Health officials in the United States are working to identify the source of the largest recorded cyclospora outbreak in the country’s history, which has sickened thousands, particularly in Michigan where more than 6,000 cases have been reported. Cyclospora is a microscopic parasite that spreads exclusively through human fecal contamination, and understanding how it entered the food supply remains a critical challenge for investigators.

Cyclospora infects the small intestine, causing watery diarrhea that releases parasite eggs known as oocysts in human waste. These oocysts can contaminate fresh produce if hygiene practices are inadequate among farmworkers or if irrigation and washing water become tainted. Standard water disinfection methods, including chlorination, usually target bacteria rather than parasites, allowing cyclospora to survive in water that otherwise appears clean by conventional tests.

Experts emphasize that contamination frequently results from poor sanitation or contaminated water sources at farms. John Guzewich, who formerly worked on foodborne disease outbreaks at the Food and Drug Administration (FDA), notes that previous outbreaks have often been linked to contaminated irrigation water or unwashed hands during harvest. Outbreaks historically involved produce typically eaten raw, such as lettuce, salad mixes, raspberries, cilantro, and snap peas.

Researchers highlight that cyclospora oocysts are particularly “sticky” and tend to adhere to certain produce with fine hairs or textured surfaces, such as raspberries and herbs, making home rinsing less effective at removing them. This characteristic complicates prevention efforts since even careful washing at home may not eliminate all parasites.

Past outbreaks have offered insight into typical contamination routes. A notable outbreak in 1996 was traced to raspberries from Guatemalan farms where insecticides were mixed with water drawn from wells and nearby rivers contaminated by latrine runoff. Similarly, a 2020 outbreak, which sickened 701 people across 14 states, was linked to Fresh Express bagged salads sourcing produce from a South Florida farm. Cyclospora was detected in irrigation canals near the farm, and genetic tests confirmed the strain matched that found in affected individuals.

Because the parasite only grows in the human gut, large-scale outbreaks suggest multiple sources of contamination rather than a single infected picker. Kalmia Kniel, a microbiologist studying cyclospora, said it’s unlikely that one person’s improper hygiene alone could have caused widespread illness. Instead, multiple infected farmworkers could have contaminated water through leaking septic systems or runoff during heavy rains, leading to widescale spread via irrigation or wash water, or through contaminated harvesting equipment.

Food safety experts stress the need for extensive environmental sampling to identify the outbreak’s origin. Frank Yiannas, former FDA official, said investigators will have to conduct thorough testing of water, fields, and facilities. Given the complexity and resources required for such investigations, experts caution against expecting quick resolutions. “I would love if it happened that way,” Guzewich said, “but outbreak investigations take time, expertise, and effort.”

As the outbreak continues, officials are focused on pinpointing how human fecal contamination infiltrated the produce supply chain to prevent future incidents and protect consumers.