A multi-country outbreak of Salmonella enterica subspecies enterica serovar Agona (S. Agona) is under investigation in the European Union (EU), with cases retrospectively identified back to 2014. Overall, 147 outbreak cases have been reported by five EU countries: 122 cases since 1 January 2017, and 25 historical cases between 2014 and 2016. The United Kingdom is reporting most outbreak cases (129), with Finland (15), Denmark, Germany and Ireland (one case each) reporting the other cases. The Irish case was probably infected while visiting the United Kingdom.
The outbreak was first detected in the United Kingdom using whole genome sequencing (WGS). All S. Agona isolates from the five countries are genetically close with a maximum difference of 2 alleles from any other single isolate by core genome multilocus sequence typing scheme (cgMLST) using the ECDC Enterobase pipeline. The cases peaked in April 2017 and 2018. The close genomic relationship and the distinct seasonal spring peaks suggest that cases are part of an intermittent common source outbreak.
Seventeen S. Agona food isolates from 2018, detected in the United Kingdom, were found to be closely genetically related to the human strains. The food isolates were from cucumbers sampled during processing before and after washing (11 isolates) and ready to eat (RTE) food products containing cucumbers (six isolates). The contaminated food isolates were sampled in the United Kingdom at four plants owned by Company A and a Company C plant.
At present, there is insufficient epidemiological information available on the consumption of contaminated products by humans to support the microbiological evidence provided by the isolation of the outbreak strain in food. The epidemiological investigations in the other affected countries did not generate any strong hypothesis about the vehicle or source of infection.
Although the cucumbers used in all final contaminated products originated from Spain for a limited period (from November 2017 to April 2018), no connection between supply chains was identified: primary producers of cucumbers were different (producers A and B), and cucumbers were delivered to different processing companies through different distributors in the United Kingdom. The laboratory results for Salmonella in all cucumber samples, taken either at primary production level in Spain or during distribution to/within UK, were negative.
Based on the information available, the microbiological evidence suggests RTE products containing cucumbers as a possible vehicle of infection but so far it has not been possible to identify the specific point in the production chain where the contamination occurred.
Further investigations along the food chain are needed to identify the source of contamination. These should include collection of information about various production and processing stages for the RTE products implicated in this event, as well as thorough sampling and testing.