Mexico has denied responsibility in the nationwide outbreak of a parasitic infection linked to lettuce that has sickened thousands across 41 U.S. states.
Laboratory-confirmed cases of cyclosporiasis, an illness that causes unpleasant symptoms such as explosive diarrhea, surpassed 4,000 as of July 21, according to the Centers for Disease Control and Prevention, with the actual number of sick estimated to be above 10,000. The U.S. Food and Drug Administration linked the outbreak back to lettuce grown by Taylor Farms in the central Mexican state of Guanajuato.
The country's health minister, David Kershenobich, refuted the FDA's assertion on July 21. The same day, Health and Human Services Secretary Robert F. Kennedy Jr. said in a news briefing the outbreak was "under control."
"So far, no evidence has been found to indicate that it originated there," Kershenobich said in a news conference, per Reuters. "However, we are taking all precautions in case that were the case."
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Signage is seen outside of the Food and Drug Administration headquarters in White Oak, Md., on Aug. 29, 2020.
Kennedy and the Trump administration have faced criticism for cuts to relevant health agencies, with some critics indicating that scaled-back surveillance and limited staffing enabled not only the cyclospora spread, but other recent outbreaks like screwworm. Last year, the CDC scaled back a decades-long federal-state partnership that monitors foodborne illnesses, including those caused by cyclospora.
Still, Kennedy called the critiques "invalid."
"We did cuts in the FoodNet program, but they were for redundant surveillance, and all the states will tell you that we're still doing forensics," he said in the July 21 briefing.
Authorities in the United States and Mexico are continuing to search for a cause behind the case clusters, a task that has proven messier than other foodborne outbreaks of the recent past.
What was the 'false positive' all about?
Taylor Farms said in a July 19 statement that the FDA had "apologized" to the brand for a "false positive" when testing its iceberg lettuce products from central Mexico for cyclospora. But the company's statement doesn't tell the whole story, said food safety expert Bill Marler, as it does little to prove culpability one way or another, something not unusual in such an investigation.
"A positive test means nothing and a negative test means nothing because the product they were testing was not the product people were consuming," Marler told USA Today. "The product that people consumed, the people ate the evidence ... or it's past its best-by date and was thrown out."
A drone view shows Taylor Farms Mexico's facilities after the U.S. Food and Drug Administration said on Monday that the basis for linking Taylor Farms lettuce from central Mexico to the largest U.S. foodborne illness outbreak in recent years remains unchanged, defending its decision to publicize a sample that a later review found was a false positive for the Cyclospora parasite, in Doctor Mora, Guanajuato state, Mexico, July 20, 2026. REUTERS/Demian Sanchez
The FDA made a similar point in a July 20 X post, saying, "To clarify, this false-positive lab sample DOES NOT change the basis for FDA’s ongoing outbreak investigation or the overwhelming epidemiological data supporting the current voluntary recall by Taylor Farms."
Outbreaks are often traced back to manufacturers, growers or distributors despite food samples that never test positive for a pathogen. When that happens it's not because they weren't the underlying source, said Marler, but because the culprits have already passed through the system or were hiding in other places, like soil or water.
"From 33 years of experience handling these cases, there is, for the most part, never a food product available to test in an outbreak," he said. "Because by the time you figure out the outbreak is going on, the product is long gone. That's especially true for perishable products."
So what does the false positive really mean? Not much either way, said Marler, who also added that Mexico's arguments of nonculpability are subject to the same uncertainty.
"If it had tested positive, it doesn't necessarily mean for sure that Taylor Farms was the source," he said. "And if it tested negative, it doesn't absolve Taylor Farms of being the cause."
So, is Taylor Farms in the clear?
No, Taylor Farms has not been definitively proven to be the source of the outbreak, but it also hasn't been proven not to be.
As of July 20, the FDA still said traceback data pointed to shredded iceberg lettuce from Taylor Farms locations in central Mexico. Traceback data, or information acquired through steps such as interviewing sick people and used by investigators to trace illnesses back through the food supply chain, is often the primary tool used in cyclospora outbreaks.
