METHODS: A case was defined as an infection with the outbreak strain of E. coli O157:H7 with a symptom onset date between March 8 to April 18, 2015 in the U.S., and on or after March 1, 2015 in Canada. Country-specific hypothesis generating and leafy-green focused interviews were administered to identify common food exposures in the week before onset and a joint traceback investigation was performed. Whole genome sequencing (WGS) was completed on 31 clinical isolates (28 outbreak; 3 historical) to further characterize isolates genetically.
RESULTS: A total of 29 confirmed cases (16 U.S.;13 Canada) were identified in persons with illness onset dates ranging from March 1 to April 18, 2015. Cases-patients’ ages ranged from 6 to 76 years, with a median age of 24 years; seventy-two percent (21/29) of case-patients were female. Thirty-nine percent (11/28) of case-patients were hospitalized, two developed hemolytic uremic syndrome, and no deaths were reported. All case-patients reported eating leafy greens. Romaine lettuce was reported by 81% (21/26) of case-patients and of those, 35% (9/21) reported eating pre-packaged Ready-to-Eat (RTE) Caesar salad kits. Fifty percent (5/10) of Canadian case-patients reported consuming a single brand of RTE kale salad kit. Multiple romaine lettuce growing areas in the U.S. were identified during the traceback investigation and no single source for this outbreak was identified. WGS analysis identified two distinct phylogenetic clusters, each comprised of closely related outbreak isolates. These two WGS clusters correlated with the geographical distribution and exposure history of case-patients. The historical isolates were genetically different from both outbreak clusters.
CONCLUSIONS: PFGE and MLVA sub-typing suggested that outbreak isolates in the U.S. and Canada were highly similar to each other, and WGS analysis indicated that historical and outbreak isolates were genetic variants of the same strain. WGS further grouped the outbreak isolates into two distinct sub-clusters which correlated with the epidemiological information. The evidence could suggest that outbreak isolates may be related to two different food exposures with a common upstream source. Real-time WGS analysis during outbreak investigations will help further delineate clusters defined by traditional methods, contributing to our ability to identify common food sources in future investigations.