Abstract
BackgroundAn outbreak of Legionnaires’ Disease took place in the Swedish town Lidköping on Lake Vänern in August 2004 and the number of pneumonia cases at the local hospital increased markedly. As soon as the first patients were diagnosed, health care providers were informed and an outbreak investigation was launched.MethodsClassical epidemiological investigation, diagnostic tests, environmental analyses, epidemiological typing and meteorological methods.ResultsThirty-two cases were found. The median age was 62 years (range 36 – 88) and 22 (69%) were males. No common indoor exposure was found. Legionella pneumophila serogroup 1 was found at two industries, each with two cooling towers. In one cooling tower exceptionally high concentrations, 1.2 × 109 cfu/L, were found. Smaller amounts were also found in the other tower of the first industry and in one tower of the second plant. Sero- and genotyping of isolated L. pneumophila serogroup 1 from three patients and epidemiologically suspected environmental strains supported the cooling tower with the high concentration as the source. In all, two L. pneumophila strains were isolated from three culture confirmed cases and both these strains were detected in the cooling tower, but one strain in another cooling tower as well. Meteorological modelling demonstrated probable spread from the most suspected cooling tower towards the town centre and the precise location of four cases that were stray visitors to Lidköping.ConclusionsClassical epidemiological, environmental and microbiological investigation of an LD outbreak can be supported by meteorological modelling methods.The broad competence and cooperation capabilities in the investigation team from different authorities were of paramount importance in stopping this outbreak.
Highlights
An outbreak of Legionnaires’ Disease took place in the Swedish town Lidköping on Lake Vänern in August 2004 and the number of pneumonia cases at the local hospital increased markedly
Eighty-four were admitted to the hospital in Lidköping during the weeks of the outbreak, which was overall a more than a twofold increase in the number of hospitalized patients with pneumonia, most of them radiologically visualized, in comparison with previous years
The cooling tower was of the open-type without a dripeliminator where condensed steam was released with a very high concentration of L. pneumophila sg 1, subtype Benidorm belonging to the virulent monoclonal antibodies (MAb) 3–1 group
Summary
An outbreak of Legionnaires’ Disease took place in the Swedish town Lidköping on Lake Vänern in August 2004 and the number of pneumonia cases at the local hospital increased markedly. It is estimated that 0.5 - 5% of community acquired pneumonia is due to Legionella infection [2,3]. Numerous community-acquired outbreaks have been published, mostly emanating from cooling towers. Meteorological factors such as wind, humidity, and dispersion modelling were discussed [4,5,6,7,8,9,10,11]. The only other known Swedish outbreak from a cooling tower occurred in a hot and humid summer [4]
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