Federal health officials confirmed Thursday that shredded iceberg lettuce served at Taco Bell restaurants in five U.S. states is the source of a widespread outbreak of cyclosporiasis, a parasitic intestinal illness that has now sickened thousands of people across more than 30 states since the outbreak was first identified in early May.

The Centers for Disease Control and Prevention issued a warning late Thursday advising consumers not to eat shredded iceberg lettuce from Taco Bell locations in Indiana, Kentucky, Michigan, Ohio and West Virginia. According to the CDC’s most recent tally, the outbreak has sickened at least 1,644 people across 34 states, including at least 94 hospitalizations, though officials cautioned that federal numbers lag behind what individual state health departments have reported. Michigan alone reported more than 5,000 confirmed cases as of Friday, including 102 hospitalizations, according to the state’s Department of Health and Human Services, a figure far exceeding the CDC’s national count and underscoring how significantly reporting delays have affected the visibility of the outbreak’s true scale.

A Food and Drug Administration trace-back investigation identified a single supplier behind the contaminated lettuce. While the federal government’s initial public warnings did not name the company, Taylor Fresh Foods, based in Salinas, California, confirmed that FDA testing had traced the contamination to a specific independent farm affiliated with the company in central Mexico. In a statement posted to social media on July 17, Taylor Farms de Mexico said it was voluntarily removing all iceberg lettuce sourced from central Mexico and had informed the FDA that it would initiate a formal recall. The company emphasized that none of its branded salads or salad kits sold in grocery stores nationwide are connected to the outbreak, noting that those retail products do not contain iceberg lettuce in the first place.

Taco Bell confirmed Friday that it had completed the removal of the affected product from its restaurants nationwide. “As of July 17, Taco Bell has completed removal of affected Taylor Farms lettuce from our restaurants,” the company said in a statement. “Based on ongoing conversations with public health officials, and out of an abundance of caution, Taco Bell worked swiftly to voluntarily remove the product from restaurants and the affected ingredient has been removed from our supply chain nationwide. We took this action to ensure our guests can enjoy their Taco Bell favorites safely. We believe public health is a shared responsibility among restaurants, their suppliers, and authorities, and we are proud to have consistently acted quickly and proactively to protect our guests.”

The CDC has said it is also investigating additional cyclosporiasis illnesses and outbreak clusters nationally that are unrelated to the Taco Bell lettuce findings, meaning not every reported case tied to the broader national surge in cyclospora infections is necessarily connected to this specific source.

Cyclosporiasis is caused by Cyclospora cayetanensis, a microscopic parasite transmitted through contaminated food or water. According to the CDC, symptoms typically begin about one week after exposure, though onset can range anywhere from as soon as two days to as long as two weeks or more after consuming contaminated food. Common symptoms include watery diarrhea, loss of appetite and weight loss, with the illness sometimes persisting for weeks if left untreated. Officials noted the infection is treatable with antibiotics once properly diagnosed, though there is no routine testing for cyclospora, meaning healthcare providers typically only test for it when an outbreak has already raised awareness among clinicians. No deaths have been reported in connection with the current outbreak.

Health officials have emphasized that thorough washing alone cannot reliably remove Cyclospora from produce, since the parasite is resistant to typical rinsing methods. The CDC recommends cooking produce to an internal temperature of at least 158 degrees Fahrenheit, or 70 degrees Celsius, as the most reliable method for killing the parasite when contamination is a concern.

The outbreak has had measurable financial consequences for companies connected to the fresh produce and fast-food supply chain. Shares of Yum Brands, Taco Bell’s parent company, fell nearly 7% over the five trading days following the outbreak’s public disclosure. Other companies that sell fresh lettuce also saw their stock prices affected by the broader health scare, even without direct ties to the confirmed outbreak source. Salad chain Sweetgreen saw its shares plunge nearly 13% over the course of the week, while fast-casual chain Cava fell more than 3%. Both companies saw a partial rebound Friday, with Sweetgreen shares rising more than 17% and Cava climbing about 2%, a reaction analysts attributed to apparent relief among investors once the CDC’s investigation confirmed that neither company’s ingredients were identified as a source of the outbreak. Analysts have generally said any near-term sales or stock price impact facing Taco Bell and similarly affected companies is likely to be limited in scope and duration, particularly once headlines about the outbreak begin to fade.

This is not the first time Taylor Farms has been connected to a significant foodborne illness investigation involving a major fast-food chain. Last year, an E. coli outbreak was traced to onions supplied to several restaurants from a Taylor Farms facility in Colorado, a situation that briefly forced McDonald’s to stop using onions on its Quarter Pounder sandwiches at some locations while the investigation unfolded. On its website, Taylor Farms describes itself as the leading global producer of salads and healthy fresh foods, operating production facilities across the United States, Canada, Mexico and Western Europe, and industry analysts have noted that a small number of large, vertically integrated companies like Taylor Farms dominate the U.S. bagged lettuce and salad supply chain, meaning contamination at a single facility or farm can quickly ripple across numerous restaurant chains and retail products nationwide.

The FDA said it continues working with Taylor Farms to determine whether lettuce from the implicated Mexican farm was distributed to any additional locations or retailers beyond the five states currently named in the CDC’s consumer warning. Federal and state health officials said the investigation remains active, and additional updates are expected as testing and case reporting continue in the coming weeks. Consumers who believe they may have symptoms consistent with cyclosporiasis are encouraged to consult a healthcare provider and report their illness to their local health department to assist ongoing surveillance efforts tied to the outbreak.