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Correlation of West Nile Virus Infection with Emergency Department Chief Complaints by Using a Passive Syndromic Surveillance Model

John Flaherty, M. Gillam
Evanston Northwest Healthcare, Evanston, Illinois; Northwestern University, Feinberg School of Medicine, Chicago, Illinois

Corresponding author: John Flaherty, Evanstone Northwestern Healthcare, 2650 Ridge Rd., Evanston, IL 60201. Telephone: 847-570-2114; Fax: 847-634-1693; E-mail:


Introduction: West Nile virus infection appeared diffusely in Illinois in 2002, with >800 cases and 63 deaths. This number of confirmed cases was the highest in the nation and resulted in triple the number of deaths of any other state.

Objectives: This study used a passive syndromic surveillance model to analyze emergency department (ED) patient chief complaints of fever and headache, influenza-related symptoms, and viral syndrome, and correlate these data with known West Nile virus cases and with the epidemic curve of confirmed cases in northern Illinois.

Methods: A passive syndromic surveillance system using a computerized patient log was implemented. A retrospective cohort study used structured query language (SQL) queries to search for patient chief complaints of fever and headache, influenza-related symptoms, or viral syndrome. Positive matches were compiled in a graphical and geographic database.

Results: SQL queries revealed a biphasic distribution, with a first peak corresponding to influenza cases during the second week of February and a second unexpected peak during the second week of September 2002 (Figure). Geocoding and frequency analysis matched the confirmed outbreak. A majority of these patients were discharged, and no deaths occurred. IgM serology was positive in 5% of cases. Statistical analysis determined no significant differences in distribution and a coefficient of determination of 0.67.

Conclusion: Passive syndromic surveillance systems can retrospectively detect West Nile virus infection. The system was able to detect an increase in syndromic cases in the ED during a confirmed outbreak of West Nile virus. Further study is needed to quantify this effect. Serologic confirmation will also aid in validation.


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