Michigan reported 4,312 cyclosporiasis cases and 121 hospitalizations since June 22, extending what officials describe as a fast-growing outbreak with no confirmed source yet.
Lettuce and other salad greens are a leading focus for investigators, but Michigan's chief medical executive said no specific product has been definitively identified.
Taco Bell earlier removed lettuce and cilantro from some Michigan locations as a precaution, saying health officials have not linked the chain or any ingredient, supplier, restaurant or retailer to the outbreak.
Chipotle shares fell nearly 5% on July 15 even though the company said it does not believe its ingredients are tied to the national wave of infections, underscoring broader fallout for restaurant brands.