The Mounting Threat of Cyclosporiasis, the “Explosive Diarrhea”

“Public health authorities in Michigan are investigating a rapidly growing outbreak of cyclosporiasis that has produced more than 170 reported cases across 7 counties over 9 days, far exceeding the state’s typical annual burden of approximately 50 cases.”

Department of Health and Human Services, July 3, 2026

“If 4 out of 5 people SUFFER from diarrhea…does that mean that 1 enjoys it?”

George Caarlin, Comedian

Hundreds of different pathogens—viruses, bacteria and parasites—can cause diarrhea (defined as 3 or more loose or watery stools per day) in humans. Cyclospora cayetanensis is one of these parasites, and cyclosporiasis is the disease it causes. Many Americans may not be familiar with cyclosporiasis, but given the rapid rise in cases in recent years that is likely to change. Cyclosporiasis is the focus of this week’s post.

What is cyclosporiasis? Cyclosporiasis is a gastrointestinal infection caused by the parasite Cyclospora cayetaneisis. Based on stool samples from patients in Papua New Guinea, in 1979 the British parasitologist R.W. Ashford was the first to recognize human cases of C. cayetanesis. In 1994, Ynes Ortega, Robert Gilman, and Charles Sterling definitively classified and formerly named the parasite C. cayetanensis. (The species name honors Cayetano Heredia University in Lima, Peru where the early research work was carried out.)

The parasite can cause severe diarrheal illness that is not usually life-threatening and, fortunately, the parasite is not typically transmitted from person-to-person.  Symptoms of cyclosporiasis include “explosive” watery diarrhea, loss of appetite, weight loss, cramping, bloating, increased gas, nausea, and fatigue. (C. cayetanesis has been nicknamed “the explosive diarrhea parasite” because of the profuse and protracted diarrhea it causes.) Severe dehydration is, however, possible and the illness is more consequential in older adults and people with weakened immunity.

Humans acquire the infection through the ingestion of food or water contaminated with feces that contain the parasite. The foods at the highest risk for transmitting C. cayetanensis are raw, fresh produce that are often eaten without cooking or peeling. The parasite thrives on these items. Because this produce is typically grown in tropical or subtropical regions, it can become contaminated via tainted irrigation water from infected field workers.

Common fresh foods frequently linked to cyclosporaisis outbreaks include:

• Fresh herbs: particularly cilantro and basil

• Leafy greens: bagged salad mixes, romaine salad, and mesclun lettuce

• Berries: especially imported raspberries

• Other veggies: snow peas and green onions.

This parasite is highly resilient; therefore, simply washing your fruits and veggies does not reliably remove or kill Cyclospora. Cooking or heating the food to proper temperatures is the only effective way to destroy it.

Currently, there is no vaccine to prevent cyclosporiasis, and none appears to be on the horizon. Most people with healthy immune systems, however, recover from the illness on their own in days or weeks but, in more severe cases, the infection is treated with antibiotics. At present, the preferred treatment for the disease is the antibiotic trimethoprim-sulfmethoxazole.

Current rise in cases in U.S. Initially, scientists thought that Cyclospora was confined to tropical and subtropical regions of the world. But outbreaks of cyclosporiasis have been reported increasingly in North America.

Since the 1990s, there have been at least 11 documented cyclosporiasis outbreaks in the U.S. and Canada. These include: (1) the 1996 outbreak that was traced to fresh raspberries from Guatemala; (2) the 2013 spike where the illness was traced to cilantro imported from the Puebla region of Mexico; (3) the 2018 multi-state wave where Cyclospora was identified for the first time in produce domestically grown in the U.S.; and (4) a 2022 Florida cluster linking the outbreak to Caesar salad kits containing bagged romaine lettuce.

The U.S. typically sees several hundred cases of cyclosporiasis annually. But, in the late 2010s, reported cases of cyclosporiasis began to rise. The increase in cases is attributed, in part, to better diagnostic tests. The Centers for Disease Control and Prevention (CDC) recorded 2,272 U.S. cases in 2023, 1,180 cases in 2025 across 38 states and an upswing in cases in 2026.

As of June 16, 2026, the CDC identified 145 domestically-acquired cases in 17 states, with 20 hospitalizations and no deaths. All these people became sick after eating contaminated food in the U.S. There were also 45 cases seen in people who ate or drank food or water while traveling outside the U.S.

In addition to the CDC’s tally, since June 22, 2026, more than 300 cases of cyclosporiasis have been reported in southeast Michigan— a state that typically identifies 50 cases a year. (Campbell, L., “CDC investigates parasite that’s caused cases of ‘explosive” diarrhea in 18 US states, The Guardian, July 3, 2026). To date, no specific food has been implicated in the current outbreak. The CDC and state departments of public health are, however, working to find the source of this parasitic infection.

In the meantime. In the U.S., cyclosporiasis is seasonal with cases usually peaking between May and August when fresh produce is more readily available. To continue to enjoy fresh produce—part of a well-balanced diet—and to reduce the risk from most causes of foodborne illness and other contaminants, the CDC recommends washing fresh fruits and vegetables with clean running water. It is also important to thoroughly wash your hands, kitchen counters and cutting surfaces when preparing food. And if you want to be super cautious until the public health sleuths determine what food is the current culprit, follow the CDC’s rule of thumb for eating foods in countries with high incidences of enteric infections, that is, “boil it, cook it, peel it or forget it.” It is a sure way to reduce your risk of infection.