An Explosion of Salmonella Infections in 2003 in the Netherlands:
Hot Summer or Side Effect of the Avian Influenza Outbreak?

Source: Wilfrid van Pelt, et al. - National Institute of Public Health and the Environment (RIVM), Bilthoven, the Netherlands
(dated 16/06/2004)

In June 2003, the Dutch National Salmonella Centre reported a significant excess isolation rate of Salmonella Enteritidis (SE) as compared to former years in most regional public health laboratories. At the end of 2003, this amounted to an extra 540 laboratory confirmed cases for the whole of the Netherlands, which would mean an estimated 7500 extra cases of gastro-enteritis caused by SE in the general population, which is an increase of 50%. The hot summer could not explain the findings. Strong evidence is found that the import of contaminated eggs is the most probable reason for the excess.

Introduction

In June 2003, the Dutch National Salmonella Centre reported a significant excess Salmonella isolation rate compared to former years in most regional public health laboratories (figure 1). Since Mai 2003 the number of laboratory confirmed cases clearly increased above expected(1) and from June up to November and again since the beginning of 2004 above the level of tolerance (a measure for the significance of an excess). This solely concerned Salmonella Enteritidis (SE), not S. Typhimurium (ST), nor other S. serotypes or Campylobacter spp.. In this article, we try to indicate the possible role of the hot summer in the 2003 excess as compared to that of imports of (contaminated) eggs.

Materials and methods

The data are from the National Salmonella Centre (NSC) and the National and European Reference Laboratory (CRL) for Salmonella at RIVM that does the sero- and phage-typing of isolates taken from humans (mostly sent by regional public health laboratories covering 64% of the Dutch population) and animals, from food, animal food and from the environment(2). The sensitivity to various antibiotics has been quantitatively determined by the minimal inhibitory concentration (MIC) at CIDC-Lelystad(3).

Results

The excess isolation rate of SE since Mai 2003 amounted to an extra 540 laboratory confirmed cases for the whole of the Netherlands at the end of 2003 (figure 2, taking into account the 64% coverage of the laboratory surveillance). This is 50% higher than found in former years. Figure 2 shows that the explosion of cases concerned SE only. According to our epidemiologic study in 1999(4), 540 extra laboratory confirmed cases would mean an estimated 7500 extra cases of gastro-enteritis caused by SE in the general population. In Denmark, with a comparable laboratory surveillance system as in the Netherlands, it was shown that, compared to controls, an extra 1.5-2.1% of the laboratory confirmed patients dies within one year probably due to the infection (5). This would mean that the 2003 excess S. Enteritidis infections caused 8-11 deaths.

Paper presented at the 5th World Congress on Foodborne Infections and Intoxications held in Berlin, Germany (7-11 June 2004)

Author Details:

Wilfrid van Pelt, Ph.D.
National Institute of Public Health and the Environment (RIVM)
Bilthoven
The Netherlands
E-mail: W.van.Pelt@rivm.nl

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