Cyclosporiasis is a parasitic illness caused by eating or drinking contaminated food or water.
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This summer, the digestive health of thousands of Americans has been severely affected by tiny Cyclospora parasites.
This marks the largest outbreak of the parasite ever recorded in the U.S. According to a July 28 update from the Centers for Disease Control and Prevention, 45 states have reported cases of cyclosporiasis, the illness caused by the parasite.
Michigan has emerged as the epicenter of this outbreak, with over 10,000 individuals consuming potentially contaminated food, possibly lettuce, carrying Cyclospora parasites.
The parasites depend solely on human digestive systems to reproduce, likely traveling from regions where they are more prevalent to unsuspecting consumers across the U.S. Their intricate biological processes contribute to the difficulty in eradicating them, causing infections that can persist for weeks or even months.
A lifecycle that requires the human gut
On herbs, vegetables, or berries, Cyclospora parasites exist in dormant pods called oocysts, which are almost invisible to the human eye. “It would take roughly a hundred of them, linked end to end, to stretch across the head of a pin,” explains Vernon Carruthers, a professor at the University of Michigan Medical School.
Once ingested, these oocysts progress through the esophagus, stomach, and into the intestines. “The oocysts react to gut signals, such as body temperature and digestive enzymes, prompting the parasites to be released,” Carruthers states.
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The parasites target the small intestine lining, infiltrating cells to replicate. “They proliferate from a few ingested parasites to millions replicating inside our intestinal cells,” Carruthers notes.
Eventually, some parasites evolve into egg- and sperm-like forms that merge to create new microscopic oocysts—a process repeated millions of times during peak infection. “It’s truly remarkable, given that sperm must find the eggs inside infected cells shielded by a mucus layer,” Carruthers adds.
This entire process disrupts the intestinal lining. “[The parasite is] invading host cells, replicating within them, and eventually destroying these cells during its life cycle,” explains Joel Barratt, a molecular parasitologist at Emory University School of Medicine, who previously led a CDC lab team addressing parasitic outbreaks.
Hardy pods in human excrement
As the parasites harm the small intestine’s lining, they hinder nutrient absorption and cause extensive damage, leading to classic cyclosporiasis symptoms like nausea and diarrhea, Barratt says.
A single milliliter of infected diarrhea may contain around a thousand parasitic pods, Carruthers mentions.
To date, scientists believe Cyclospora only infects people when they consume food or water tainted with human waste traces.
However, fresh waste isn’t infectious. “When oocysts are freshly expelled, they are not infectious,” Barratt explains. “They need time in the environment to mature.”
The maturation period for infectiousness is uncertain. It might take days or weeks and could be seasonally influenced.
The parasite can enter the food supply through contaminated soil or irrigation water, or through unclean hands handling produce. It might also spread during the industrial washing process, where multiple produce items are rinsed in the same water.
The Cyclospora pods are resilient, remaining viable in water or on crops, and can withstand chlorine, sunlight, and cold temperatures.
Infections can linger
Within the body, Cyclospora infections may last for weeks or months. “It’s far more complex than a typical viral stomach bug, which often resolves in a day or two,” states Dr. Bobbi Pritt, a microbiologist at Mayo Clinic and director of its Clinical Parasitology Laboratory.
A combination of specific antibiotics—sold as Bactrim, Septra, or Cotrim—can help by halting parasite replication, allowing the immune system a better chance to eliminate them, Carruthers explains.
Cyclosporiasis isn’t typically fatal, and individuals in regions where the parasite is more common, such as tropical and subtropical areas, seem to build resistance.
In areas where Cyclospora is endemic, research shows that younger individuals are more likely to exhibit symptoms, while older individuals are less affected. “This suggests that immunity can be developed, but it may take several exposures,” Carruthers says.
It’s a “mystery” why this is a big outbreak year
In recent years, Cyclospora has caused increasing issues in the U.S.
According to Barratt, cyclosporiasis outbreaks in the U.S. have been “fluctuating somewhat, with significant outbreaks in 2018 and 2019, but this year is unprecedented.”
A significant factor in U.S. cyclosporiasis outbreaks is the imported food supply. “We import produce and food products from around the world, many from regions lacking proper sanitation,” Pritt explains. The human waste containing Cyclospora “enters groundwater or contaminates the plants being grown.”
Nonetheless, the widespread nature of cyclosporiasis cases in the U.S. this year remains a mystery. “It might just be bad luck,” Pritt suggests, “with a large parasite load introduced into widely consumed products,” affecting major chain restaurants like Taco Bell, whose shredded lettuce from Taylor Farms in central Mexico has been linked to illnesses in a nine-state cluster. Taylor Farms voluntarily recalled the product on July 17.
Public health officials are still investigating additional sources. In North Carolina’s outbreak, parsley and cilantro are potential culprits, according to the state health department.
Meanwhile, health authorities advise washing and cooking fresh vegetables to reduce the risk of a severe gut infection.
