An outbreak of Shiga toxin-producing Escherichia coli O157:H7 associated with contaminated salad leaves: epidemiological, genomic and food trace back investigations

Abstract

In August 2015, Public Health England detected an outbreak of Shiga toxin-producing Escherichia coli (STEC) serotype O157:H7 caused by contaminated salad leaves in a mixed leaf prepacked salad product from a national retailer. The implicated leaves were cultivated at five different farms and the zoonotic source of the outbreak strain was not determined. In March 2016, additional isolates from new cases were identified that shared a recent common ancestor with the outbreak strain. A case-case study involving the cases identified in 2016 revealed that ovine exposures were associated with illness (n = 16; AOR 8·24; 95% CI 1·55-39·74). By mapping the recent movement of sheep and lambs across the United Kingdom, epidemiological links were established between the cases reporting ovine exposures. Given the close phylogenetic relationship between the outbreak strain and the isolates from cases with ovine exposures, it is plausible that ovine faeces may have contaminated the salad leaves via untreated irrigation water or run-off from fields nearby. Timely and targeted veterinary and environmental sampling should be considered during foodborne outbreaks of STEC, particularly where ready to eat vegetables and salads are implicated.

Microbial Contamination of Fresh Produce: What, Where, and How?

“…Considering individual pathogens, E. coli contamination was mainly linked to the application of contaminated manure (nine of 22 outbreaks whose contamination route was documented; Ackers et al., ; Breuer et al., ; Mikhail et al., ). HAV was mainly spread by infected handlers, who were responsible for 10 of the 12 outbreaks with a reported contamination pathway (Calder et al., ; Dentinger et al., ; Heywood et al., ).…”

“…The complete dataset of considered outbreaks is available in Table S1. Ackers et al, 1998;Breuer et al, 2001;Mikhail et al, 2017). HAV was mainly spread by infected handlers, who were responsible for 10 of the 12 outbreaks with a reported contamination pathway (Calder et al, 2003;Dentinger et al, 2001;Heywood et al, 2007).…”

Whole genome sequencing and metagenomics for outbreak investigation, source attribution and risk assessment of food‐borne microorganisms

“…Retrospectively, historical outbreaks have been investigated applying WGS, and there is evidence that WGS analysis helps define a more targeted case definition when compared to methods used previously for cluster detection (Gymoese et al., ; Morganti et al., ; Revez et al., ,b; Simon et al., ; Ung et al., ). The level of certainty offered by WGS provides the impetus to drive outbreak investigations and direct trace‐back enquiries and has led to the successful resolution of outbreaks (Byrne et al., ; Gobin et al., ; Sinclair et al., ; Mikhail et al., ; Jenkins et al., ).…”

Assessing the genomic relatedness and evolutionary rates of persistent verotoxigenic Escherichia coli serotypes within a closed beef herd in Canada

“…Verotoxigenic Escherichia coli (VTEC) are food-and waterborne pathogens associated with sporadic cases and outbreaks of severe enteric disease, of which non-O157 serotypes are recognized as emerging and previously under-surveilled pathogens [1]. With the continual integration of wholegenome sequencing (WGS) into public-health laboratories and national surveillance programs [2][3][4][5][6], the degree of genomic variation among VTEC isolates is now commonly used for cluster identification during outbreak investigations [7][8][9], routine surveillance [10][11][12], and retrospective characterization of isolates to address broader questions of epidemiology and population structure [13][14][15].…”