FDA is investigating a new cyclospora parasite outbreak — and hasn't identified a source

The new cluster is one of at least four active cyclospora outbreaks the FDA is currently investigating. The largest, which has been linked to shredded iceberg lettuce supplied by Taylor Farms and served at Taco Bell locations in five states, involves at least 1,644 confirmed cases, according to the Wall Street Journal. The FDA said it has not identified a source for any of the other active outbreaks.

Across 41 states, at least 4,173 cases of cyclosporiasis have been confirmed, with at least 308 people hospitalized, according to the Journal. The Centers for Disease Control and Prevention is also reviewing thousands of additional suspected cases.

The FDA describes cyclospora as a microscopic parasite that reaches people through food or water that has been tainted with human fecal matter. The resulting illness, cyclosporiasis, can cause diarrhea, stomach cramps, nausea, and fatigue. The incubation period ranges from as few as two days to well over two weeks, and the illness can drag on for weeks if left untreated.

The new outbreak and the broader surge in cases follow a multistate outbreak tied to Taco Bell that investigators linked to shredded iceberg lettuce from Taylor Farms de Mexico. Last week, Taylor Farms pulled iceberg lettuce that had been grown and processed in central Mexico from distribution, a recall that extended to retail outlets, restaurants, and food service customers in 27 states. Taco Bell said it stopped using lettuce supplied by Taylor Farms de Mexico as of July 17.

The investigation has had complications. The FDA announced Sunday that a sample of Taylor Farms iceberg lettuce it had reported as testing positive for cyclospora the previous day was a false positive. After re-reviewing the results, FDA laboratory experts determined the reading was erroneous and notified Taylor Farms. The recall remains in effect despite the retraction.

Michigan has recorded more cyclospora cases than any other state, yet health officials there have been unable to pinpoint what is driving the infections. Officials have flagged lettuce and salad greens as a likely vehicle for the parasite, but they say the evidence is insufficient to hold any single source responsible or to exclude the possibility that several separate outbreaks are unfolding at once.

The parasite follows a seasonal pattern in the U.S., with case counts rising each spring and summer when heat and humidity create conditions favorable to its transmission, frequently drawing investigators from multiple states.