Cyclosporiasis: What’s Behind This Summer’s Lettuce-Linked Outbreak (And Why the Culprit Keeps Slipping Away)

Summer in the US usually means sunshine and beach trips. This year, for thousands of people across at least 34 states, it’s also meant abdominal cramping, nausea, and explosive diarrhea, the result of ingesting a microscopic parasite called Cyclospora. Nearly a hundred people have been hospitalized since the outbreak began in May, and while no deaths have been reported, health officials are still working to pin down exactly where the contamination came from.

A parasite that hijacks its host from the inside

Cyclospora belongs to the same broad group of parasites as malaria. According to parasitologist Malcolm Jones of the University of Queensland, these organisms can only grow and reproduce inside a host cell, effectively hijacking its internal machinery to complete their life cycle. The species behind this outbreak, Cyclospora cayetanensis, is a major driver of gastrointestinal illness worldwide.

Why this outbreak is so hard to trace

C. cayetanensis has an unusual life cycle that makes it especially tricky to track. It doesn’t spread directly from person to person, and it doesn’t need an intermediate animal host either. Instead, an immature form of the parasite is shed in human feces and takes at least a week in the environment to mature into a hardy, infectious form called an oocyst, which can then persist for weeks. If contaminated water or improperly treated fecal material comes into contact with crops, oocysts can end up on produce. As few as ten oocysts are enough to cause disease.

Once swallowed, the parasites break free from their cyst walls in the gut, invade the lining of the small intestine, and go through rounds of asexual reproduction before switching to sexual reproduction and forming new cysts that pass back out in feces to start the cycle again. Symptoms can take up to two weeks to appear after eating contaminated food, and in untreated cases they can resolve and then return in a relapsing pattern over a month or longer, all of which makes it genuinely difficult for investigators to work backward to the source.

Lettuce, a recall, and then a walk-back

Using epidemiological data and genotyping of Cyclospora from stool samples, CDC investigators identified clusters of related cases and linked the outbreak to shredded iceberg lettuce sourced from Mexico, sold at major chains including Taco Bell. The FDA recalled the implicated lettuce products, and officials advised anyone with recalled lettuce at home to discard it and disinfect any surfaces it may have touched.

Then came a twist: on re-review, the FDA announced the lettuce results were a false positive, leaving the true source of the outbreak unidentified for now.

Staying safe in the meantime

For anyone concerned about fresh produce in general, Jones recommends thoroughly washing fruits, vegetables, and leafy greens in clean water, then cooking or microwaving them, since heating food above 70°C (158°F) kills the oocysts. He also points to surveillance as the longer-term fix: catching outbreaks early lets health authorities trace the source and pull the specific product off shelves before it spreads further.

Original paper: Hadjilouka A, Tsaltas D. Cyclospora Cayetanensis — Major outbreaks from ready-to-eat fresh fruits and vegetables. Foods. 2020;9(11):1703. 

Source: Cyclosporiasis: What’s Behind This Summer’s Lettuce-Linked Outbreak (And Why the Culprit Keeps Slipping Away)