An outbreak of Shiga toxin-producing E.coli linked with contaminated salad leaves

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.

Fig. 2. Temporal and geographic distribution of cases within the 10 SNP cluster incorporating the outbreak attributed to consumption of contaminated prepacked salad and sporadic cases detected following the outbreak, UK, July 2015 – December 2016. Sub-cluster A (highlighted green) – salad outbreak; sub-cluster B (highlighted pink) – contaminated lamb mince; sub-cluster C – contaminated lamb sausage (highlighted yellow); sub-cluster D (highlighted blue) – direct contact with sheep or lambs or their environment in the northwest of England.

 

For the full article, entitled: 'An outbreak of Shiga toxin-producing Escherichia coli O157:H7 associated with contaminated salad leaves: epidemiological, genomic and food trace back investigations', see the January issue of: Epidemiology and Infection.