The Parasite Behind Summer’s “Explosive Diarrhea” Outbreaks Appears to be Cyclospora
Every spring, as bagged salads and fresh berries fill grocery coolers across North America, public health agencies brace for the return of a microscopic troublemaker. Cyclospora cayetanensis is a single-celled parasite that causes cyclosporiasis, an intestinal illness best known for producing prolonged, watery, and sometimes explosive diarrhea. Virtually unknown in wealthy countries until a series of dramatic outbreaks in the 1990s, it has since become a recurring seasonal hazard tied to fresh produce, a familiar cause of traveler’s diarrhea, and a persistent puzzle for the scientists who study it. This article traces what is known—and what remains stubbornly unknown—about the parasite’s life cycle, the illness it causes, the foods and water that carry it, and the outbreaks that have made it a household name during summer.
A Coccidian Parasite With a One-Host Life Cycle
Cyclospora cayetanensis belongs to a group of organisms called coccidian parasites, placing it in the phylum Apicomplexa alongside more famous relatives such as the agents of malaria and toxoplasmosis. Within that phylum it sits in the family Eimeriidae, whose members share a characteristic pattern: a one-host, fecal-oral cycle in which the parasite is passed in feces and picked up again through the mouth. Its closest biological cousins are the Eimeria species that cause coccidiosis in poultry and livestock, and researchers have proposed using those chicken parasites as laboratory surrogates precisely because C. cayetanensis has proven so difficult to grow and study directly.
What makes Cyclospora unusual, and what shapes nearly everything about how it spreads, is that humans are the only known host. Unlike many parasites that cycle through animal reservoirs, no animal reservoir has ever been identified for the species that infects people. The parasite completes its entire developmental journey inside a single human being—and then, crucially, requires a stint in the outside environment before it can infect anyone else.
The cycle begins when a person swallows a sporulated (mature) oocyst in contaminated food or water. Inside the small intestine, chemical signals in the gut trigger the oocyst to excyst, releasing the infective forms known as sporozoites—generally two per oocyst. These sporozoites invade the epithelial cells lining the duodenum and jejunum, the upper reaches of the small bowel. Once inside a host cell, each sporozoite transforms into a trophozoite and begins to multiply.
That multiplication proceeds in stages. The parasite first undergoes asexual reproduction, forming two types of schizonts (also called meronts): Type I schizonts containing eight to twelve merozoites, and Type II schizonts containing four. Merozoites from the first type appear to keep the asexual cycle going, while those from the second type switch to sexual development. In this sexual phase, the parasites differentiate into male microgamonts, which bear two flagella, and female macrogametes containing wall-forming bodies. Fertilization produces a zygote that becomes an oocyst, which is then shed in the feces in an immature, unsporulated form. The interval between infection and the appearance of oocysts in stool—the prepatent period—is thought to be about one week.
Here lies the single most important quirk of Cyclospora biology. When freshly passed, the oocyst is not yet infectious. It must first mature in the environment through a process called sporulation, during which the single-celled sporont divides into two sporocysts, each holding two sporozoites. This maturation is thought to require at least one to two weeks under favorable conditions, typically warm temperatures in the range of 22°C to 32°C. Only after sporulation can the oocyst infect a new person.
The practical consequence is profound: because the parasite needs days to weeks outside the body to become dangerous, direct person-to-person transmission is essentially impossible. You cannot catch cyclosporiasis by shaking hands with, caring for, or sharing a bathroom with an infected person the way you might catch norovirus. This distinguishes Cyclospora sharply from its coccidian relative Cryptosporidium, whose oocysts are infectious the moment they leave the body. The delay explains why cyclosporiasis is almost always a foodborne or waterborne disease rather than a contagious one—and why outbreak investigators look for a contaminated crop or water supply rather than a sick food handler passing germs directly to diners.
Despite this detailed sketch, much of the life cycle remains genuinely uncertain. The picture assembled above rests on only a handful of biopsy specimens, because the parasite has never been reliably cultured in the laboratory. Its stages inside human tissue measure less than 10 micrometers, making them hard to identify with confidence. Questions about how oocysts survive, disperse, and endure in soil and water are, as one recent review put it, still largely unanswered.
