Thirteen STEC O157:H7 cases were identified in nine US states: Minnesota (five) and one each from Iowa, Illinois, Kansas, North Carolina, Nebraska, New Jersey, South Dakota and Wisconsin in late 2015 and early 2016.
Among 12 patients, eight were hospitalized and two developed hemolytic uremic syndrome (HUS).
Link to Pizza Ranch restaurants
Nine ate at one of nine Pizza Ranch restaurants during the week preceding illness onset, including eight who ate a dessert pizza made with dough mix provided by an unnamed manufacturer.
The ninth patient consumed bread sticks made from the same dough mix.
Evidence from one restaurant showed that dessert pizzas were made with the same dough mix used in traditional pizzas, but used thicker dough and might have been undercooked.
Pizza Ranch stopped using dough mix from the manufacturer in early February, 2016.
Flour is a raw agricultural product and does not undergo processing to kill bacteria and other pathogens.
A separate outbreak of STEC O121 and STEC O26 from flour occurred in 24 states later in 2016.
Flour produced at a General Mills facility in Kansas City, Missouri was the likely source.
CDC said it was a reminder it is not safe to taste or eat raw dough or batter and such items should be baked first.
Sampling of dough mix
STEC infections with the identified Pizza Ranch outbreak pulsed-field gel electrophoresis (PFGE) pattern are commonly reported to PulseNet, with an average of 40-50 illnesses annually.
Eighty-eight samples of dry dough mix from five restaurants where patients reported eating were collected by public health officials in Illinois, Iowa, Minnesota, Nebraska and Wisconsin.
The Minnesota Department of Agriculture identified non-O157 STEC in seven of 17 samples, including one Shiga toxin-1–producing non-O157 STEC isolate and six Shiga toxin-2 (stx2)–producing non-O157 STEC isolates.
FDA collected six samples of dry dough mix from the manufacturer and all tested negative for STEC O157:H7 but one yielded a stx2-producing STEC O8:H28.
The study said although no lab evidence identified contaminated flour as the outbreak source, identification of other enteric pathogens in multiple samples of dry dough mix consumed by patients implicates flour as the possible source of pathogen introduction.
“The small number of cases and the lack of additional restaurant clusters suggest that this was a low level contamination event or that contamination only affected a limited amount of product,” it added.
“This outbreak serves as a reminder that consumers, industry, and government should consider that flour, a raw agricultural product, might be contaminated with pathogens and, when consumed raw or undercooked, might pose a risk to human health.”
The PFGE pattern was previously isolated from a sample of bulk flour collected during a 2009 investigation.
Seventy-seven patients fell ill during March to July 2009 from 30 states; 35 were hospitalized and 10 developed hemolytic-uremic syndrome.
In the case-control study, 33 of 35 patients consumed ready-to-bake commercial pre-packaged cookie dough.
“Despite instructions to bake…cookie dough before eating, case patients consumed the product uncooked. Manufacturers should consider formulating ready-to-bake commercial pre-packaged cookie dough to be as safe as a ready-to-eat product,” according to an outbreak analysis.
“Notes from the Field: Multistate Outbreak of Escherichia coli O157:H7 Infections Linked to Dough Mix - United States, 2016”
Authors: Gieraltowski L, Schwensohn C, Meyer S, et al.