Why is the cyclospora parasite behind the US outbreak difficult to trace?

CHICAGO – Cyclospora, the parasite linked to thousands of suspected foodborne illness cases in the United States, has proven especially difficult for health officials to trace because of its long incubation period and the challenges involved in laboratory testing.

To identify the source of infections, laboratory scientists and epidemiologists interview patients about what they ate, where they ate, and when symptoms began. The U.S. Food and Drug Administration (FDA) then uses that information to trace potentially contaminated food products.

Based on those interviews, public health officials linked a major outbreak affecting Michigan, Indiana, Kentucky, Ohio, and West Virginia to shredded iceberg lettuce served at Taco Bell restaurants and supplied by Taylor Farms' operations in Mexico.

Unlike bacteria such as Salmonella or E. coli, Cyclospora cannot be cultured in laboratories, making it impossible to use whole-genome sequencing to match the parasite directly to contaminated food sources. As a result, investigators rely heavily on patient interviews rather than laboratory confirmation.

The FDA also tested lettuce shipments from Taylor Farms at the U.S.-Mexico border to prevent additional contaminated produce from entering the country. Although an initial test indicated the presence of Cyclospora, the result was later determined to be a false positive after quality-control checks.

Despite the false-positive result, the FDA said its investigation continues to point to shredded iceberg lettuce from a Taylor Farms facility in central Mexico as the likely source of the outbreak. The company has since recalled all iceberg lettuce products sourced from that region.

Health officials noted that Taco Bell accounts for only about 60% of the reported cases in the multistate outbreak. Investigations into other potentially unrelated outbreaks remain ongoing, according to the U.S. Centers for Disease Control and Prevention (CDC).

Former FDA Commissioner Scott Gottlieb said tracing Cyclospora is far more difficult than tracking bacterial foodborne illnesses because the parasite cannot be analyzed using whole-genome sequencing, a method commonly used to identify the source of outbreaks.

Difficult to Detect

Cyclospora infections, known as cyclosporiasis, can take up to two weeks to cause symptoms, making it difficult for patients to accurately remember what they ate or where they consumed contaminated food. By the time symptoms develop, fresh produce such as lettuce may already have been removed from stores or restaurants.

Because Cyclospora is a parasite rather than a bacterium, scientists cannot grow it in laboratory cultures. Instead, they must extract its genetic material and create synthetic copies of its DNA for testing, a process that can occasionally produce false-positive results.

"It's extremely difficult to detect in food, even with the highly sensitive methods that we use," Donald Prater, the FDA's acting deputy commissioner for food, said during a briefing.

More Cases Expected

The CDC recently urged healthcare providers to consider testing patients for Cyclospora, a move expected to improve diagnosis and ensure infected individuals receive the recommended antibiotic treatment.

However, food safety experts said increased testing could also lead to a surge in reported cases, even if the outbreak itself is no longer expanding.

Craig Hedberg, a food safety expert at the University of Minnesota, said the contaminated lettuce responsible for the Midwest outbreak has likely already moved through the distribution system because of the short shelf life of fresh produce.

Still, heightened public awareness is expected to encourage more people with symptoms to seek medical testing, resulting in additional reported infections that may reflect increased detection rather than ongoing contamination. -Reporting by Julie Steenhuysen; additional reporting by Yasmeen Abutaleb in Washington; Editing by Will Dunham/Reuters