Cyclospora Cases Are Rising Across the US—Why Health Officials Are Struggling to Trace the Source

More than 1,200 Cyclospora infections have now been reported in Michigan alone as health officials investigate outbreaks across several US states. Yet despite hundreds of patient interviews and ongoing food safety investigations, authorities have not identified a single source linking every case.

Experts say the delay is not unusual. Unlike bacteria such as Salmonella or Escherichia coli (E. coli), Cyclospora is a microscopic parasite whose biology makes outbreaks significantly harder to trace. Investigators often have to reconstruct meals eaten weeks earlier while examining multiple food supply chains before identifying a likely source.

The US Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) are investigating several clusters of cyclosporiasis across states including Michigan, Ohio, Pennsylvania, Texas, Illinois, and New York. While some investigations have pointed to fresh produce served at restaurants, catered events, and grocery retailers, officials have not concluded that all of the reported cases originate from a single nationwide outbreak.

Why Cyclospora Is Harder to Trace Than Other Foodborne Illnesses

Unlike many foodborne bacteria, Cyclospora cannot be easily linked to contaminated food through standard genetic sequencing. Scientists say the parasite reproduces sexually, creating greater genetic variation that makes matching infections to a common source considerably more difficult.

Timing also complicates investigations. Symptoms usually appear about a week after exposure and can take even longer in some cases. By then, many patients struggle to remember exactly what they ate, forcing epidemiologists to rely on detailed interviews, grocery receipts, and purchase histories to piece together possible exposures.

Modern food supply chains add another layer of complexity. Fresh herbs, leafy vegetables, and other produce may pass through several suppliers before reaching consumers, making traceback investigations both time-consuming and labour-intensive.

Investigators Are Examining Multiple Food Sources

Federal investigators are tracing several ingredients that have been linked to individual clusters, including cucumbers, cilantro, white onions, and green onions. Other investigations have focused on Mexican-style restaurants, catered events, and grocery retailers where infected people reported eating before becoming ill.

So far, however, authorities have stressed that no single food item has been identified as the definitive source of the broader rise in cases.

Michigan Has Become the Largest Known Outbreak

Michigan has recorded the largest concentration of cases, reporting more than 1,200 infections since late June compared with roughly 50 cases during a typical year. Dozens of patients have required hospital treatment.

Ohio has also reported an unusually high number of infections, while additional cases continue to be investigated in several other states. Public health officials say the investigations remain active and additional information may emerge as more patient interviews and laboratory testing are completed.

What People Should Know About Cyclospora

Cyclospora causes an intestinal infection known as cyclosporiasis. Common symptoms include prolonged watery diarrhoea, stomach cramps, bloating, nausea, fatigue, and loss of appetite. Unlike many common stomach bugs, untreated infections can persist for several weeks and symptoms may disappear before returning again.

Diagnosis can also prove challenging because routine stool tests do not normally screen for Cyclospora. Doctors often need to request specialised laboratory testing to confirm the infection before prescribing antibiotics, which are generally effective once the parasite has been identified.

Health officials advise anyone experiencing persistent diarrhoea, particularly in areas affected by recent outbreaks, to seek medical advice.

Why the Investigation Could Take Weeks

Public health experts caution that Cyclospora outbreaks rarely produce quick answers. Identifying the source requires extensive interviews, laboratory analysis, and traceback investigations across complex food distribution networks.

While investigators continue searching for a common source, experts say the parasite's unique biology means these outbreaks often take far longer to solve than illnesses caused by bacteria such as Salmonella or E. coli. Until more evidence becomes available, health agencies will continue monitoring new cases while working to determine whether the recent infections are connected.