Symptoms and the Course of Illness
Once a person ingests infective oocysts, symptoms typically take about a week to appear. The incubation period averages roughly one week, though it can range from as short as two days to two weeks or more. The hallmark of the disease is watery diarrhea—frequent, and often described by clinicians and patients alike as frequent and sometimes explosive bowel movements. During the large 2026 surge, one microbiologist told reporters the most common presentation was simply explosive watery diarrhea.
Beyond diarrhea, cyclosporiasis produces a cluster of other gastrointestinal and systemic symptoms: loss of appetite, unintended weight loss, stomach cramps and pain, bloating, increased gas, nausea, and fatigue. A low-grade fever is common, while vomiting occurs but is less frequent. The infection specifically targets the small intestine, where biopsies reveal villous blunting and inflammation of the lining—damage that helps explain the malabsorption, weight loss, and profuse watery output.
A distinctive feature of untreated cyclosporiasis is its duration and its tendency to wax and wane. Left untreated, the illness may last anywhere from a few days to a month or longer, and it characteristically follows a remitting-relapsing course in which symptoms seem to resolve, only to return one or more times. Fatigue and a lingering sense of being unwell can persist for weeks.
Not everyone who is infected becomes sick. Some people carry the parasite without any symptoms at all, a pattern that is especially common in regions where the parasite is endemic and repeated exposure may build partial immunity. For the great majority of otherwise healthy people, the disease is uncomfortable but not life-threatening and is ultimately self-limiting. The picture changes for immunocompromised individuals—people undergoing cancer treatment, organ-transplant recipients, or those living with HIV—for whom the illness can become severe, protracted, or chronic and can even spread beyond the intestine to colonize other organs such as the biliary tract. Health officials during the 2026 outbreak specifically flagged concern for people who are immunocompromised due to cancer therapy or a transplant.
Diagnosing cyclosporiasis is not straightforward, which is one reason case counts almost certainly understate the true burden. The parasite is not detected by routine stool tests, so a clinician must specifically request testing for it, and not every gastrointestinal PCR panel includes a target for Cyclospora. Complicating matters further, oocysts may be shed intermittently and at low levels, even in someone with dramatic diarrhea, so a single negative stool specimen does not rule out the diagnosis; patients may need to submit several samples collected on different days. In the laboratory, the oocysts can be identified by their characteristic blue autofluorescence under ultraviolet light as well as by acid-fast and safranin staining and, increasingly, by DNA-based methods.
Treatment, fortunately, is well established. The drug of choice is the antibiotic combination trimethoprim-sulfamethoxazole (TMP-SMX), sold under brand names such as Bactrim, Septra, and Cotrim. A typical course for an immunocompetent adult is one double-strength tablet twice a day for seven to ten days, with people living with HIV sometimes needing longer courses and secondary prevention. For patients who cannot take sulfa drugs, options are limited; no highly effective alternative has been firmly established, though ciprofloxacin is considered an acceptable, if less effective, substitute. In a randomized trial among HIV-infected patients in Haiti, TMP-SMX cleared diarrhea in all treated patients, outperforming ciprofloxacin. Alongside antibiotics, rest and fluids to prevent dehydration remain essential supportive care. There is currently no vaccine.
Where the Parasite Comes From: Food, Water, and Warm Climates
Because sporulation happens outside the body, cyclosporiasis is fundamentally a disease of contaminated food and water. People become infected by ingesting food or water contaminated with sporulated oocysts, and in industrialized countries the overwhelming culprit is fresh produce—particularly items eaten raw. Over three decades of investigations, the foods repeatedly implicated in U.S. outbreaks form a consistent roster: raspberries, basil, snow peas, mesclun lettuce, and cilantro, joined more recently by bagged salad mixes and kits, and pre-packaged vegetable trays. A striking pattern unites them: nearly every implicated food is a raw item that never passes through a cooking step. By contrast, foods that are frozen, cooked, or peeled have essentially never been linked to an outbreak.
