(Bloomberg) -- Wendy’s Co. restaurants were linked to an outbreak of dangerous E. coli bacteria in an investigation of dozens of cases, the US Centers for Disease Control and Prevention said Friday.

Wendy’s removed some romaine lettuce from stores as a precaution after 22 people in a midwestern E. coli outbreak reported eating at the restaurants in the week before their illnesses began, the CDC said. 

The CDC isn’t telling people to avoid eating at Wendy’s or to stop eating romaine lettuce and said a specific food hasn’t been confirmed as the source of the outbreak. Wendy’s shares fell as much as 4.3% after US markets closed following the CDC report. 

Wendy’s is cooperating with health authorities and removing sandwich lettuce as a precaution, a spokesperson said in an emailed statement. 

The E. coli outbreak has sickened 37 people in Indiana, Michigan, Ohio and Pennsylvania, the agency said. Nine people were hospitalized, including three with kidney failure. No deaths have been reported. Of the 26 people CDC interviewed, 22 reported eating at Wendy’s the week before falling ill. Most reported eating sandwiches with romaine lettuce.

Wendy’s uses a different type of lettuce for its salads. The CDC said it’s still working to confirm whether the romaine lettuce is the source of the outbreak, and whether the same supply of lettuce is also sold at other businesses. The agency said there’s no evidence that romaine lettuce sold in grocery stores or other restaurants is linked to the outbreak.

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