U.S. health officials have identified shredded iceberg lettuce served at Taco Bell as one source of contamination behind an outbreak of parasitic infections affecting over 1,000 people.

The outbreak has been caused by the protozoan parasite Cyclospora cayetanensis, which can enter the body when someone consumes food or water contaminated with the parasite. Since May 1, there have been 1,644 confirmed cases across Indiana, Kentucky, Ohio, West Virginia and Michigan, which is currently the hardest-hit state.

(Health officials are also investigating additional cases of Cyclospora infection in the U.S. that have yet to be linked to this specific lettuce-related outbreak. Those other potential cases remain under investigation and total over 5,100, per the Centers of Disease Control and Prevention (CDC).)

In the five-state outbreak, there have been 94 hospitalizations and no deaths to date.

Through an investigation, health officials found that all 1,644 of the sickened people with confirmed cases of Cyclospora reported eating Taco Bell in Indiana, Kentucky, Michigan, Ohio or West Virginia before becoming ill. Interviews with the affected individuals suggested that shredded lettuce might be a common source of exposure.

The Food and Drug Administration (FDA) then investigated the source of the lettuce, identifying a single supplier of iceberg lettuce from Mexico that was used by Taco Bell locations where the people ate before becoming ill. News reports have named California-based Taylor Farms as the supplier.

"FDA is working directly with the identified supplier to determine if potentially contaminated shredded iceberg lettuce remains on the market," an FDA notice states. "As part of this investigation, FDA and state partners have initiated collection of product samples for testing and analysis."

The investigation is ongoing, and additional sources of contamination or affected restaurants could possibly be identified later, the FDA noted. In the meantime, Taco Bell has said it will stop using lettuce from the implicated supplier.

"Taco Bell is working to stop use of all lettuce implicated by this investigation. Not all Taco Bell locations in these states received implicated product," the FDA statement says.

Currently, the CDC is warning consumers not to eat shredded iceberg lettuce served at Taco Bell locations in Indiana, Kentucky, Michigan, Ohio or West Virginia.

Case counts reported by individual states may differ from the FDA's and CDC's counts, because states may include both probable and confirmed cases. The FDA and CDC will update their totals once they can confirm additional cases reported to them.

Symptoms of Cyclospora infection typically emerge two to 14 days after a person consumes the parasite. These symptoms include watery diarrhea, fatigue, cramping, bloating, gas and loss of appetite, and without treatment, they can last between a few days to a month or more. The infection is treated with an antibiotic, which blocks the parasite's ability to multiply.