Een legionella -epidemie onder bezoekers van een beursin Bovenkarspel 1. Beschrijving van de epidemie 2. Patient-controle onderzoek naar de bron

摘要

In March 1999 an unexpected high number of patients with atypicalpneumonia was hospitalised in Hoorn. The diagnosis legionellosis wasconfirmed by the urine antigen test. An exploratory case control studysuggested that the source should be found at the Westfriesian Flora (WF,a yearly exhibition organised from February 19 to 28 at Bovenkarspel withabout 80,000 visitors). Subsequently, a national alert was sent outthrough the media, a case register was established and the outbreakinvestigation was started. This report describes the outbreak using thedata in the case-register, as well as the results of the case-controlstudy. Separate reports are published on the environmental investigationand the cohort study.The case-register includes 318 persons among whom133 confirmed cases and 55 probable cases. The median age was 66 years(range 20-91 years), the male:female ratio 1.4. The day of onset wasbetween February 25 and March16; the longest incubation period was 19days, considerably longer than reported in literature. Except for onepatient with COPD and recurrent pneumonia, all cases -regardless ofearlier visits- visited the Flora on February 23 or later. Theattack-rate per 100,000 visitors according to day of visit increased from1.1 (February 21) to 53.4 (February 27). The highest number ofhospital admissions was on March 12, the day of the national alert. Thehospitalisation frequency was 86.7%; 19.1% of the patients requiredartificial respiration. Twenty-nine persons who were included in thecase-register and had visited the Flora died, among whom 17 confirmed andfour probable cases. The case-fatality rate for confirmed and probablecases amounted to 0,11.The findings strongly indicate that one or moresources of infection were contaminated with Legionella spp on or beforeFebruary 21; subsequently multiplication of the agent in these sourcesled to increasing concentrations of Legionella spp in the exhibitionhalls. Three variables were included in the final models of all threemeasurements (questionnaire, sketches and floorplan) in the case-controlstudy: the number of hours visiting the WF (OR 1.6, 2.2 and 1.8 respectively), smoking (OR 3.8, 6.0 and 10.9 respectively) and thenumber of hours visiting the consumer products fair with equal durationof the total visit (OR 0.7, 0.5 and 0.5 respectively). Age wasincluded in the final model of questionnaire data (OR 1.1) and offloorplan data (OR 1.2). Halting at the whirlpool in hall 3 was anexplanatory variable in the model of the sketch data (OR 2.6) as well asthe floorplan data (OR 5.8). The variable 'whirlpool' was alsoassociated with legionellosis in the analysis of questionnaire data. However, no distinction was made between the two whirlpools and the twobubblemats.Both from these results and from the results of theenvironmental investigation and the cohort study, it was concluded withreasonable certainty that the whirlpool in hall 3 was the source of theoutbreak.