Cyclosporiasis: What to Know About the Current Outbreak 

Summer in the US means sunshine, warm temperatures, and trips to the beach. Unfortunately, though, many could not enjoy their time this summer due to abdominal cramping, nausea, and explosive diarrhea—the result of ingesting microscopic Cyclospora parasites.

Spread via fresh produce that has been contaminated with wastewater, the parasites cause a severe gastrointestinal disease called cyclosporiasis; cases of the illness have been reported in thousands of individuals across at least 34 states since the beginning of May. While no deaths have been reported, nearly a hundred people have been hospitalized due to cyclosporiasis.

With health officials now homing in on sources of the illness and recalling lettuce products in multiple states, parasitologists can offer some clarity about the current foodborne outbreak.

What Kind of Parasite Is Cyclospora?

Cyclospora parasites are protists belonging to the subphylum Apicomplexa—the same group that includes malaria.¹ “These are parasites that are obligate intracellular parasites, [which means] they live within cells and can only grow and reproduce within a cell,” said Malcolm Jones, a parasitologist at the University of Queensland. “They hijack the internal machinery of that cell to achieve their reproductive goals.”

Jones, who is also group leader at the Queensland Institute of Medical Research Berghofer’s molecular parasitology laboratory, said the Cyclospora genus contains several different species that are specific to different hosts. The species responsible for human outbreaks is Cyclospora cayetanensis, a significant contributor to the global spread of gastroenteritis.¹

How Does Cyclospora Infection Spread?

Unlike other parasites, C. cayetanensis does not have an intermediate host, nor is it transferred directly between humans—the result of a quirk in its life cycle. The immature form of the parasite is shed from the human gastrointestinal tract and takes at least one week to mature into specialized cysts called oocysts; these hardy oocysts can persist in the environment for weeks and are the infectious agent of the disease.

If farmers use improperly treated human fecal material to fertilize crops or if contaminated water irrigates fields due to flooding or human error, humans can ingest the oocysts that land on the food. As few as ten oocysts can cause disease. “The cyst walls are adapted to be broken down by host digestive conditions, enzymes, and so forth, and that liberates two parasites from each cyst,” Jones explained. “These parasites then will invade cells generally of the middle region of the small intestine, and they will penetrate the lining cells of the small intestine and then undergo a whole round of asexual reproductive phases.”

After heavily infecting the lining of the intestines, the parasites switch back to sexual reproduction. “The zygote will start to form a little cyst around the outside of it, which will get liberated from the host cell as it eventually degrades, and it'll pass out with feces,” Jones added. The disease is often treated with antibiotics but tends to run its course over a couple of days to two weeks in immunocompetent people. However, if left untreated symptoms can also return after having seemed to resolve, following a remitting-relapsing pattern over a month or longer, according to the CDC. The onset of symptoms begins up to two weeks after ingesting contaminated produce, adding to the complexity of identifying the source of the outbreak.

Tracing the Culprit Behind the Widespread Explosive Diarrhea

Using epidemiological data and targeted genotyping of Cyclospora from stool samples of infected individuals, scientists at the Centers for Disease Control (CDC) identified clusters of related cases, CDC representatives said in a press conference last week. They subsequently linked the outbreak to shredded iceberg lettuce sourced from Mexico and sold at major franchises such as Taco Bell, which led to the US FDA recalling the products.

In the wake of these discoveries, the CDC urged anyone who has purchased lettuce products on the recall list to throw them away or return them to stores and thoroughly wash or disinfect any surfaces in their homes that have come into contact with the recalled lettuce. For anyone who is still concerned or has doubts about other fresh produce, Jones said the best bet is to thoroughly wash all fruits, vegetables, and leafy greens in clean water and then cook or microwave them. “Cooking at temperatures above 70 degrees Celsius [158 degrees Fahrenheit] will kill the oocysts on the food,” he said.

However, upon re-reviewing the samples yesterday, the FDA announced that the results were a false positive, suggesting that the exact source of the pathogen still remains unknown.

Even as experts continue to hunt the culprit behind this outbreak, Jones remarked that increased surveillance will be key in addressing and minimizing such occurrences in the future. “If the authorities are vigilant, they [can] detect this disease early and do their tracing and find the source of disease,” he said. “Then [they can identify] a particular line of product in the supermarkets and pull them off the shelves.”