Explosive diarrhoea’ outbreak grips US: how researchers are hunting its source

US Health Officials Scramble to Trace Source of Growing Cyclospora Outbreak

A microscopic parasite known as Cyclospora cayetanensis is causing a significant uptick in cases of watery, explosive diarrhea across the United States. While the Centers for Disease Control and Prevention (CDC) is currently tracking 145 confirmed cases across 17 states as of June 16, health experts warn that these figures represent only a fraction of the actual infections.

The illness, known as cyclosporiasis, is a nationally notifiable disease. Because it often requires specific testing not included in standard diagnostic panels, many cases go undetected or unreported. Patients in the current outbreak range in age from 5 to 86, with 61% of those infected being women. While at least 20 people have required hospitalization, no deaths have been reported.

How the Parasite Hijacks the Human Gut

Cyclospora cayetanensis is an intracellular parasite, meaning it invades and hijacks cells lining the intestines. Once inside, the parasite multiplies rapidly, damaging the intestinal wall and triggering severe inflammation. This damage impairs the body’s ability to absorb water and nutrients, leading to the hallmark symptom of watery diarrhea. Physicians typically treat the infection with the combined antibiotic trimethoprim–sulfamethoxazole, commonly sold as Bactrim. This medication is effective because it starves the parasite of an essential vitamin required for replication.

The Seasonal Nature of Contamination

The current outbreak coincides with the peak season for fresh produce, which runs from May 1 through August 31. During these months, Americans consume more raw items like salads, herbs, berries, and vegetables. Unlike some foodborne pathogens, Cyclospora is not transmitted person-to-person. Instead, it is contracted by consuming food or water contaminated with human feces.

The parasite reproduces through egg-like structures called oocysts, which are shed in human stool. These oocysts are not immediately infectious; they must mature—or sporulate—in a warm environment, typically between 22 °C and 32 °C, for one to two weeks. This environmental requirement explains why outbreaks are most common during the summer months.

Contamination generally occurs due to sanitation failures, such as raw sewage run-off entering agricultural soil or farm workers lacking adequate access to toilet facilities. While past U.S. outbreaks were frequently tied to produce imported from tropical regions, recent years have seen an increase in infections linked to produce grown domestically within the United States.

Tracking an Elusive Source

Identifying the specific source of an outbreak requires rigorous coordination between state and federal health authorities. When a patient tests positive for Cyclospora, officials collect information on the individual’s recent diet to look for commonalities.

To improve the speed and accuracy of these investigations, the CDC has implemented molecular genetic analyses to examine the DNA of the parasites found in patients. “They could type the parasite from the people infected in all those different states and see if it’s even coming from the same source,” Dr. Pritt explained.

If genetic testing shows that the parasites are not related, it may indicate that multiple, separate contaminated produce supplies have entered the food chain.

Key Facts About the Outbreak

| Category | Details |
| :— | :— |
| Primary Symptom | Explosive, watery diarrhea |
| Transmission | Contaminated food or water (not person-to-person) |
| High-Risk Foods | Raw produce (salads, herbs, berries, vegetables) |
| Treatment | Antibiotic (trimethoprim–sulfamethoxazole) |
| Seasonality | May 1 through August 31 |

While health officials in Michigan have pointed toward lettuce or salad greens as a potential culprit, the investigation remains ongoing. Experts like Joel Barratt, a molecular parasitologist formerly with the CDC, note that the increasing size of these annual outbreaks may be linked to climate change, which provides warmer conditions for oocysts to mature. Further research into farm hygiene practices and surveillance of asymptomatic workers may be necessary to better understand how these outbreaks begin.

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