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An investigation into a multistate E. coli outbreak has been linked to food served at several Pizza Ranch restaurants, health officials concluded.
The Centers for Disease Control and Prevention confirmed that nine of the 13 cases of the illness affected people who reported eating at a Pizza Ranch restaurant prior to contracting the virus.
Health investigators have so far been unable to determine how the strain was contracted, a CDC spokeswoman told CNBC.
Health officials on Wednesday declared the outbreak to be over.
Pizza Ranch said it has been working with investigators.
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"Pizza Ranch independently ran over 40 tests on different products to test for the presence of E. Coli O157, and it was not found in any products tested," Ryan Achterhoff, chief administrative officer for Pizza Ranch, said in a statement.
"We provided public health investigators with a list of all of our ingredients, as well as contact information for our ingredient suppliers. We also contacted the supplier of our dough mixes regarding this issue, with the request that they cooperate with state and federal health officials."
The restaurant has temporarily discontinued its use of its Skillet Dough and Original Dough mix due to reports that the products were a possible factor in the contraction of the illness.
"In addition, we instructed all Pizza Ranch locations to complete a special, precautionary cleaning of all surfaces and equipment used in dough preparation or service," Achterhoff said.
He said the likely source of the bacteria is an ingredient supplier and not the restaurant; the CDC confirmed that several people living multiple states away from a Pizza Ranch location have contracted the same strain of E. coli.
The states affected by this particular E. coli outbreak were Iowa, Illinois, Kansas, Nebraska, North Carolina, New Jersey, South Dakota, Wisconsin, and Minnesota.
Pizza Ranch has more than 180 locations in 13 states, all of which are open and serving the company's full menu.