A Public Health Alerts report today highlights more than 6,000 cases of Cyclospora infection tied to iceberg lettuce detected in Michigan through July 22. It reflects the initial weeks of the largest known US outbreak of cyclosporiasis (a diarrheal illness caused by infection with the Cyclospora parasite).

Public Health Alerts, a collaboration between NEJM Evidence and CIDRAP, fills a gap in reliable data, offering expert-reviewed reports that translate frontline observations into actionable public health evidence. An NEJM Evidence editorial explains the initiative further. Read all Public Health Alerts reports here. 

4% hospitalization rate

In the report, scientists from the Michigan Department of Health and Human Services (MDHHS), Michigan Department of Agriculture and Rural Development, the Center for Disease Control and Prevention (CDC), and the University of Michigan described how the outbreak was first detected in June.

On June 25, MDHHS notified the CDC of an increase in cyclosporiasis cases in Michigan. The agency alerted the public on July 1. A July 4 MDHHS alert recommended specific food safety guidance for restaurants and the public, and on July 13 the agency notified the public of possible lettuce contamination and recommended additional precautions. 

The CDC issued a Health Alert Network advisory on July 14, but it did not mention lettuce. Within a week, the Food and Drug Administration linked the outbreak to Taylor Farms iceberg lettuce.

The report includes Cyclospora cases reported through the state’s disease surveillance system but excludes illnesses that began within 14 days after international travel. From June 22 to July 22, 6,065 cases had been reported to the MDHHS from 69 of Michigan’s 83 counties. 

Of the 3,114 patients who were interviewed, their median age was 42 years, 54.4% were female, and 4.4% reported being hospitalized. Illness-onset dates ranged from May 10 through July 18, with distinct peaks from June 21 through 27 and July 5 through 11. Of those interviewed, 69.6% reported eating food at a restaurant chain the report calls “Chain A.”

The authors conclude, “Findings in this ongoing investigation suggest that iceberg lettuce at restaurant Chain A was a strong epidemiologic indicator, although the possibility of additional exposure pathways cannot be excluded. For example, consumption rates of iceberg lettuce among patients who did not recall eating at restaurant Chain A were higher than comparable background estimates, and a notable proportion of patients did not recall either Chain A or lettuce consumption.

They add, “Cyclospora contamination typically occurs during farming or processing. After contamination, products might be distributed to multiple destinations. In Michigan, this outbreak’s bimodal distribution of illness-onset dates suggests two distinct batches of contaminated produce or a single contaminated lot with staggered distribution.”

As of August 27, the MDHHS has confirmed 14,510 cyclosporiasis cases, including 356 hospitalizations and two deaths. Not all are linked to contaminated lettuce.