Trace-back and trace-forward tools developed ad hoc and used during the STEC O104:H4 outbreak 2011 in Germany and generic concepts for future outbreak situations

Abstract

The Shiga toxin-producing Escherichia coli O104:H4 outbreak in Germany in 2011 required the development of appropriate tools in real-time for tracing suspicious foods along the supply chain, namely salad ingredients, sprouts, and seeds. Food commodities consumed at locations identified as most probable site of infection (outbreak clusters) were traced back in order to identify connections between different disease clusters via the supply chain of the foods. A newly developed relational database with integrated consistency and plausibility checks was used to collate these data for further analysis. Connections between suppliers, distributors, and producers were visualized in network graphs and geographic projections. Finally, this trace-back and trace-forward analysis led to the identification of sprouts produced by a horticultural farm in Lower Saxony as vehicle for the pathogen, and a specific lot of fenugreek seeds imported from Egypt as the most likely source of contamination. Network graphs have proven to be a powerful tool for summarizing and communicating complex trade relationships to various stake holders. The present article gives a detailed description of the newly developed tracing tools and recommendations for necessary requirements and improvements for future foodborne outbreak investigations.

FIG. 1.

Scheme of tracing strategies: (1) Disease clusters are identified (by public health authorities). (2) Trace-back is performed in order to identify common nodes of different clusters (straight solid arrow). (3) Trace-forward is performed in order to identify yet unknown or potential clusters (straight dotted arrow). This strategy is usually followed within the context of an ongoing process that involves company audits, local sampling, and further trace-back and trace-forward investigations.

FIG. 2.

Database structure. The database consists of five tables that are relationally interconnected by the gray shaded items.

FIG. 3.

Combined network of all relevant fenugreek seed and sprout supplies. Supplier network of the STEC O104:H4 outbreak in 2011; results of combined forward and backward tracing of the supply chain. Here, all the companies are represented that have come in contact with the suspected batch of seeds or produced sprouts (created with the R-Package “network,” available at http://statnet.org) (Butts, 2008).

FIG. 4.

Representation of the delivery quantities and routes of all examined seeds (six varieties, including fenugreek). The rectangles contain anonymized names of companies and provide distributors, producers, and retailers (foreign companies are represented in color). The delivery quantities vary between a minimum of 50-g pack and a maximum of 15 tons—indicated by the thickness of the arrows. The most important spots in the network are circled and colored: horticultural farm in Lower Saxony (yellow), French retailer (green), German importer (blue), and Egyptian exporter (red) (created with graphviz, available at www.graphviz.org).

FIG. 5.

County-based geographical projection of the known distribution of sprouts from the horticultural farm in lower Saxony using Google Earth. The horticultural farm in Lower Saxony is located in the county shown in light gray. Sprouts produced there were delivered into the counties shown in white.