Researchers have been unable to pinpoint what was behind a number of E. coli O55 outbreaks in England between 2014 and 2018.

Over the 5-year period, there were 43 confirmed and three probable infections of Shiga toxin-producing E. coli (STEC) O55:H7. In 2014, 21 people were sick, 10 in 2015, six in 2016 and 2017 and three in 2018.

Patients ranged in age from 6 months to 69 years old with 30 under the age of 18 and 28 were female. There were 36 symptomatic cases, 25 developed hemolytic uremic syndrome (HUS) and two children died.

No common food or environmental exposures were identified, although most sick people lived in rural or semi-rural environments and had contact with wild and domestic animals.

Severe illness and two deaths
The initial outbreak investigation in 2014 and 2015 was in Dorset followed by Surrey in 2016 and 2017. East Midlands and east of England was involved in 2018, according to the study published in the journal Epidemiology and Infection.

STEC O55:H7 had not been observed in England prior to 2014. No cases have been detected in Dorset since 2015 and none with the same Shiga toxin profile have been noted in the UK since 2018. However, experts are continuing to monitor for signs of re-emergence.

In the Dorset outbreak, 31 sick people ranged in age from 8 months to 69 years old and in Surrey, 12 people aged 1 to 34 years old were ill. Three sporadic infections in children aged 6 months to 3 years old were found in 2015, 2016 and 2018.

Person-to-person transmission occurred at a nursery school during the Dorset outbreak. Overall in the county, 15 people developed HUS and 16 were hospitalized. In Surrey, seven got HUS and eight needed hospital treatment. Of the sporadic cases, three had HUS and two were hospitalized. Two children, aged 6 months and 2 years old, died as a result of infection.

Researchers from Public Health England said despite the decline in patients over the years, clinical outcomes of people reporting symptoms remained severe, including the two deaths in 2018.

Environmental link
Exposure questionnaires were available for 43 of the 46 confirmed or probable cases between 2014 and 2018. No common food and drink company, premises or supply chain was identified between patients and they did not visit a common area, site or venue. The most frequent exposures were to rural and semi-rural environments and contact with domestic and wild animals. Several reported walking in local recreation grounds, woods, paddocks and gardens.

In Dorset, more than 100 food, water and environmental samples were taken but none were positive for STEC O55:H7. Positive samples were identified from fecal samples from two cats — one pet belonged to a confirmed case and the other was from outside the home of another confirmed patient. The hypothesis being the strain was imported from outside the UK, possibly by wild birds.

In the East Midlands, 31 samples were examined for STEC using real-time PCR testing but all were negative. The strain behind the two cases in the East Midlands was not closely related to the earlier strains and may have been caused by a separate importation event, according to the study.

A case-case study highlighted the severity associated with STEC O55:H7 compared to infection with STEC O157:H7. Outbreaks of STEC O55:H7 have not been described in any other country, although there is evidence to suggest that STEC O157 evolved from E. coli O55:H7.

Researchers cautioned against the use of extensive resources required to carry out environmental sampling but recommended comprehensive testing of all household contacts in such outbreaks.