An outbreak of stomach infections in the U.S., how the Cyclospora parasite makes people sick, and what researchers know so far about its origins

The United States is currently battling growing cyclospora outbreaks in various states. Researchers say this is the largest such outbreak in the country’s history, and health officials have yet to determine the source. Since May 1, 1,645 cases of parasitic infection have been confirmed in 45 states, with more than 5,100 more cases under investigation.Joel Barratt, a molecular parasitologist at Emory Medical School in Atlanta, Georgia, said the numbers quickly exceeded the total number of cases that would typically occur in a year. More than 100 people infected with the parasite have been hospitalized, but no deaths have been reported.

What is Cyclospora and how does it affect humans?

The main symptom is watery diarrhea, as the parasite inflames the intestinal wall

Cyclospora cayetanensis is an intracellular parasite, meaning it invades and hijacks host cells, especially those lining the intestines, Nature reports. Once inside, it quickly begins to multiply, damaging cells. It can take a week or more for the parasite to complete its life cycle and cause enough damage to cause symptoms.Infection means someone has consumed food or water contaminated with human feces. Cyclospora cayetanensis reproduces through egg-like structures called oocysts that are shed in human feces. These oocysts are not immediately contagious: they must first sporulate or mature in a warm environment of 22°C to 32°C for a week or two, which is one reason summer outbreaks tend to occur.The main symptom is watery diarrhea, as the parasites inflame the lining of the intestines, disrupting the body’s ability to absorb water and nutrients, causing excess fluid to pass through the intestines. The infection clears up when the immune system flushes the parasite out of the body, but the illness can linger for weeks or even months.The main treatment doctors use to fight parasites is the combination antibiotic trimethoprim-sulfamethoxazole (sold as Bactrim). Antibiotics are not usually used to fight parasites, but Bactrim can starve Cyclospora of the essential vitamins it needs to replicate.

How are researchers trying to pinpoint the origins of the outbreak?

Health officials are still trying to pinpoint the source of the infections, but Michigan, which has reported the most infections (4,300 cases), has suggested lettuce or salad greens may be to blame. On Thursday, the U.S. Food and Drug Administration identified shredded iceberg lettuce from a supplier in Mexico that was sent to Taco Bell stores in five states as a source of the outbreak, but there may be other food sources driving the spread.Tracking the parasite requires coordination among U.S. state and federal health officials. First, state labs collect stool samples from people with symptoms of the disease and send them to federal agencies. If a sample tests positive for Cyclospora, health officials will determine when the person began experiencing symptoms and record what the person ate in the weeks prior.

How to prevent infection?

Produce is safest when heated to 158°F or higher, which actually kills Cyclospora.

The best way to prevent illness is to follow standard sanitation and food safety practices. It is important to wash your hands with soap and water after using the bathroom and before and after preparing food because alcohol-based hand sanitizers do not kill Cyclospora; however, soap and water do. Cleaning produce with vinegar is more effective than just using tap water. Produce is safest when heated to 158°F or higher, which actually kills Cyclospora.

Foods to avoid during Cyclospora infection

Because Cyclospora embeds itself into the texture of leafy, fragile, and moist surfaces, past outbreaks have been linked to:

Typical cyclospora season lasts until late August, and cases may continue to rise until then, according to the CDC.