Science1 publisher2 min readPublished
Health officials closed the largest U.S. cyclospora outbreak without finding where it started
Nearly 13,000 reported cases were tied to recalled iceberg lettuce grown in central Mexico. Federal officials still cannot say how sewage contaminated enough food to sicken that many people.
The Scientist · Science desk
What happened
- U.S. health officials said Friday that the largest cyclospora food poisoning outbreak in the country's history is over, with nearly 13,000 reported cases tied to a 21-state cluster linked to iceberg lettuce.
- Investigators first looked at lettuce served at Taco Bell, then focused on Taylor Farms, which recalled iceberg lettuce grown in central Mexico across thousands of packaged salad products sent to chains including Yum Brands.
- Michigan reported more cases than any other state and both of the two deaths tied to the outbreak.
- The FDA has ended its on-site inspections and sample collection at the iceberg lettuce growers and the processing facility in Mexico, and said samples from those visits are pending analysis.
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Why it matters
- constraint A supplier audit at those farms can only re-verify generic soil and water controls, since the investigation identified no contamination point and no specific failure to close out.
- decision These outbreaks cluster in late spring and summer, so buyers sourcing iceberg from central Mexico have to set next season's contracts on the strength of the epidemiological link alone.
- contradiction The declaration closes the lettuce cluster, yet thousands of cases from the same surge sit outside it, so the outbreak that just ended does not account for the year's rise in illness.
The counts in this investigation follow three different rules. They do not add into a single number. The nearly 13,000 cases tied to the outbreak are reported cases [3]. The nearly 20,000 logged since May 1 are lab-confirmed [7], and more than 6,100 further cases are recorded as suspected [8]. Add those two and the government has taken in more than 26,100 reports since May 1 [1]. Subtract the outbreak total from the lab-confirmed total and roughly 7,000 are left over [2]. Because the two figures are counted differently, that difference is an approximate size and not a case count. Thousands of cases in this year's surge sit outside the lettuce cluster entirely [9].
For scale, the worst previous year for this illness in the United States was 2019, with about 4,700 illnesses reported nationally [10]. The lab-confirmed total since May 1 of this year is about four times that full-year figure [3]. The 16-fold comparison federal officials drew puts last year's same-period total below roughly 1,250 [4].
What was declared over is narrower than the surge. FDA officials said they are confident that all recalled iceberg lettuce related to this specific outbreak is off the market [2]. Where the contamination came from is still unknown [4]. The parasite spreads through human feces, and it is not clear how sewage could have contaminated enough food to sicken so many people [5].
"This contamination can happen if produce is grown in soil or comes into contact with water that has been contaminated," said Ellen Shumaker, director of outreach for Safe Plates, a food safety program at North Carolina State University [6]. Shumaker was describing the general route, not a field, a water source or a step in this supply chain.
Investigations of this parasite often end here. Cyclosporiasis is less common than foodborne illness caused by salmonella or E. coli, and many cases are never linked to a specific food or other source [12]. The recalled lettuce came out of central Mexico [13], and the Associated Press reports that U.S. regulators are conducting fewer international inspections to catch contaminated produce before it lands on American plates [17].
What to watch
- Results of the FDA samples collected at the Mexican growers and processing facility. The results may name a contamination point or rule one out.
- Whether the thousands of cases outside the lettuce cluster are ever tied to a source, or keep accumulating through the season.
- Whether the FDA's count of international produce inspections changes after this outbreak.