Researchers have described an outbreak and ongoing transmission of a strain of E. coli O157 in England and Scotland in 2019.

Investigators concluded the source of infection was likely Scottish cattle and the outbreak strain was found in ground (minced) beef in July 2019. However, only half of 14 patients linked to the outbreak could be explained by exposure to raw beef products sold at one retailer.

In August 2019, public health surveillance systems in Scotland and England identified seven people infected with the same strain of Shiga toxin-producing E. coli (STEC) O157:H7. Four of them lived in England and three in Scotland.

The outbreak investigation team included Public Health England, Public Health Scotland, the Food Standards Agency (FSA) and Food Standards Scotland (FSS).

Source found thanks to retail sampling survey
Four patients were male and they ranged from 1 to 46 years old with the median age of 19. The earliest onset date was late July and the last was end of August 2019. All seven cases reported diarrhea or bloody diarrhea, five were admitted to hospital, two reported fever, and none developed hemolytic uremic syndrome (HUS). No deaths were reported.

The outbreak strain had not been detected in the past three years of routine whole genome sequencing for STEC isolates in England. It was similar to STEC O157:H7 from UK beef cattle isolated during a study in 2015, according to the research published in the journal Epidemiology and Infection.

Epidemiological analysis of enhanced surveillance questionnaire data identified handling raw beef and shopping at the same national retailer as the common exposure.

Six of the seven cases in July and August reported shopping at the same retailer and five consumed ground meat, burgers or sliced ham from the delicatessen. Two people reported handling raw ground beef or raw beef burgers.

A microbiological survey of ground beef at retail identified the same strain in a product sample sold by the supermarket, providing microbiological evidence of the link. Two samples from the same branch of the retailer in late July 2019 were positive for the outbreak strain, although this was not a location any of the patients reported shopping at.

A review of surveillance data at Public Health Scotland identified a further sick adult from early April 2019 from the north of Scotland who reported contact with farm animals including cattle.

Second wave of infections
Between September and November 2019, another four primary and two secondary patients infected with the same strain were identified. One person reported consumption of beef and none shopped at the implicated retailer but they did visit the same petting farm. All four people who visited the farm denied direct contact with cattle and didn’t eat there.

All six patients lived in England. All but one were female and ages ranged from 2 to 30, with the median being 4 years old. Three people were admitted to hospital, two reported bloody diarrhea, one had fever, and two children developed HUS. No deaths were recorded. onset dates ranged from mid-September to early November 2019.

An inspection by environmental health officers at the petting farm found the facilities and hygiene standards were satisfactory. A total of 26 fecal samples from different species at different locations across the farm were tested in early December but were negative for STEC O157:H7. Boot sock samples from walking through the premises were also negative.

These six cases appear to be the result of ongoing transmission of the outbreak strain either via the food chain, contact with animals or via person-to-person contact, said researchers.

The food sample that tested positive for STEC O157:H7 during the microbiological survey of ground beef was sourced from a Scottish cow, slaughtered in that country and minced at a cutting plant in England owned by the retailer.

Movement data on sheep and cattle from the petting farm revealed no commonalities with that of cattle from the matching ground beef isolate.

Investigators could not identify other stores that were supplied by the cutting plant behind the outbreak cluster, or whether cattle from the same herd linked to the ground beef sample were slaughtered at a different abattoir and cutting plant supplying other retailers and food outlets.

“Although the catering supply chain for the petting farm did not align with the established distribution chain of the implicated mince beef, alternative distribution chains may have been in operation,” said researchers.