An article recently published in Eurosurveillance provides an analysis of a 2021 multinational foodborne illness outbreak investigation involving imported melons contaminated by Salmonella Braenderup in order to exemplify collaboration across countries, rapid information sharing, and harmonized data collection, including key lessons learned, to potentially form the basis of standard practices in multinational outbreak investigations.
At present, there is no standard approach to the often resource-intensive multinational outbreak investigations for food and waterborne diseases. However, the 2021 outbreak of S. Braenderup linked to melons shows that collaboration across countries, especially when supported by a global organization, can help solve outbreaks more rapidly through the sharing of genomic data and strengthening of conclusions where findings differ. Organized cooperation can also help conserve resources by distributing the workload between countries.
The outbreak exemplified in the article was first detected on May 3, 2021, when Denmark notified the European Center for Disease Prevention and Control (ECDC) of a genomic cluster of S. Braenderup ST22 via the Epidemic Intelligence Information System, which is now called the European surveillance portal for infectious diseases (EpiPulse). On the same day, Belgium reported an increase in S. Braenderup cases, including four isolates that had been sequenced and clustered genetically with the Danish outbreak strain.
Two days later, on May 5, 2021, the UK reported an outbreak of a specific single nucleotide polymorphism (SNP) single-linkage cluster of S. Braenderup through the European Commission’s Early Warning and Response System (EWRS). By May 20, 2021, more than 200 S. Braenderup isolate sequences clustering within 0–6 allelic differences with the Danish outbreak strain had been reported from nine countries (later 12) in the EU/European Economic Area (EEA), the UK, Canada, Switzerland, and the U.S. On May 12, 2021 ECDC declared a multinational outbreak and initiated coordinated outbreak investigations by holding the first international coordination meeting with affected countries on May 12, 2021.
With support from the affected countries in the EU/EEA (Austria, Belgium, Czechia, Denmark, Finland, France, Germany, Ireland, Luxembourg, the Netherlands, Norway, Sweden, and the UK), ECDC established an international team to investigate the outbreak and identify the source. Under ECDC coordination, affected countries participated in international coordination meetings where epidemiological and laboratory findings, developments in national investigations, and working hypotheses were discussed, and information was shared by each country’s outbreak investigation team.
Prior to hosting the first international coordination meeting, on May 10, 2021, ECDC sent out the first official communication to the affected EU/EEA countries and the UK via an outbreak notification summary, and after the first meeting, two follow-up meetings were convened on May 19 and June 15, 2021.
To initiate a standard data collection, ECDC sent out a line list template, and the affected countries contributed to the shared line list with case data. As the outbreak continued to evolve, an update of the outbreak notification summary was distributed to affected EU/EEA countries and the UK on June 14, 2021. ECDC also shared the first descriptive epidemiology analysis with affected countries on July 6, 2021. On August 18, 2021, ECDC facilitated the fourth international coordination meeting with affected countries, summarizing lessons learned from the outbreak.
A common case definition was agreed upon, and countries shared their raw whole genome sequencing (WGS) data with ECDC for a centralized WGS analysis, which enabled a standardized cluster analysis methodology where a common case definition for a laboratory confirmed case could be verified. ECDC monitored the development of the outbreak in close collaboration with the European Food Safety Authority (EFSA).
Initial questionnaires from case interviews in the UK and Denmark, along with preliminary epidemiological studies in the UK, suggested fruits, specifically melons (other than watermelons) as a possible vehicle of infection. “Small melons” formed a working hypothesis. A common questionnaire template, modified from the original questionnaires, was developed with a focus on melons and distributed to all affected countries to support their case interviews.
Additionally, the UK, Denmark, and Germany performed country-specific case-control studies with the main objective of exploring the hypothesis of melons or other similar types of fruits being the vehicle of infection in the outbreak. Other countries shared sequencing and case interview data to contribute to ongoing investigations. To take into account national differences, data from each country were interpreted separately and fed back to international meetings coordinated by ECDC to be discussed in the context of further epidemiological, microbiological, and traceback evidence.
Traceback activities involved supply chain investigations and supermarket loyalty card information analysis in the UK, and receipt and credit card documentation in Denmark. The UK also conducted sampling and testing of 200 Galia melons (also known as honeydew melons). Each country involved in the outbreak conducted its own phylogenetic analyses.
The melons were identified as being imported from Honduras. The international-level coordination between countries facilitated by ECDC was key in concluding the source of the outbreak in a quick and effective manner, especially given the absence of confirmatory microbiological evidence of Salmonella in melons and the approaching end of seasonal supply of Galia melons from Honduras. Having a supranational coordinating function improved national investigations in terms of information-sharing, coordination, and consistency in methodology. Additionally, the international coordination spared resources since it removed the need for full investigations in all affected countries. This was particularly important given that the outbreak occurred during the COVID-19 pandemic.