Researchers have described the first national outbreak of Shiga toxin-producing E. coli in the United Kingdom associated with burgers that affected 12 people in 2017.

It was also the first known outbreak in the UK linked to frozen burgers. Four small, local outbreaks occurred in England and Wales between 2009 and 2015 and they were likely due to the consumption of undercooked fresh burgers or cross-contamination outside the home.

In November 2017, Public Health England (PHE) identified a suspected outbreak through routine surveillance, when four cases with Shiga toxin-producing E. coli (STEC) O157: H7 isolates with the same phage type were detected, according to the study published in the journal Epidemiology and Infection.

Whole-genome sequencing (WGS) indicated cases were likely linked to a common source and investigations included a review of surveillance data, a case-case study, and trawling interviews.

The outbreak strain had not been detected in the past three years of routine WGS for STEC isolates in England. Around 700 cases of STEC O157: H7 are reported annually in England.

Small outbreak with severe illness
Twelve people were affected, eight were hospitalized and four developed hemolytic uremic syndrome (HUS) but nobody died. Frozen beef burgers supplied by Sainsbury’s were the outbreak vehicle.

Testing of two leftover burger samples from the freezers of two separate cases and a retained sample from the production site was positive for the outbreak strain. Product trace-back indicated burgers were produced at a single meat preparation factory, who made frozen burgers for all major UK supermarkets.

Burgers were produced on Sept. 5, 2017, from the same batch of material. Contamination may have been limited to one mix prepared in the space of one hour on that day. In total, 30,252 packs had been produced from that batch. A number of customers returned the product and 19,000 units were destroyed.

Investigations at the production site identified no breaches of food safety legislation. Cooking guidance on the packaging was deemed adequate and interviews with seven food handlers who prepared burgers for eight outbreaks cases self-reported good storage and cooking practices at home.

Eleven cases lived across England and one person was from Scotland. Nine were male and three were female. They ranged in age from 1 to 65 years old and illness onset dates were from Sept. 28 to Nov. 23, 2017.

Outbreak investigation
A case-case study compared exposures among the 11 outbreak cases in England against 537 primary non-outbreak cases (control group) who were not associated with travel abroad or other known outbreaks.

This study identified that Sainsbury’s was reported by 10 of 11 outbreak cases compared to 106 people in the control group. Among food exposures, 13 categories were reported by more than half of outbreak cases.

Outbreak cases were re-interviewed using a trawling questionnaire to get a more detailed food history. Consumption or handling of raw beef intended for cooking was reported by seven patients interviewed and included one or more products of burgers, minced beef, beef pies, steak, and roast beef.

Following hypothesis generation, cases were again re-interviewed using a focused questionnaire and asked about consumption or handling of suspected products. Of 10 cases re-questioned in December about burger and mince products, nine reported eating cooked frozen burgers from Sainsbury’s and one from another retailer.

Sainsbury’s reviewed purchase history for six cases with loyalty card data. This confirmed that four of them had purchased the retailer’s own brand frozen burgers between August and November 2017. Online shopping history from another person indicated they had ordered a different type of Sainsbury’s own branded fresh burgers.

From loyalty card data and patient interviews, product details were available for 10 of 12 cases. Nine reported burgers from Sainsbury’s, seven a specific own-brand frozen burger, and two an own-brand fresh burger product.

The retailer did additional laboratory tests on cooking instructions. The FSA concluded that if cooking instructions were adequately followed, the necessary temperature would be achieved and that guidance had an adequate safety margin.

“Due to variation in cooking practices and performance of cooking appliances, and failure to follow cooking instructions or avoid cross-contamination in the home, we suspect that sporadic STEC cases linked to frozen burgers cooked at home are underestimated,” said researchers.

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