Dr. Karen Neil and colleagues at the Centers for Disease Control and Prevention and state health departments, said during the 2009 outbreak, 77 patients with illnesses were identified in 30 states, and 35 people were hospitalized.
The investigation, which involved extensive traceback, laboratory and environmental analysis, led to a recall of 3.6 million packages of the prepackaged commercial cookie dough.
However, the investigation, published in Clinical Infectious Diseases, found no single source, vehicle or production process associated with the dough could be identified to have contributed to the contamination.
Neil and colleagues suspected one of the ingredients used to produce the dough was contaminated. Their investigation didn't conclusively implicate flour, but it remains the prime suspect.
Flour does not ordinarily undergo a "kill step" to kill pathogens that may be present, unlike the other ingredients in the cookie dough like the pasteurized eggs, molasses, sugar, baking soda and margarine.
The study authors concluded "foods containing raw flour should be considered as possible vehicles of infection of future outbreaks of Shiga toxin-producing Escherichia coli."
"Manufacturers should consider using heat-treated or pasteurized flour, in ready-to-cook or ready-to-bake foods that may be consumed without cooking or baking, despite label statements about the danger of such risky eating practices," the authors concluded. "In addition, manufacturers should consider formulating ready-to-bake prepackaged cookie dough to be as safe as a ready-to-eat food item."