Among the 31 symptomatic, confirmed cases, 22 reported bloody diarrhea and 18 were admitted to the hospital. Overall, 19 confirmed and probable cases developed hemolytic uremic syndrome (HUS). Ages of HUS patients ranged between 12 weeks and 15 years with a median age of three years, although the majority (74 percent) were five years old or younger.

Epidemiological Evidence

Interviews of patients revealed a dried fruit multipack product marketed for children as a commonly consumed item. Product traceback identified a singular supplier in a non-European country that exported to the five major UK retailers that sold the multipack to the outbreak patients. However, all food samples that were tested yielded negative results for STEC.

Still, epidemiological and food chain data pointed to the dried fruit product as the most plausible vehicle of infection. The temporal distribution of cases and the product’s long shelf life also allowed for a reasonable level of investigative evidence to be compiled by supply chain tracing.

Difficulty of Confirmation and Importance of Weight-of-Evidence Frameworks

Although microbiological testing of the food failed to detect the pathogen, the infectious dose of STEC is low, so contamination of the product may have been present but below the limit of detection. There are also no selective enrichment methods for non-O157 STEC. Moreover, specific batch numbers could not be obtained for testing, limiting the ability to conduct targeted testing to enhance the likelihood of obtaining microbiological evidence.

The researchers highlight the importance of relying on multi-source weight-of-evidence frameworks, such as the framework offered by the European Food Safety Authority (EFSA), which enable the classification of vehicles of infection in outbreaks as “strong” even in the absence of testing confirmation. 

The Presence of Pathogenicity Genes

Despite the relatively small number of cases in this outbreak compared to other recent foodborne STEC outbreaks in the UK, it posed a substantial public health concern due to the high proportion of patients who developed HUS. The high rate of HUS was likely due to the number of young children involved and the pathogenicity of the outbreak strain; PCR revealed the strain had stx2a and eae, which is the combination of pathogenicity genes most frequently associated with the most severe clinical outcomes.