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Chapter 69 : Legionellosis Outbreak at a Commercial Fair in Kapellen, Belgium, 1999: a Case-Control Study
A Legionnaires' disease outbreak was detected among visitors (50,000) and workers (830) of a commercial fair being held in Kapellen from 29 October to 7 November, 1999. An environmental investigation and a cohort study among stand workers were initially carried out to trace the source of the outbreak. A case-control study was launched in February 2000 to identify the source of the outbreak. All cases (n = 93) were persons attending the fair who developed symptoms of Legionnaires' disease within 2 weeks after their fair visit. Due to time (3 months after the outbreak) and logistical constraints, controls were selected from an available register from a nearby hospital. This register collected information about medical complaints of fair visitors who consulted the hospital, mostly to get reassured about their health status. Data were analyzed with the SPSS 9.0 statistical package. The analysis was repeated on confirmed cases to address the issue of potential misclassification of presumptive and possible cases. Additionally, the analysis was repeated on cases and controls who visited the fair once during the first long weekend (4 days) of the fair to account for a different risk of getting the disease related to the day of the fair visit. A total of 45 (48%) cases were confirmed, whereas 48 cases (52%) were presumptive or possible. The response rate for cases was 74.2% (69 of 93), and for controls was 46.3% (162 of 350). In Legionella outbreaks, timely epidemiological investigations are crucial, particularly to guide environmental investigations.