have made it a tragedy for many. It is important to analyse this outbreak scientifically in order to learn from this unique event and to be prepared for comparable infections in the future. In particular, all the steps regarding detection of cases, diagnostic procedures, identification of vehicle and origin, and infection control measures, all the way to therapy, should be reflected carefully. Usually, even experienced physicians encounter only a few cases of EHEC-induced HUS in adults in their whole career. Therefore, the large number of cases in Germany represents a valuable source of information for future epidemics.

This manuscript summarises the work of the HUS investigation team of the Robert Koch Institute (RKI) and gives an overview of the work done by the colleagues in the Department of Infectious Disease Epidemiology at the RKI (G. Krause, C. Frank, D. Werber, K. Stark, and U. Buchholz), the Department for Infectious Diseases (M. Mielke and A. Fruth), and the RKI-Consultant Laboratory for HUS/EHEC at the University of Münster (H. Karch). Many additional colleagues were involved.

Epidemic Profile and Development of the Outbreak

The extent of the outbreak becomes apparent by comparison with the average annual numbers of EHEC cases or HUS in Germany. In previous years about 1,000 patients per year were identified, with a median age of about 5 years. Of these patients about 70 per year developed HUS, with a median age of about 2 years (Frank et al., 2011a). In the outbreak from May to September 2011, approximately 3,000 EHEC cases were observed with a median age of 46 years, 58 percent of those patients were female, and 18 deaths were observed among the EHEC patients (0.6 percent). An additional 855 EHEC patients who developed HUS were identified (Frank et al., 2011b). This represents more than 20 percent of the total number of patients (3,842). The large majority of these patients were adults, the average age was 42 years, 68 percent of the HUS cases were female, and 35 deaths were observed among the HUS patients (4.1 percent). The total death toll was 53 patients (Figure A1-1).

Analysis of the incidence of HUS by the likely county of infection revealed that northern Germany was mainly affected. The same is true for cases with travel history; also for these patients the county of residence at the time of infection was northern Germany. Most cases were observed in the states of Schleswig-Holstein, Mecklenburg-Western Pomerania, Hamburg, Bremen, and Lower Saxony. Later in the epidemic, cases were found in all of the 16 German states. The incidence in the five northern German states varied from 1.8 to 10 cases per 100,000 persons. All other states had incidence rates with less than 1 case per 100,000 persons (Frank et al., 2011b; Wadl et al., 2011).

A substantial number of EHEC or HUS cases occurred also internationally during this time, particularly in the European Union, but also a few cases in the United States and Canada. Particularly affected was Sweden with 35 EHEC

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