Cyclospora infections are surging across the U.S. this summer, driven in part by contaminated fresh produce and the parasite's seasonal peak.
Experts say the illness is difficult to diagnose and cannot be prevented simply by washing produce, making outbreaks especially challenging to control.
Unlike many stomach bugs, Cyclospora is not spread directly from person to person, but through food or water contaminated with human waste.
The United States is experiencing one of its largest outbreaks of cyclosporiasis in recent years, with thousands of confirmed and suspected cases linked to contaminated produce. As health officials continue investigating the outbreak, infectious disease experts say several factors make the parasite unusually difficult to detect, trace and prevent.
Cyclosporiasis is an intestinal illness caused by the microscopic parasite Cyclospora cayetanensis. People become infected by eating food or drinking water contaminated with human feces carrying the parasite. Fresh produce—including leafy greens, herbs and berries—has been implicated in previous outbreaks because these foods are often consumed raw.
This year's outbreak has largely been associated with iceberg lettuce distributed from Mexico, although investigators continue to examine whether other foods also played a role. Although there have been no deaths, the U.S. Centers for Disease Control and Prevention reports nearly 2,000 people in nine states have been sickened, with nearly 100 hospitalized.
Epidemiologic evidence continues to point to contaminated iceberg lettuce, even after the FDA determined an earlier positive laboratory test from one sample was a false positive.
A seasonal disease
Cyclospora infections tend to peak between May and August in the United States. According to experts, the seasonal increase occurs because warmer weather helps the parasite mature in the environment while imports of fresh produce from regions where the parasite is more common also increase during the summer months.
Unlike viruses such as norovirus, Cyclospora is not usually passed directly from one infected person to another. The parasite must spend one to two weeks outside the body before it becomes infectious, making contaminated food and water the primary sources of infection.
Symptoms can last for weeks
Symptoms usually begin about a week after exposure but can take as long as two weeks to appear. The hallmark symptom is frequent watery diarrhea, often accompanied by stomach cramps, nausea, bloating, fatigue, loss of appetite and weight loss.
Without treatment, symptoms may persist for several weeks or even recur after temporarily improving. Some infected people, however, experience no symptoms at all.
Doctors typically prescribe the antibiotic trimethoprim-sulfamethoxazole (Bactrim) to treat the infection. Maintaining hydration is also important, particularly for older adults and people with weakened immune systems, who may face a higher risk of complications from severe diarrhea.
Hard to diagnose — and hard to prevent
Scientists say one reason the current outbreak has become so widespread is that Cyclospora is not detected by routine stool tests. Physicians generally must order specialized testing when they suspect the infection.
Prevention is also challenging. Because the parasite can cling tightly to fresh produce, simply rinsing fruits and vegetables may not remove it completely. Cooking foods thoroughly is the most reliable way to kill the parasite, but that's not practical for many fresh fruits and salad ingredients.
Health experts recommend washing hands before handling food, rinsing fresh produce under running water even though it is not foolproof, and paying close attention to food recalls and public health advisories during the summer outbreak season. People who develop prolonged watery diarrhea after eating fresh produce should seek medical care and ask whether testing for Cyclospora is appropriate.