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Syndromic Approach to West Nile Virus --- The Netherlands, 2002--2003

Cees van den Wijngaard, L. van Asten, B. Rockx, W. van Pelt, G. Godeke, M. Koopmans
National Institute for Public Health and the Environment (RIVM), Bilthoven, the Netherlands

Corresponding author: Cees van den Wijngaard, RIVM, Pb 1, 3720 BA, Bilthoven, the Netherlands. Telephone: 31-0-30-27429-10; Fax: 31-0-30-274-44-09; E-mail:

Disclosure of relationship: The contributors of this report have disclosed that they have no financial interest, relationship, affiliation, or other association with any organization that might represent a conflict of interest. In addition, this report does not contain any discussion of unlabeled use of commercial products or products for investigational use.


Introduction: In October 2002, after the West Nile virus (WNV) epidemic in the United States and reports on a changing epidemiology for flaviviruses in Europe, the National Institute for Public Health and the Environment (RIVM) launched a project to study trends in neurologic infectious disease in the Netherlands.

Objectives: This study used a prospective syndromic approach to initiate surveillance for WNV by monitoring patients with unexplained meningoencephalitis and conducting additional laboratory testing for WNV during June--November, when mosquitoes are active in the Netherlands.

Methods: Because neurologic illness of possible viral etiology other than acute flaccid paralysis is not notifiable in the Netherlands, RIVM 1) examined medical registration data from 103 (99%) of 104 Dutch hospitals covering 16 million persons to identify all discharge diagnoses for unexplained meningoencephalitis; 2) examined data from 11 laboratories covering 2.5 million persons to study trends in submissions of cerebrospinal fluid (CSF) for virologic testing for common neurotropic viruses (e.g., herpes and enteroviruses); and 3) actively collected CSF samples from six virologic laboratories for further exclusion of WNV infection.

Results: Hospital surveillance for 2002 and 2003 indicated that approximately 500 patients per year had meningitis or encephalitis (unspecified viral or unexplained) diagnosed during June-November. In 2002, of 158 CSF submissions, 137 (87%) tested negative for common viruses; none of these samples had been tested for WNV. Samples that were subsequently collected by RIVM for further WNV testing (150 in 2002, 294 in 2003, and 337 in 2004) tested negative for antibodies to WNV. Because WNV can cause meningoencephalitis, a patient with unexplained meningoencephalitis might be infected by WNV. In 2003, a total of 500 hospital patients received such a diagnosis, but only 294 CSF samples were further tested for WNV. At this level of testing, the probability of detecting WNV meningoencephalitis would have been 0.99 if five WNV-caused meningoencephalitis cases had occurred among the 500 hospital patients but only 0.59 if one WNV-caused case had occurred.

Conclusion: No endemic WNV transmission has been detected in the Netherlands since 2002. CSF submission data for 2003 and 2004 and hospital discharge data for 2004 are not yet available. On the basis of available data, no substantial endemic transmission of WNV occurred. However, a limited outbreak of WNV meningoencephalitis might not be detected. Ruling out WNV as an etiologic agent in all CSF samples when no common pathogen has been detected will improve surveillance.

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Date last reviewed: 8/5/2005


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