The United States just came through an unprecedented Cyclospora season (2, 7). Nearly 20,000 people were sickened, with the biggest cluster traced to recalled iceberg lettuce (2, 3). Which means somewhere a kid who refused to eat his greens is feeling extremely vindicated.

Cyclospora is not the pathogen most people picture when they think of summer stomach trouble. That is usually a bacterium like Salmonella or E. coli, or a virus like norovirus. Among parasites, Cryptosporidium, the leading cause of waterborne disease in the US and a familiar culprit in pool outbreaks, is the more common name (5). Which makes this season the exception: the rarer of the two parasites is the one that exploded, and the one the news zeroed in on this year.

Since May 1, the CDC has counted 19,883 laboratory-confirmed cases acquired domestically, across 49 states and the District of Columbia, with 1,064 hospitalizations and two deaths, up from just 1,180 cases in the same period a year earlier (2). The largest single cluster was an outbreak traced to recalled iceberg lettuce sourced from central Mexico, which accounted for 12,883 of those cases across 21 states (3). The CDC declared that outbreak over on September 11, and the FDA closed its investigation on October 2 without conclusively determining how the contamination occurred (3). With recent cases dwindling, the CDC moved off weekly surveillance updates on September 15, shifting to periodic reporting (2).

Note: Death from cyclosporiasis is rare; both fatalities were in Michigan, in people with significant underlying conditions (3, 4).

The illness

Cyclospora cayetanensis is a single-celled parasite endemic to tropical and subtropical regions (1). It colonizes the small intestine and causes watery, sometimes frequent diarrhea that can subside and return, along with nausea, fatigue and loss of appetite (1). Symptoms typically begin about a week after infection, though the range runs from two days to two weeks or more (1). Untreated cases can drag on for weeks (1). In otherwise healthy people the infection can clear on its own (8), and a prescription antibiotic works for those who need it (1). Immunocompromised patients face a higher risk of serious illness (8).

This can be easy to mistake for an ordinary stomach bug. Easy to miss in testing, too.

How it spreads

Cyclospora spreads through food or water contaminated with human feces, and is unlikely to spread directly from person to person (1, 7). Humans are thought to be the only definitive host, meaning the only organism in which the parasite can complete its sexual reproduction cycle (8). The reason it moves so poorly from person to person lies in its life cycle: the oocyst is not infectious when it is shed and must spend at least a week or two maturing in the environment before it can infect a new host (1). That delay is why fresh produce, often imported from regions where the parasite is endemic, tends to drive US outbreaks (1, 8).

This year’s outbreak fit that pattern. On July 17, a supplier recalled all the iceberg lettuce it had sourced from central Mexico, covering both retail and food-service products in dozens of states (3).

Note: The CDC also tracked other, separate clusters not tied to the lettuce (2). FDA found Cyclospora in wastewater at the processing facility and in a drainage ditch at one grower, but could not conclusively determine how the lettuce became contaminated (3).

Finding it and tracing it

The reported case total belies the true scale, and detection is a large part of why. The parasite leaves the body as an oocyst, a microscopic hard-shelled capsule shed in low numbers. Routine stool examinations often miss it because most US labs do not test for Cyclospora unless it is specifically requested, and patients may not shed enough oocysts to detect (1, 7). Labs can spot them with specialized stains or under ultraviolet light, where the oocysts naturally glow, but only if someone orders those tests (6). Commercial multiplex PCR panels that screen for a range of gut pathogens now include both Cyclospora and look-alikes such as Cryptosporidium (6, 9). But molecular testing is not available everywhere, and a clinician who does not suspect the parasite may never order the right test (1, 7). On top of that, it takes about six weeks from when symptoms start until a case reaches the CDC, and the true number of illnesses is likely higher than any count (2).

Tracing outbreaks is harder still. Pathogens such as Salmonella can be whole-genome sequenced and tied to a single source with high precision. Cyclospora genotyping is less established: weeks after this outbreak ended, FDA was still working out how its environmental results related to patient samples (3). The parasite also cannot be grown in a lab dish the way bacteria can, and there is no animal model to study it outside of human patients, which leaves researchers with fewer tools to work with (8). Both jobs, detecting the parasite and tracing it, begin at the same step: extracting its nucleic acid from stool, a complex sample whose preservatives and inhibitors can compromise a reaction (9, 10). Cryptosporidium poses the same challenge at the lab bench, which is precisely why telling the two apart matters.

