A surge in cyclosporiasis cases across multiple U.S. states this summer is raising alarms at federal health agencies as investigators scramble to identify common sources of infection.
The parasite Cyclospora cayetanensis, which causes severe gastrointestinal illness, has appeared in unexpectedly high numbers in at least 13 states, with Michigan reporting the most dramatic spike. As of late June, Michigan had documented 681 cases of cyclosporiasis — more than 13 times its typical annual average of 50 cases.
Ohio reported 177 cases for the year, with 171 occurring in June alone, predominantly after June 20. New York State outside New York City recorded 112 cases for 2024, with 107 of those arriving since May 1. These numbers represent a marked departure from historical seasonal patterns, suggesting multiple simultaneous outbreaks rather than a single contaminated food source.
The CDC's investigation has identified at least eight distinct clusters spanning Alabama, Kentucky, Michigan, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia. Earlier investigations uncovered separate case clusters in Illinois, New York City and Pennsylvania linked to Mexican-style restaurants, a grocery chain and catered events.
The FDA's Coordinated Outbreak Response and Evaluation network is conducting traceback investigations on white and green onions, cucumbers and cilantro as potential vectors, though federal officials stress there is no evidence yet that these clusters represent a unified multi-state outbreak.
Unlike bacterial pathogens such as E. coli or salmonella, cyclospora parasites do not lend themselves to the rapid genetic matching techniques that have become standard in foodborne illness investigations. The PulseNet system, which sequences bacterial DNA across a national surveillance network to identify outbreaks in near-real time, cannot be effectively applied to cyclospora because of fundamental biological differences.
"Bacterial pathogens maintain genetic consistency across transmission, while cyclospora parasites undergo sexual reproduction and exchange genetic material between generations, creating significant variation in their genomes," said Dr. Jennifer McEntire, a microbiologist and founder of Food Safety Strategies.
The CDC has developed methods to categorize parasites into rough "strains," but experts acknowledge that cyclospora genotyping lacks the precision available for investigating other foodborne pathogens.
"Cyclospora is a weird one," said Dr. Max Teplitski, former head of Food Safety at the U.S. Department of Agriculture and now chief science officer for the International Fresh Produce Association.
In the absence of reliable genetic tracking, outbreak investigations depend heavily on epidemiological legwork. Investigators conduct detailed interviews with infected individuals, asking about food consumption patterns from days or weeks prior to symptom onset. Because human memory proves unreliable for dietary recall, investigators cross-reference grocery receipts and credit card transaction data to reconstruct exposure histories.
Teplitski has urged investigators to expand their inquiries beyond food sources to include water exposures — recreational swimming, water parks and wells. Cyclospora parasites contaminate multiple environmental reservoirs.
Cyclosporiasis typically emerges during warm months but historically occurs in predictable seasonal patterns. The infection causes weeks of debilitating diarrhea, abdominal cramps and bloating, making rapid outbreak identification and containment a public health priority.
The CDC plans to publish updates to its cyclosporiasis tracking website this week as investigations progress. State health departments continue to interview additional patients and conduct traceback analyses on implicated food items and establishments.
