(Reuters) -The E.coli bacteria outbreak in four Midwest states from an unknown source has affected 47 more people, the U.S. Centers for Disease Control and Prevention said on Thursday, taking the total tally to 84.

As of Wednesday, 38 people have been hospitalized. Although many strains of E.coli are found in the intestines of healthy people and animals and are harmless, certain types can cause severe abdominal cramps, bloody diarrhea and vomiting.

The agency said among the 62 people it interviewed, 52 were reported to have eaten at a Wendy's (NASDAQ:WEN) restaurant in the week before their illness started.

"We are fully cooperating with public health authorities on their ongoing investigation of the regional E. coli outbreak," the burger chain told Reuters in an email.

The new cases are an increase from last Friday when the health agency launched an investigation after 37 people across Michigan, Ohio, Indiana and Pennsylvania took ill.

Many of them had sandwiches with romaine lettuce at the burger chain a week before they fell ill, the agency had said.

The CDC, however, is not advising against eating at Wendy's restaurants or asking people to avoid romaine lettuce as it is yet to confirm the exact cause of the outbreak.

As a precaution, Wendy's has discarded and replaced the sandwich lettuce at some restaurants in the affected regions, the company confirmed.

"The lettuce that we use in our salads is different, and is not affected by this action," the company said.

The agency said the true number of sick people is likely to be higher than reported and the outbreak may not be limited to the states with known illnesses.