The US health authorities have identified shredded iceberg lettuce served at the country’s largest Mexican-inspired fast-food chain as the source of an outbreak of cyclosporiasis, a parasitic illness that causes severe, explosive diarrhea.

More than 1,600 laboratory-confirmed cases of cyclosporiasis have been reported nationwide since early May, with 94 people hospitalized. No deaths have been reported, although the Centers for Disease Control and Prevention (CDC) has warned that the actual number of infections is likely much higher because many cases go undiagnosed.

The CDC on Friday identified the chain as Taco Bell and said a traceback investigation linked the contaminated produce to a single supplier in central Mexico. The CDC urged people not to eat shredded iceberg lettuce served at Taco Bell restaurants in Indiana, Kentucky, Michigan, Ohio, and West Virginia.

The agency said the implicated supplier, Taylor Farms de Mexico, has voluntarily removed all iceberg lettuce from the US market. Officials are continuing to investigate whether the contaminated produce was distributed to other restaurants or retailers.

Cyclosporiasis is caused by the microscopic parasite Cyclospora cayetanensis. The illness typically causes explosive watery diarrhea, along with bloating, gas, stomach cramps, nausea, and vomiting. Some people also experience fatigue and fever, while others develop no symptoms at all.

The infection is usually contracted by consuming food or water contaminated with feces, with symptoms typically appearing around a week after exposure, according to the CDC.

While the illness can be treated with antibiotics, it is not detected by most routine laboratory tests, making it difficult to diagnose. If left untreated, the infection can last for more than a month, with recurring bouts of diarrhea being the most characteristic feature.

The parasite is endemic in tropical and subtropical countries. There is no evidence that it is capable of human-to-human transmission.

Previous cyclosporiasis outbreaks in the US have been linked to cilantro, raspberries, sweet peas, lettuce, and basil imported from Mexico.