Shiga Toxin-Producing E. coli Infections Associated with Flour

Abstract

Background: In 2016, a multijurisdictional team investigated an outbreak of Shiga toxin-producing Escherichia coli (STEC) serogroup O121 and O26 infections linked to contaminated flour from a large domestic producer.

Methods: A case was defined as infection with an outbreak strain in which illness onset was between December 21, 2015, and September 5, 2016. To identify exposures associated with the outbreak, outbreak cases were compared with non-STEC enteric illness cases, matched according to age group, sex, and state of residence. Products suspected to be related to the outbreak were collected for STEC testing, and a common point of contamination was sought. Whole-genome sequencing was performed on isolates from clinical and food samples.

Results: A total of 56 cases were identified in 24 states. Univariable exact conditional logistic-regression models of 22 matched sets showed that infection was significantly associated with the use of one brand of flour (odds ratio, 21.04; 95% confidence interval [CI], 4.69 to 94.37) and with tasting unbaked homemade dough or batter (odds ratio, 36.02; 95% CI, 4.63 to 280.17). Laboratory testing isolated the outbreak strains from flour samples, and whole-genome sequencing revealed that the isolates from clinical and food samples were closely related to one another genetically. Trace-back investigation identified a common flour-production facility.

Conclusions: This investigation implicated raw flour as the source of an outbreak of STEC infections. Although it is a low-moisture food, raw flour can be a vehicle for foodborne pathogens.

Figure 1. Number of Case Patients, According to State of Residence

Figure 2. Number of Case Patients, According to Week of Illness Onset (December 21, 2015, through September 5, 2016)

Figure 3. Phylogenetic Tree of Selected Shiga Toxin–Producing Escherichia coli Serogroup O121 Isolates Involved in the Outbreak

The tree is based on 40 clinical isolates (case) and 9 isolates from flour samples (flour). The source of each isolate (type of sample and state abbreviation) is provided after the identification number of each isolate. The numbers at the tree nodes are bootstrap values that indicate the confidence in the clustering on repeated analysis of random subsets of the data (the closer the value is to 100, the higher the confidence is in the clustering). Additional details are provided in the Supplementary Appendix. SNP denotes single-nucleotide polymorphism.