At the end of September 2005, an outbreak of Salmonella Typhimurium DT104 infections was detected in the Netherlands by the Dutch National Salmonella Centre. From 19 September to 7 November 2005, 165 extra cases of S. Typhimurium DT104 infection were recorded, by far exceeding the usual number for this time period of about 15 cases (Figure). The Dutch national salmonella surveillance relies on the regional public health laboratories, which cover about 64% of the Dutch population, submitting bacterial isolates to the National Institute for Public Health and Environment (Rijkinstituut voor Volksgezondheid en Milieu, RIVM) for sero- and phage typing The number of new cases declined during November and reached normal levels by the end of the month.

Figure. The number of laboratory confirmed Salmonella Typhimurium DT104 infections recorded by the Dutch National Salmonella Centre at the RIVM in the Netherlands, depicted per week and excess patients cumulatively for 2005.

Investigation
The cases were dispersed throughout the Netherlands and males and females were equally represented. The age distribution of cases was shifted towards the ages 6–20 years compared to data from previous years. Data from preliminary interviews with 19 patients using a structured questionnaire suggested beef as the infection source; however, no particular food items stood out as undisputable vehicles.

The majority of the bacterial isolates had the same antibiotic resistance profile, i.e. resistant to amoxicillin, tetracycline, chloramphenicol, sulphamethoxazole and sometimes trimethoprim but sensitive to ciprofloxacin and cefotaxime. A subset of isolates was typed by pulsed-field gel electrophoresis (PFGE) and multiple-locus variable-number tandem-repeats analysis (MLVA). The latter method was recently presented as more discriminatory than PFGE when typing DT104 isolates [1]. The molecular typing indicated clonality of the outbreak isolates and suggested a link to a recent outbreak of S. Typhimurium DT104 in Denmark, linked to beef from Italy [2]. A portion of the incriminated original beef batch (from Italy) has now been traced in Holland and is currently being sampled.

Implications
With 165 laboratory-confirmed cases, this is a large outbreak - even more so considering that this is probably only the tip of the iceberg. In earlier studies in the Netherlands it was found that laboratory-confirmed cases should be multiplied by about 23 to reach the estimated total number of clinical cases due to the infection in the general population (i.e., close to 4000 in the present outbreak). Notwithstanding a possibility for international spread, two Enter-Net postings have not elicited any reports of increased incidence of DT104 salmonellosis from any other European countries apart from the recent outbreak in Denmark.

As salmonellosis is not notifiable in the Netherlands, various permissions have to be obtained before a patient can be contacted, which is why only a small initial group of patients have so far been interviewed with the structured questionnaire. A more specific beef oriented case-control study (120 cases, 480 controls) has been started this week.

Acknowledgements
We thank Bjørn-Arne Lindstedt at the Norwegian Institute of Public Health for the MLVA analyses. We are grateful for the willing assistance and advice from Carolien de Jager, Agnetha Hofhuis, Anneke Westerhof and Yvonne van Duynhoven