A community outbreak of Legionnaires' disease: evidence of a cooling tower as the source

Abstract

A community outbreak of Legionella pneumonia in the district of Cerdanyola, Mataró (Catalonia, Spain) was investigated in an epidemiological, environmental and molecular study. Each patient was interviewed to ascertain personal risk-factors and the clinical and epidemiological data. Isolates of Legionella from patients and water samples were subtyped by pulsed-field gel electrophoresis. Between 7 August and 25 August 2002, 113 cases of Legionella pneumonia fulfilling the outbreak case definition criteria were reported, with 84 (74%) cases being located within a 500-m radius of the suspected cooling tower source. In this area, the relative risk of being infected was 54.6 (95% CI 25.3-118.1) compared with individuals living far from the cooling tower. Considering the population residing in the Cerdanyola district (28,256 inhabitants) as a reference population, the attack rate for the outbreak was 399.9 cases/100,000 inhabitants, and the case fatality rate was 1.8%. A single DNA subtype was observed among the ten clinical isolates, and one of the subtypes from the cooling tower matched exactly with the clinical subtype. Nine days after closing the cooling tower, new cases of pneumonia caused by Legionella ceased to appear. The epidemiological features of the outbreak, and the microbiological and molecular investigations, implicated the cooling tower as the source of infection.