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Investigation into de source of the legionellose epidemic after the Westfriese Gardenshow at Bovenkarspel

Onderzoek naar de bron van een epidemie van legionellose na de Westfriese Flora in Bovenkarspel

Synopsis

In March 1999 results from an exploratory case-control study indicated that the most probable origin of a large epidemic of legionnaire's disease had to be directly located at or in the surroundings of the Westfriese Flora (WF) in Bovenkarspel (Province of Noord-Holland, Netherlands). The WF is a yearly exhibition of flowers, and agricultural and consumer products, which was held in 1999 from 19 to 28 February. After inspecting the premises and making an inventory of all exhibits, a list of potential sources was drawn up. The water supply system and all the still available equipment using water were sampled. A risk assessment was done for each potential source by interviewing exhibitors. In total, sixteen products were found potentially hazardous: two whirlpools, two bubblemats (perforated rubber mats meant for the bottom of a water-filled bathtub through which air can be blown at high speed), eleven fountains and a sprinkler installation. Legionella pneumophila was isolated from two whirlpools and a sprinkler installation. With the use of molecular-biological techniques (PFGE, ERIC/REP-PCR and AFLP), three different genotypes were identified. These were B-1, B-2 and B-3 (B for Bovenkarspel). To enable comparison of these isolates with patient isolates, all microbiology laboratories in the Netherlands collaborated in collecting patient isolates related to the epidemic. In total 29 isolates were collected, 28 of which were genetically identical to B-1; the other one was genetically identical to B-2. Through the risk assessment based on the reported use and estimating the degree of contamination from the microbiological investigation of the three culture-positive products, the most probable source contributing the most to the spread of L. pneumophila was found to be the whirlpool located in hall 3 of the WF premises. It is not possible to rule out the contribution made to spreading by the whirlpool in hall 4, whereas the sprinkler's contribution to spreading is not probable. The contribution to spreading by the fountains and bubblemats is also unlikely. Although Legionella spp was not isolated from the water supply system, the organism was likely introduced into the three products from this system. These results were confirmed by the outcome of a case-control study and a cohort study, both designed to pinpoint the exact location of exposure to Legionella spp on the WF premises. They showed the whirlpool in hall 3 to be the most probable source of infection (reported separately).
 

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