EXPLAINER: What has made the cyclospora behind the US outbreak so challenging to trace?

CHICAGO, Illinois - Cyclospora, the parasite behind thousands of suspected U.S. cases of a foodborne illness, has a long incubation period, and samples cannot be cultured in labs, making it especially challenging to trace.

To investigate, laboratory scientists have worked with epidemiologists to gather information from infected patients about what they ate, where and when, and then the U.S. Food and Drug Administration has tried to identify the tainted food products.

From those interviews, public health officials have traced a large outbreak in Michigan, Indiana, Kentucky, Ohio and West Virginia to shredded iceberg lettuce from Taco Bell restaurants supplied by Taylor Farms' operations in Mexico.

The officials often must rely on those interviews instead of testing because of the particular characteristics of cyclospora.

The FDA did test Taylor Farms' product at the U.S.-Mexican border — aimed at preventing more tainted produce from entering the United States. That initially produced a positive result for the presence of cyclospora, but it turned out to be a false positive following quality-control checks, the agency said.

Still, its investigation and the outbreak data continue to point to shredded lettuce from a Taylor Farms location in Central Mexico, the agency said in a briefing on Monday, and the company has issued a recall of all iceberg lettuce products sourced from that region of Mexico.

Even so, Taco Bell accounts for only about 60% of all cases in the multistate outbreak, according to former FDA Commissioner Scott Gottlieb. Investigations of other, possibly unrelated outbreaks are still underway, according to the U.S. Centers for Disease Control and Prevention.

"Cyclosporiasis is very hard to test for. There's no whole genome sequencing," Gottlieb said. "FDA relies on whole genome sequencing to effectively fingerprint a pathogen to try to isolate a particular pathogen to a particular outbreak to a particular grower. You don't have that in this case."

Cyclospora testing is difficult

Some better-known foodborne pathogens, such as Salmonella or E. coli, cause symptoms within a few days of consumption. But cyclosporiasis symptoms can take up to two weeks to appear — too long for people to easily recall exactly what they ate, and where. By that time, fresh produce items such as tainted lettuce may already be out of stores or restaurants.

The tiny parasite that causes cyclosporiasis cannot be traced using the most advanced testing methods in which scientists grow pathogens in their labs and sequence the full genome of the organism. This produces a genetic fingerprint that can be matched with contaminated food sources.

Because this is a parasite and not a bacterium, cyclospora cannot be grown or cultivated in labs — a process that increases the amount of genetic material used to confirm its presence.

Instead, scientists have to extract the genetic material from the organism and make synthetic copies of its DNA — a process that can sometimes result in false-positive results, as it did with the FDA's testing.

"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, told the briefing.

More cases expected

Last week, the CDC urged doctors to consider testing for cyclospora, which would ensure that more people get the proper treatment with a course of antibiotics. But it also could cause unintended consequences.

Craig Hedberg, a University of Minnesota food safety expert, said because of the harvest cycle and perishability of fresh produce, the contaminated lettuce that has infected so many people in the Midwest likely already has moved through the distribution system.

But because of the publicity generated by this outbreak, Hedberg said, many more people are likely seeking out testing.

"We'll probably see a huge influx of new cases, and it doesn't necessarily reflect that the actual outbreak is continuing to grow," Hedberg said. — Reuters