Researchers have described the first outbreak linked to flour in Canada that occurred a few years ago.

It was also the first national outbreak associated with a non-O157 strain of E. coli, according to the study published in Epidemiology and Infection.

In December 2016, a cluster of six E. coli O121 cases were identified in Canada. In total, 30 confirmed cases were found with symptom onset dates between November 2016 and April 2017. A total of 28 were further typed as O121:H19.

Patients ranged from 2 to 79 years old and half were female. Eight were hospitalized and one person developed hemolytic uremic syndrome. No one died. Canadian clinical isolates carried the stx2 gene.

Early enquiries pointed to beef
When initial interviews using a closed-ended hypothesis-generating questionnaire did not point to a common source, people were re-interviewed using an open-ended approach. This allowed exploration of sources of illness not initially considered and not included in hypothesis-generating questionnaires.

Flour was considered from the start because of a 2016 U.S. outbreak. However, most initial cases did not report baking or cooking. It was only after probing specifically about exposure to flour and licking the spoon while baking that flour became an item of interest. Many cases had to be contacted multiple times and asked multiple questions.

Early interviews identified ground beef as a possible source of infection, as all six initial cases reported this exposure. Three cases had eaten hamburgers at two different restaurant chains in the same province. Public health investigators found the restaurants got hamburgers from the same supplier. As more infections were reported, people continued to report ground beef consumption, but other hypotheses also emerged, including sausage style deli-meats, bacon, pizza, pork pieces or parts, and oats. As additional cases were recorded, ground beef and deli meats were no longer frequently mentioned.

In total, 25 people were re-interviewed by the Public Health Agency of Canada (PHAC) using an open-ended approach. Just more than half of these were re-interviewed by PHAC two separate times.

Zeroing in on flour
Open-ended interviewing on cooking and baking behaviors in the home revealed exposures to raw flour and consumption of raw batter and dough. Further questioning on brand names uncovered Robin Hood flour, made by Ardent Mills, as a commonly used ingredient in baking and cooking.

In the end, 12 cases had used or probably used Robin Hood flour during the exposure period and 16 people reported it was used or probably used in the home during their exposure period. Twelve cases had both direct and indirect exposure to this flour in the home and four had indirect exposure to it though baking done by others at home.

Of patients that did not report exposure to the flour, one person had pizza at a restaurant that used flour produced by the implicated mill. An additional three people may have had occupational exposure: one case was a baker in a restaurant and two were grocery store cashiers who may have had contact with flour at work.

Positive product testing
In March 2017 a positive E. coli O121 test came from an open sample of Robin Hood brand flour from a patient’s home. Presence of bacteria in open samples does not automatically confirm the product as the outbreak source, but provides evidence to be considered in the investigation, according to the report. Both flour samples collected from retail tested positive for E. coli O121 and matched clinical results from patients.

Finding the outbreak strain in an open sample resulted in a hypothesis tested by re-interviewing cases to ask specifically about flour as well as a food sampling plan.

In total, 109 samples from 257 bags of Robin Hood flour were sampled and 12 percent were positive for STEC. The outbreak strain was isolated in several Robin Hood brand flour products of different sizes and production dates between September and November 2016.

Different E. coli serotypes, including O8:H19, O8:H28, O15:H4, O88:H25 and O187:H52, were identified during testing of closed and open samples of flour, but no clinical cases were matched to them.

No deviations were noted at the mill wher the recalled product was manufactured. There was no kill step in flour processing and no testing for pathogens at the mill. Investigation at the site did not identify a specific source of E. coli contamination in the flour products so the root cause of contamination was not discovered.

“These recent E. coli O121 flour outbreaks, combined with the finding of other E. coli non-O157 serotypes in closed flour samples, suggest that flour is an emerging vehicle for non-O157 STEC infections and should be considered as a potential source in non-O157 STEC outbreak investigations,” said researchers.

Further flour research
Meanwhile, scientists have looked at the occurrence of bacterial pathogens and levels of indicator organisms in wheat flour in Canada.

Non-O157 STEC were isolated from six of 347 samples, according to a study in the Journal of Food Protection.

Results suggest the occurrence of STEC O157 and Salmonella is low but the rate of non-O157 STEC in wheat flour with the potential to cause illness is relatively common. Consumption of raw flour could increase the likelihood of STEC infections, said researchers.

Another study, published in the same journal, did online surveys on consumers’ flour-handling practices and their knowledge about food safety risks related to this product.

From 1,045 flour-using consumers in the United States, less than 1 percent kept a record of product identification details, like lot numbers, and less than 11 percent kept brand and use-by-date information.

If a recall affected the flour they bought, nearly half would buy the same product from a different brand for a few months before they returned to the recalled brand. Among consumers who use flour to bake, two-thirds said they ate raw cookie dough or batter. Food safety messages were less impactful on those raw dough “eaters” than “non-eaters.”