Legionnaires' disease from a cooling tower in a community outbreak in Lidköping, Sweden- epidemiological, environmental and microbiological investigation supported by meteorological modelling

Abstract

Background: 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. As soon as the first patients were diagnosed, health care providers were informed and an outbreak investigation was launched.

Methods: Classical epidemiological investigation, diagnostic tests, environmental analyses, epidemiological typing and meteorological methods.

Results: Thirty-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.

Conclusions: Classical 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.

Figure 1

Epidemic curve of the Legionella outbreak in Lidköping 2004.

Figure 2

Areas that the cases mainly had visited within two weeks before onset of symptoms in the Legionella outbreak in Lidköping 2004. The Market place is the central part in town. The four stray visitors points of visit are indicated.

Figure 3

a and 3b demonstrate the modelled main concentration plume from the suspected cooling tower as an average during the first and the last half part of the Legionella outbreak in Lidköping 2004, respectively. Bright red color stands for the highest concentration.

Figure 4

a, 4b and 4c demonstrate the modelled main concentration plume from the suspected cooling tower during the short time three of the four stray visitors visited Lidköping during the Legionella outbreak 2004. The fourth stray visitor spent two hours within 50 meters from the cooling tower. Bright red color stands for the highest concentration. Figure  4a, Stray visitor 1 August 04. Figure 4b, Stray visitor 2 August 05. Figure 4c, Stray visitor 3 August 09.