This post is part of the Meet a Microbe series on the blog, which serves as a primer on the many members of the microbial world. Check it out to meet other microbes!
Cyclospora cayetanensis is making headlines lately for an outbreak in the U.S. “Explosive diarrhea” is the language that many are using to describe the manifestations of this parasitic infection. The parasite is believed to come from ingesting contaminated fruits, vegetables, or water. Cyclosporiasis, which is the name for the illness, rarely takes hold in the U.S. to this magnitude so the outbreak this year is quite unusual. So after sauteing my salad greens up to a wilty little blob for lunch, I took to my blog to write about this unpleasant parasite.
How was Cyclospora cayetanensis discovered?
Cyclospora cayetanensis infections were first reported in 1977 from patients in New Guinea. However, at the time, the infections were thought to be caused by a different parasite. In the next decade, doctors noticed unexplained diarrheal illnesses in people after travel, but it wasn’t quite clear what the cause was. It wasn’t until the mid 1980s and the 1990s when scientists began to understand its waterborne and foodborne transmission routes. The parasite got its name in 1993 and 1994. Since then there have been several outbreaks in the U.S. linked to contaminated produce – particularly raspberries, basil, cilantro, snow peas, and mesclun lettuce.
Where does Cyclospora cayetanensis exist?
Cyclospora cayetanensis is most common in warmer climates such as tropical and subtropical areas. Scientists have proposed that climate change could broaden the distribution of the parasite and cause it to survive over the winter.
Why the source of cyclosporiasis is so hard to track down
Cases from this year’s outbreak began cropping up in May and still, there hasn’t been a single source of illness identified. This is for several reasons. First, the time between when someone ingests Cyclospora cayetanensis and when they first show signs of disease is long – up to two weeks. This makes it hard for patients to remember what they ate or where they ate leading up to symptoms. Secondly, many people with cyclosporiasis may not seek treatment as it can often be managed at home. Symptoms start similarly to those of other gastrointestinal infections like norovirus or food poisoning with Escherichia coli or Salmonella, which people typically ride out at home without seeing a doctor. On the diagnosis front, because cyclosporiasis has been quite rare in the U.S., it isn’t included in routine tests and doctors need to specifically order it. Because of the overlap in symptoms, it makes it more difficult for public health officials to know how big the outbreak is and where it is occurring to pinpoint a source.
Last year, the CDC’s Foodborne Diseases Active Surveillance Network (FoodNet) made tracking many foodborne pathogens optional. Cyclospora tracking was one of the pathogens cut among other bacteria and only E. coli and Salmonella now require tracking.
The parasitic lifestyle of Cyclospora cayetanensis
Cyclospora spends part of its life cycle inside the human small intestine and another part in the environment. The lifecycle of Cyclospora depends on human infection. Those with Cyclospora infections first excreted unsporulated oocysts in their stool. The oocysts are noninfectious and are in an environmentally resistant stage of the cell with thick walls to protect the parasite. After one to two weeks, the parasite sporulates in the right conditions (ex: 23 to 27°C). These sporulated cells are infectious and when ingested by food or water, they reach the intestines of the human. There, the infectious cells exit the cyst and can infect the cells of the intestinal lining. There it multiples and wrecks havoc on our intestinal barrier causing inflammation, malabsorption, chronic diarrhea, and other unpleasant symptoms.
Fun fact: Cyclospora can’t be spread by direct person-to-person contact
Because Cyclospora needs time to sporulate in the environment before they can become infectious, it can’t be spread directly from person to person (ex: fecal-oral route).
How long will the current Cyclospora outbreak last?
Because of the long incubation period of the parasite before an individual shows symptoms, new diagnoses can keep appearing even if the source of exposure has been identified. How long the current outbreak lasts comes down to identifying the source of the parasite so it could take weeks to months more.
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Further reading
Cyclosporiasis. Cleveland Clinic. 2026.
- Read for an overview of symptoms, diagnosis, treatment, etc.
Surveillance of Cyclosporiasis. CDC. 2026.
- Read for updates on the current 2026 outbreak. Case reporting does seem to lag behind though compared to state reporting.
Update on Cyclospora cayetanensis, a Food-Borne and Waterborne Parasite. Clinical Microbiology Reviews. 2010.
- Read for an overview of the history of Cyclospora, its biology, clinical presentation, diagnosis, and epidemiology
Featured image: Cyclospora oocysts from a stool sample. Source: CDC/ DPDx – Melanie Moser