Concurrent outbreaks of Escherichia coli O157:H7 and O157:H39 with high asymptomatic carriage of other Shiga toxin-producing E. coli in nursery children, south-east Scotland, United Kingdom, July to October 2022

We managed a complex outbreak of Shiga toxin-producing <i>Escherichia coli</i> (STEC) and <i>stx</i>-negative <i>E. coli</i> O157 (non-STEC) cases in four nurseries in two neighbouring towns in south-east Scotland, with epidemiological links through shared management, staff or other visits. There were 57 confirmed cases between July and October 2022. Nurseries voluntarily closed to support outbreak control. Subsequent whole genome sequencing identified two separate, unlinked outbreaks of <i>stx</i>2a-positive <i>E. coli</i> O157:H7 (19 confirmed cases in Nursery 1) and <i>stx-</i>negative <i>E. coli</i> O157:H39 (17 confirmed cases in Nursery 2). Smaller numbers of six additional STEC and <i>E. coli</i> O157 (non-STEC) strains were identified in the four nurseries. Five children from Nursery 1 who tested positive for <i>stx</i>2a-positive <i>E. coli</i> O157:H7 required hospitalisation, one of whom developed haemolytic uraemic syndrome. Children with other STEC and <i>E. coli</i> O157 (non-STEC) strains had few or no symptoms. Overall, five of 19 cases with s<i>tx</i>2a subtypes were asymptomatic, compared with seven of nine for <i>stx</i>2f subtypes, and 14 of 25 for <i>stx</i>-negative subtypes. Given the findings in this setting, further information on the prevalence of asymptomatic STEC carriage by strain, age and geography, and in other contexts, will support understanding and risk management of future outbreaks.