Cystodiasis: What False Positives Mean on FDA Taylor Farms Lettuce

The Food and Drug Administration’s retraction of a key part of a study on cysticercosis has caused confusion in the United States, but the agency announced Monday that it had not changed its main conclusions about the outbreak.

The agency announced Sunday that a test that showed samples of iceberg lettuce supplied by Mexico’s Taylor Farms harbored the Cyclospora parasite returned a false positive. But this only applied to one particular shipment of lettuce supplied by the company, and the agency does not believe it was the vehicle that sickened more than 1,600 people and likely thousands of others.

The agency said in a statement Monday that the false positives do not change its previous conclusion that the outbreak was linked to shredded iceberg lettuce from central Mexico supplied by Taylor Farms and also available at some Taco Bell locations. The agency still advises Americans not to eat the recalled iceberg lettuce.

“FDA’s tracking and outbreak data continue to focus on Taylor Farms’ shredded iceberg lettuce in central Mexico,” the statement said. “FDA continues to work with our federal and state partners to investigate this multi-state outbreak and ensure that the products involved in this outbreak are removed from the market.”

Investigators are continuing to investigate the exact cause of the outbreak, which can cause symptoms similar to severe abdominal pain for days or even weeks. CNBC reached out to the FDA for details on the next steps in the investigation, including whether additional ingredients or suppliers are being scrutinized and whether the agency plans to issue further guidance. The agency has not yet responded.

The false-positive tests come as the developing investigation creates uncertainty for both consumers and the food industry. The FDA says not to eat Taylor Farms’ iceberg lettuce, but some customers are skipping the salad altogether as cases rise.

“Unfortunately, this latest development has the potential to add further confusion to an already complex situation for consumers,” said Frank Yiannas, FDA’s deputy commissioner for food policy and response.

Industry analysts don’t expect the outbreak to have a long-term impact on Taco Bell or other restaurant chains, but based on foot traffic data, it could at least temporarily hurt sales, and businesses linked to the outbreak could suffer a one- to two-quarter hit. Taco Bell traffic on Fridays was down about 19% compared to the average day of the week so far this year, according to data from research firm Placer.ai.

In food poisoning outbreak investigations, false positives can occur if the initial screening signal is not reproduced during confirmatory testing. Some doctors note that Cyclospora is particularly difficult to reproduce in the lab.

However, this has not changed the way government officials view the origins of the outbreak.

As the outbreak continues to explode, the Michigan Department of Health and Human Services says it continues to recommend buying whole lettuce instead of pre-washed, pre-bagged or pre-mixed salad kits.

The agency added that, based on interviews with more than 2,000 infected people, many patients did not report eating lettuce at a restaurant, but frequently said they ate leafy vegetables in some situation.

Taylor Farms announced Monday morning that it will continue its voluntary recall of suspected iceberg lettuce from central Mexico, and Taco Bell has already begun removing suspect lettuce from restaurants in affected states.

The ripple effects extend beyond those companies.

walmart The company announced Monday that it has proactively removed four bagged salad kits supplied by Taylor Farms under the Marketside brand from its stores in 27 states, although there is no indication that its products are contributing to the spread of infection.

“At this time, there are no confirmed illnesses associated with these products,” the company said in a statement. “We are working closely with our suppliers and have taken immediate action to remove the product from sale.”

Former FDA Commissioner Dr. Scott Gottlieb suggested to CNBC that the test results do not reveal a link between Taylor Farms and the outbreak. He said Friday’s recall was for a product that was imported several weeks ago and was not the same lettuce that tested positive for FDA testing.

He also said that because Cyclospora has an incubation period of up to two weeks and lettuce has a short shelf life, it is important to test multiple shipments, even if they come from the same farm.

Dr. Norman Beatty, an associate professor at the University of Florida School of Medicine, said the source may be a single vendor for now, but the contamination may already be widespread.

“This outbreak may have led to further spread due to the nature of this parasite,” Beatty told CNBC. “The reason this is important for this outbreak is because it is isolated in more than 30 states, and there is potential for small outbreaks to occur in other areas.”

For restaurants, uncertainty is proving to be as difficult as the outbreak itself. Salad-focused chains and other operators that rely heavily on fresh produce are left in the dark about whether other ingredients still pose a potential risk.

Taco Bell told CNBC in a statement Saturday that it voluntarily removed the suspect lettuce from its stores within 72 hours of beginning the work and adjusted its supply chain accordingly. The company added that it is “confident” that consumers can safely dine at its stores.

“Clearly, this is an industry-wide issue and not unique to Taco Bell,” the company said. “We hope that other members of the industry will take the same level of immediate action and accountability to consumers that we have.”

Taco Bell isn’t the only restaurant underperforming, according to Placer.ai. Other restaurant chains that sell lettuce, including Chopt, Panera Bread and Chipotle, also saw foot traffic decline Friday, the company said.

Amid headlines last week about the outbreak, the industry’s top advocacy group stressed that it takes food safety seriously.

“Food standards require all restaurants to have a food safety certified manager on every shift,” Michelle Colmo, CEO of the National Restaurant Association, said in a LinkedIn post.

“We continue to help public health authorities investigate outbreaks and ensure our members have the information they need to evaluate food safety decisions,” she added.

Despite the false positive results, Taylor Farms and Taco Bell’s parent company yum brand The company is still weathering the reputational damage caused by the outbreak, which led to product removals and headlines. Still, restaurant analysts previously told CNBC they don’t expect businesses linked to the outbreak to suffer long-term financial damage.

Yum Brands stock has fallen nearly 9% over the past five days.