," Prater said.

During the question-and-answer session, FDA officials reiterated that Taylor Farms remains the primary supplier identified through the investigation and confirmed the company continues to cooperate with regulators through its voluntary recall. They also emphasized that the false-positive laboratory result does not weaken the epidemiologic evidence linking the outbreak to the recalled lettuce.

Officials said increased import screening of lettuce at the southern border will continue as the investigation progresses and pledged to update the public should additional products, brands, restaurants, retailers, or commodities become implicated.

Read Infection Control Today for the latest information on this ongoing outbreak.