Warnings are now appearing on the flaps of flour packages. “Cook before sneaking a taste,” says one. Another says “Flour is raw. Please cook fully before enjoying.”

And the April 23 edition of Morbidity and Mortality Weekly Report (MMWR) turned its “Note from the Field” section to a closer look at outbreaks involving flour. “Multistate Outbreak of Escherichia coli O26 infections linked to Raw Flour — the United States, 2019” examines one such instance.

Here’s how it came down, according to the MMWR authors:

PulseNet, the molecular subtyping network for foodborne disease surveillance, on Feb. 20, 2019, identified six Shiga toxin–producing Escherichia coli (STEC) O26:H11 infections with the same pulsed-field gel electrophoresis (PFGE) pattern combination. 

The PFGE pattern combination matched that of infections from a July 2018 outbreak associated with ground beef. In response, CDC initiated an investigation with federal, state, and local partners to identify the outbreak source and implement prevention measures.

The CDC defined the case as STEC O26 infection with an isolate matching the outbreak strain by PFGE or related by core genome multilocus sequence typing scheme (cgMLST), with dates of illness onset from Dec. 11, 2018 – May 21, 2019.

 Investigators initially hypothesized that ground beef was the outbreak cause because of the PFGE match to the July 2018 outbreak and because in early interviews, patients commonly reported eating ground beef and leafy greens. Investigators used cgMLST to compare the genetic sequences of isolates from both outbreaks and determined that they fell into separate genetic clades, or groups, differing by 6–11 alleles, suggesting that something other than ground beef caused the illnesses in 2019. 

The CDC noted that one patient consumed raw cookie dough and that most patients were young adult females, similar to demographic distributions of past flour-associated STEC outbreaks. Investigators developed a supplemental questionnaire focusing on beef, leafy greens and flour exposures.

Twenty-one cases were reported from nine states. The median age of patients was 24 years with a range of 7 to 86 years; 71 percent were female. Three patients were hospitalized, and none died. 

Among 13 patients asked about flour exposures, six reported eating, licking, or tasting raw homemade dough or batter during the week before illness onset. Three patients reported eating raw dough or batter made with the same grocery store brand of all-purpose flour, including a patient who reported eating raw dough at a bakery in Rhode Island. Overall, of 18 patients with store information, 11 reported shopping at this same grocery store chain.

The Rhode Island Department of Health visited the bakery reported by the patient and collected flour for testing. On May 21, 2019, testing identified STEC O26 from an intact bag of all-purpose flour, which was the same grocery store brand reported by other patients. 

PulseNet confirmed that the STEC O26 isolated from the flour was highly related to clinical isolates using cgMLST (0–1 alleles). Product distribution records collected by the Food and Drug Administration found six patients in three states purchased flour from a single milling facility in Buffalo, New York. 

The flour brand was recalled from retail outlets in 11 states.

The milling company also recalled all lots of this product and several other lots of flour produced in that facility, resulting in the recall of additional brands and products distributed to multiple states.

The MMWR authors say flour is recognized as a cause of STEC outbreaks (1–5). Raw flour is not a ready-to-eat product, and this outbreak highlights the continuing risk for illness associated with the consumption of flour and raw dough or batter.