Two features of the parasite make contaminated produce especially hazardous. First, Cyclospora oocysts are resistant to routine chemical disinfection: the chlorine and iodine treatments that kill many bacteria do not reliably destroy the oocysts. Second, and sobering for anyone who relies on rinsing their salad, washing does not remove the parasite with any reliability. The oocysts cling tenaciously to the surfaces and crevices of leaves and berries. This is why food-safety experts describe cyclosporiasis as fundamentally a contaminated-water-and-field problem rather than a grocery-cooler one—the contamination is baked in at the source, on the farm.
That source is usually traced to fecal contamination of irrigation or wash water in regions where the parasite circulates. In the landmark Guatemalan raspberry outbreaks, investigators concluded that the likely origin was exposure of field workers to contaminated irrigation water. A detailed epidemiologic study in Guatemala found that people with Cyclospora infection were far more likely to have drunk untreated water in the two weeks before falling ill, and other endemic-area studies have tied risk to porous or shallow wells and contact with soil. Contaminated water can seed a crop directly, or oocysts in soil and water can find their way onto produce during growing, harvesting, or washing.
Geography and season matter enormously. Cyclosporiasis is endemic in many tropical and subtropical regions, including countries such as Guatemala, Mexico, Peru, Haiti, the Dominican Republic, and Nepal. In wealthy countries, historically, cases split into two groups: travelers returning from endemic areas, and domestic consumers who ate fresh produce imported from those same regions. For that reason, travelers to endemic areas are warned that ordinary precautions may not be enough—chemical disinfection of water is unreliable against this particular parasite.
Everywhere it occurs, the disease shows a strong seasonal rhythm, though the reasons remain poorly understood. In the United States, cases climb in spring and summer, so much so that the CDC defines the cyclosporiasis season as May 1 through August 31. Warm, wet conditions favor the sporulation the oocysts need to become infectious, which likely helps explain the summertime concentration—but the precise environmental drivers are still an open question, one of many the parasite has yet to yield.
Three Decades of Outbreaks
Before the mid-1990s, Cyclospora was an obscurity. It had been described as a human pathogen only in the early 1990s, and prior to 1996 just three small outbreaks had been reported in North America, at least one linked to drinking water. Cyclosporiasis was largely regarded as an affliction of travelers and of children in poor sanitary settings abroad. That changed dramatically in the spring of 1996.
The Guatemalan raspberry outbreaks, 1996–1997
Between May and mid-July 1996, the CDC received reports of nearly 1,000 laboratory-confirmed cases across at least 15 U.S. states, the District of Columbia, and Canadian provinces—counting both countries, roughly 1,465 confirmed infections in what remains one of the largest cyclosporiasis outbreaks ever recorded. Investigators initially suspected California strawberries, causing an unwarranted scare, before painstaking epidemiologic and traceback work pointed instead to fresh raspberries imported from Guatemala. A landmark study published in the New England Journal of Medicine found that eating raspberries was strongly associated with illness, and the investigation definitively established that Cyclospora could be a foodborne pathogen—a genuinely new insight at the time.
Despite improvements in water quality and sanitation on Guatemalan berry farms afterward, the parasite returned in 1997, and Guatemalan raspberries were implicated again. Follow-up fieldwork revealed that the parasite was endemic in Guatemala’s raspberry-growing regions, with surveillance finding infection most common among children and people with gastroenteritis, peaking in June. The upshot for the food supply was decisive: in 1998 the FDA barred imports of fresh Guatemalan raspberries, and by the end of the decade cyclosporiasis was made a nationally notifiable disease in the United States. These twin outbreaks transformed Cyclospora from a curiosity into a recognized threat and spurred sustained research into its biology.
A steady drumbeat, 2000–2017
The raspberry episodes were not the end. From 2000 through 2017, the United States recorded about 39 foodborne cyclosporiasis outbreaks totaling roughly 1,730 cases, a median of two outbreaks per year with individual outbreaks ranging from a handful of cases to more than 500. Across these years the implicated vehicles broadened well beyond raspberries to include basil, cilantro, snow peas, mesclun, and various lettuces; reviews of the global record identify basil and raspberries as the two most frequent vehicles overall.
