U.S. regulators are set to tie lettuce from Taco Bell to a parasite outbreak that’s sickened thousands in Michigan and nearby states, according to people familiar with the matter.

Taco Bell said it has voluntarily removed lettuce from a supplier and will replace the ingredient within 24 hours at restaurants in affected states. The supplier’s lettuce is being removed indefinitely from its supply chain nationwide as a precaution, despite no official advisory, the company said in a statement.

The U.S. Food and Drug Administration is poised to issue a public statement recommending consumers avoid shredded lettuce at Taco Bell locations in Michigan, Ohio, West Virginia, Kentucky and Indiana, said the people, who weren’t authorized to speak publicly.

The cyclospora parasite that causes severe diarrhea has sickened people in 34 states, according to the U.S. Centers for Disease Control and Prevention. Michigan has the largest number of cases, with more than 4,000 identified.

Taylor Farms is a potential source of the contamination, the Washington Post reported.

Earlier this week, the Mexican-inspired chain owned by Yum! Brands Inc., said it would remove some ingredients at select restaurants “voluntarily and temporarily” as a precaution, without giving more specifics. The company also said earlier that public health officials hadn’t confirmed a link between the outbreak and Taco Bell. Several independent restaurants also pulled lettuce and greens.

Yum shares were unchanged at 6:04 p.m. in extended trading in New York after initially falling 4% on the news.

The outbreak has also hit the shares of the fast-casual salad chain Sweetgreen Inc., with the stock declining about 25% this week amid reports that lettuce was the culprit. The company has said that no ingredients in its supply chain have been identified in connection with investigations into cyclospora.

The burgeoning outbreak has been difficult for health authorities to track. The disease doesn’t spread from person to person, and it can take as long as two weeks between the time someone ingests tainted food or water and when symptoms appear. Health authorities in Michigan had earlier pointed to contaminated lettuce or salad greens as a potential source.

On Thursday, White House Press Secretary Karoline Leavitt said the Trump administration is “closely monitoring” the outbreak and “committed to providing the CDC and FDA the resources that they need.”

(With assistance from Michael Hirtzer, Kristina Peterson, Elizabeth Elkin, John Tozzi and Marc Davies.)

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