The United States is in the midst of the largest outbreak of cyclosporiasis ever recorded. At least 1,645 laboratory-confirmed cases of Cyclospora cayetanensis infection have been reported across 34 states since May 2026, with more than 5,100 additional cases under investigation, bringing the potential total above 6,700. 141 people (9% of confirmed cases) have been hospitalized; no deaths have been reported.
The numbers represent a dramatic escalation over previous seasons. By the same date in 2025, only 249 cases had been reported nationally, a roughly 6.6-fold difference.
“The outbreaks seem to be getting larger year by year,” said Joel Barratt, a molecular parasitologist at Emory School of Medicine who formerly led the CDC’s parasite surveillance team.
Michigan at the epicenter
Michigan has been hit hardest, with approximately 1,251 confirmed cases, roughly 25 times the state’s annual average. Monroe County (215 cases), Detroit/Wayne County (160), and Ann Arbor/Washtenaw County (159) form the outbreak’s core. A concentrated multistate cluster of more than 400 cases has been identified across Michigan, Ohio, West Virginia, and Kentucky.
After more than 1,000 patient interviews, the Michigan Department of Health and Human Services has pointed to lettuce and salad greens as the likely vehicle. But no specific product, brand, grower, or supplier has been definitively identified, and the CDC has cautioned that “a specific food item has not yet been confirmed as the source.”
The investigation is using whole-genome sequencing of parasite isolates from patients to link cases to a common source, combined with traceback investigations through the food distribution chain. Both the CDC and FDA are coordinating with state health departments.
A parasite that thrives in warmth
Cyclospora cayetanensis is a microscopic, intracellular protozoan parasite that invades the epithelial cells of the small intestine. Infection occurs through ingestion of food or water contaminated with human feces containing sporulated Cyclospora oocysts. The disease is not transmitted person-to-person, the oocysts must mature (sporulate) in the environment for one to two weeks at temperatures between 22 and 32 degrees Celsius (72-90 degrees Fahrenheit) before becoming infectious.
Symptoms, profuse watery diarrhea, abdominal cramping, nausea, fatigue, and weight loss, typically appear about a week after ingestion. Illness can last weeks to months if untreated. The standard treatment is trimethoprim-sulfamethoxazole (Bactrim), which is FDA-approved for cyclosporiasis.
Diagnosis requires specific stool testing; Cyclospora is not detected by routine bacterial stool culture. The CDC has noted that cyclosporiasis is “often underdiagnosed and underreported,” meaning the true number of illnesses is likely higher than confirmed counts. A typical 6-week lag between illness onset and case reporting means the final tally will rise substantially as June and July cases are processed.
Climate change and range expansion
The outbreak’s scale has drawn attention to the role of climate change in expanding the parasite’s geographic reach. Historically considered a tropical pathogen in the United States, Cyclospora was primarily linked to imported produce. But outbreaks in 2018 and 2020, tied to domestically grown US produce, signaled a shift, and the current outbreak reinforces that trend.
“Warming temperatures are making conditions more favorable for the parasite to complete its life cycle in parts of the country where it could not previously survive,” said Barratt, citing a 2025 study in Sustainable Microbiology (Rees and Threlfall) that directly links climate change to the expanding range of Cyclospora.
Investigation challenges
The CDC’s ability to respond has been complicated by staffing constraints. Barratt noted that the agency’s parasite surveillance team has been weakened by recent government downsizing, which “encouraged or forced employee departures.” The CDC did not directly comment on staffing levels but stated that “multiple teams are engaged in the multistate investigation.”
Washing produce is not a reliable defense: Cyclospora oocysts adhere tightly to surfaces, and the FDA states that rinsing under water is unlikely to be fully effective.
The investigation also faces the possibility that what appears to be a single national outbreak may involve multiple separate contamination events. The CDC has stated there is “no confirmed evidence of a single multistate outbreak sharing one food source,” leaving open the possibility that the national picture reflects several independent outbreaks occurring simultaneously in a favorable climatic window.
Sources
Kozlov M. “‘Explosive diarrhea’ outbreak grips US: how researchers are hunting its source.” Nature News, July 16, 2026. https://www.nature.com/articles/d41586-026-02225-w
CDC Health Alert Network Advisory HAN-00531. July 14, 2026.
Rees & Threlfall. “Climate change and the expanding geographic range of Cyclospora cayetanensis.” Sustainable Microbiology 2, qvaf019 (2025).