Cousins, not twins

Cyclospora and Cryptosporidium look nearly identical from the outside. Both are single-celled gut parasites, both shed oocysts, both cause the same watery illness and both slip past the same routine tests. But getting the diagnosis wrong has real consequences: the two are treated with different drugs and spread in different ways, since Cryptosporidium passes readily from person to person and from animals while Cyclospora does not (1, 5). One is mostly a produce problem, the other mostly a water problem. One is treated with an antibiotic; the other usually clears on its own and calls for a different drug when it doesn’t (1, 5).

The most reliable way to tell them apart is at the molecular level, which is also where the science gets interesting. How their genomes differ, what those differences mean for detection and treatment, and why the tools to trace one outbreak have outpaced the tools to trace the other: those are the questions this series takes on next.

Note: national season figures reflect CDC surveillance updated September 15, 2026 (data as of September 14); outbreak figures reflect the CDC’s September 11 final update and FDA’s October 2 update.

Updated October 6th, 2026, to reflect FDA’s October 2 investigation findings and clarify testing details.

References

1. CDC. Clinical Overview of Cyclosporiasis (symptoms, life cycle, transmission, treatment, testing). Updated September 18, 2026. https://www.cdc.gov/cyclosporiasis/hcp/clinical-overview/
2. CDC. Cyclospora Case Data — 19,883 laboratory-confirmed U.S. cases, 1,064 hospitalizations, 2 deaths, across 49 states and DC, versus 1,180 in 2025; CDC describes the season’s case count as unprecedented. Updated September 15, 2026 (data as of September 14). https://www.cdc.gov/cyclosporiasis/php/surveillance/index.html
3. CDC and FDA. Cyclospora Outbreak Linked to Iceberg Lettuce (Taylor Farms de Mexico, central Mexico) — 12,883 cases across 21 states; outbreak declared over September 11, 2026; FDA investigation closed October 2, 2026. https://www.cdc.gov/cyclosporiasis/outbreaks/07-26/index.html and https://www.fda.gov/food/outbreaks-foodborne-illness/investigation-multistate-outbreak-cyclospora-illnesses-iceberg-lettuce-july-2026
4. Michigan Department of Health and Human Services. Infectious Disease Outbreaks, cyclosporiasis (Michigan case data and the two deaths). https://www.michigan.gov/mdhhs/keep-mi-healthy/infectious-diseases/infectious-disease-outbreaks
5. CDC. About Cryptosporidium Infections; transmission, treatment, recreational water. https://www.cdc.gov/cryptosporidium/about/index.html and https://www.cdc.gov/cryptosporidium/causes/index.html and https://www.cdc.gov/cryptosporidium/treatment/index.html
6. CDC DPDx. Cyclosporiasis (laboratory diagnosis, staining, UV autofluorescence, molecular panels). https://www.cdc.gov/dpdx/cyclosporiasis/index.html
7. CDC. Health Alert Network notice on the 2026 increase in domestically acquired cyclosporiasis — describes the surge as substantially above prior years. https://www.cdc.gov/han/php/notices/han00531.html
8. Michigan Medicine Health Lab. The Hidden Life of Parasites Like Cyclospora (expert quotes on host specificity, no lab culture system, no animal model). https://www.michiganmedicine.org/health-lab/hidden-life-parasites-cyclospora
9. CDC DPDx. Cryptosporidiosis. https://www.cdc.gov/dpdx/cryptosporidiosis/index.html
10. Ortega Y.R., Sanchez R. Update on Cyclospora cayetanensis, a food-borne and waterborne parasite. Clinical Microbiology Reviews 23(1), 218–234 (2010). https://pmc.ncbi.nlm.nih.gov/articles/PMC2806662/

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Shannon Sindermann

Shannon earned her B.S. in Molecular and Cellular Biology with double minors in Chemistry and Psychology, as well as a Technical Writing Certificate from the University of Wisconsin–La Crosse. As part of the Marketing Team, she enjoys researching scientific advances and helping make complex topics accessible to broader audiences. Outside of work, she can be found on the trails snowmobiling or kayaking across the lake—depending on the season.

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