The summer of 2013 brought an unusually large wave. By late September, the CDC had been notified of 643 cases from 25 states—against just 123 cases reported nationally the year before. Investigators concluded there had actually been more than one outbreak that summer: illnesses in Iowa and Nebraska were linked to a salad mix of iceberg and romaine lettuce, red cabbage, and carrots traced to a supplier in Guanajuato, Mexico, while cases in Texas were tied instead to fresh cilantro from Puebla, Mexico. The 2013 season underscored how difficult these investigations are, since a single reported “outbreak” can conceal multiple distinct contamination events.
The record-setting years, 2018–2020
In 2018 the United States logged about 2,299 cases, the year defined by two major foodborne outbreaks. The first sickened 511 laboratory-confirmed people across 15 states and New York City who had eaten salads from McDonald’s restaurants in the Midwest; notably, the FDA later confirmed the presence of Cyclospora in an unused package of romaine-and-carrot mix supplied to the chain—rare direct laboratory proof of contamination in an implicated food. The second 2018 outbreak involved 250 confirmed cases in four states tied to pre-packaged Del Monte vegetable trays containing broccoli, cauliflower, carrots, and dill dip.
In 2019 the country recorded an even higher tally: an unusually large 4,703 infections, according to a later CDC report. The following summer, a 2020 multistate outbreak sickened 690 people across 13 states, with investigators pointing to specific bagged salad products made by Fresh Express and sold under numerous store brands.
The 2026 surge
As of mid-2026, another season was well underway. The CDC’s surveillance reported that, as of June 16, 2026, 145 cases had been recorded in people who acquired cyclosporiasis within the United States, spread across 17 states, with at least 20 people hospitalized and no deaths. Notably, none of these patients had traveled internationally beforehand, so the CDC and FDA turned to hunting for a domestic food source. In keeping with recent years, the agency stressed there was no evidence of a single multistate outbreak linking all the cases—rather, a national surveillance count encompassing several separate clusters under investigation, with no specific grower, supplier, or produce type identified as of that update.
Litigation (Cyclospora lawsuits) are pending awaiting source identification.
Even more striking was a separate, large and growing cluster in Michigan, which is not among the CDC’s 17 states. The Michigan Department of Health and Human Services reported case counts that climbed rapidly through the summer—more than 300 cases by early July, then 681 cases by July 6 in a state that typically sees only about 50 cases in an entire year. Because the situation was still evolving, these figures should be read as snapshots of an active investigation. Infectious-disease specialists noted a longer-term shift worth watching: whereas U.S. cases were once dominated by infections acquired abroad or from imported produce, the country is increasingly seeing domestic cases as well.
Prevention and the Limits of What We Can Do
For consumers, the prevention advice is honest about its own limits. Public health agencies recommend washing hands with soap and water before handling raw produce, thoroughly rinsing fruits and vegetables under running water, scrubbing firm produce such as melons and cucumbers with a clean brush, and cutting away bruised or damaged areas. Cooking, where feasible, is the most reliable safeguard, since heat destroys the parasite. But because washing does not dependably dislodge oocysts and disinfectants do not kill them, the deeper solution lies upstream—in cleaner irrigation water, better sanitation on farms, and improved surveillance and testing along the supply chain. The FDA’s validated real-time PCR method for detecting Cyclospora in fresh produce, developed through a research program launched in 2015, represents one such upstream advance, giving investigators a far more sensitive tool for source-tracking.
The recurring pattern of these outbreaks—arriving each spring, tied to raw produce, defying easy laboratory confirmation—reflects how much about Cyclospora remains unresolved. Scientists still cannot grow it in culture, still debate exactly how oocysts survive and travel through the environment, and still lack a full account of why the illness is so sharply seasonal. What is clear is that as global food markets grow more interconnected and fresh produce travels farther from field to plate, this small parasite with a one-host life cycle and a mandatory detour through the environment will keep testing the vigilance of food-safety systems. For now, the humble advice to wash, scrub, and cook where possible—combined with a prompt visit to a healthcare provider for anyone with sudden, prolonged, watery diarrhea—remains the front line against a pathogen that has quietly earned its place among the most closely watched of summer’s foodborne threats.