The logo of Taylor Farms Mexico is seen at the company's facilities after the U.S. Food and Drug Administration said on Monday that the basis for linking Taylor Farms lettuce from central Mexico to the largest U.S. foodborne illness outbreak in recent years remains unchanged, defending its decision to publicize a sample that a later review found was a false positive for the Cyclospora parasite, in Doctor Mora, Guanajuato state, Mexico, July 20, 2026. REUTERS/Demian Sanchez
In a voluntary recall initiated July 17, Taylor Farms de Mexico of Guanajuato, Mexico, said it was removing all iceberg lettuce sourced from central Mexico from the U.S. market "out of an abundance of caution" in an action "prompted" by the outbreak. "We are actively removing the implicated products. The company has stopped receiving product from the implicated lot, suspended distribution of the iceberg lettuce from Central Mexico, notified our customers, and we are continuing to work with the FDA, CDC, and state authorities," the company said.
Taylor Farms' growing and processing facilities in Mexico were the source of a major U.S. cyclosporiasis outbreak in 2013 that sickened more than 600 people in 25 states and was traced to salad mix from Taylor Farms de Mexico in Guanajuato, according to the CDC.
The techniques used to identify Taylor Farms in this outbreak are the same as those being used currently, explained Marler.
Why health authorities are having trouble finding the culprit
The source of a cyclosporiasis outbreak is more difficult to track than foodborne illnesses for several reasons, chief among them the inability to use the same genetic tracking tools available for pathogens like E. coli and listeria, as previously reported by USA Today.
Most clusters of foodborne illness and infectious disease today are traced via whole-genome sequencing (WGS), a technique in which the unique genetic code of a pathogen is tested and compared in an effort to establish a unique "fingerprint." This DNA "fingerprinting" lets scientists determine whether germs making people sick are closely related, helping investigators connect cases and trace outbreaks to a common source.
The FDA remains focused on Taylor Farms' Mexico facilities as the likely source of a multistate cyclospora outbreak, despite an earlier false-positive test. The outbreak has sickened more than 1,600 people and pressured restaurant traffic.
Because cyclospora is difficult to culture, can't be grown in a lab and is commonly seen in low quantities in stool samples, the CDC instead relies on a different sequencing system that targets select genetic markers. This targeted genotyping helps determine which cyclospora cases belong to the same outbreak, however, unlike WGS, it usually cannot identify the contaminated food or its origins on its own.
In a July 14 media briefing, Dr.Gwen Biggerstaff, deputy director of the CDC's Division of Foodborne, Waterborne, and Environmental Diseases, called the method "useful for detection but not as fast and precise as whole-genome sequencing," acknowledging that it also cannot establish transmission links with the same confidence as WGS. This is why in some cases, such as the most recent outbreak in 2024 attributed to parsley, a single grower, manufacturer or distributor is never identified.
How the FDA, CDC are trying to track and trace the diarrhea bug
In this and previous outbreaks, investigators relied heavily on something called traceback, which is essentially the food-safety equivalent of detective work.
Once authorities identify a group of people who appear to be part of the same outbreak, investigators work backward through the food supply chain to a common source. This is done first and foremost through interviewing sick people about what they ate before becoming ill – another matter that is complicated by the parasite's nature. Because cyclosporiasis can have a long incubation period of two or more weeks, it can be difficult for people to remember what they ate during that time.
Taylor Farms sells salad kits, lettuce and other fresh-food products through retailers nationwide.
After trends are identified, health agencies "triangulate" by combining patient interviews, genetic testing and food-supply-chain records. The answer lies in where these three types of evidence overlap.
"Cyclospora investigations are particularly challenging and take a significant amount of time and effort," said Biggerstaff, who added that the CDC and FDA are combining the pillars of surveillance and case counting, patient interviewing and data tracing, as well as laboratory testing and clinical alerts to find a cause. "We want to be transparent that this is an active and evolving investigation."
