Abstract
In July 2018, a large outbreak of Legionnaires’ disease (LD) caused by Legionella pneumophila serogroup 1 (Lp1) occurred in Bresso, Italy. Fifty-two cases were diagnosed, including five deaths. We performed an epidemiological investigation and prepared a map of the places cases visited during the incubation period. All sites identified as potential sources were investigated and sampled. Association between heavy rainfall and LD cases was evaluated in a case-crossover study. We also performed a case–control study and an aerosol dispersion investigation model. Lp1 was isolated from 22 of 598 analysed water samples; four clinical isolates were typed using monoclonal antibodies and sequence-based typing. Four Lp1 human strains were ST23, of which two were Philadelphia and two were France-Allentown subgroup. Lp1 ST23 France-Allentown was isolated only from a public fountain. In the case-crossover study, extreme precipitation 5–6 days before symptom onset was associated with increased LD risk. The aerosol dispersion model showed that the fountain matched the case distribution best. The case–control study demonstrated a significant eightfold increase in risk for cases residing near the public fountain. The three studies and the matching of clinical and environmental Lp1 strains identified the fountain as the source responsible for the epidemic.
Highlights
IntroductionThis infection represents 1.9% of all community-acquired pneumonia cases, 4.0% of hospitalised cases and 7.9% of cases requiring admission to intensive care units [1]
Legionella pneumophila (Lp) is a Gram-negative bacterium responsible for a severe pneumonia namedLegionnaires’ disease (LD)
A probable outbreak-associated case was defined as a person with confirmed or probable LD according to the European Union (EU) case definition [11] with symptom onset between 10 and 31 July 2018, who lived in, or visited, the outbreak area in the 10 days before symptom onset
Summary
This infection represents 1.9% of all community-acquired pneumonia cases, 4.0% of hospitalised cases and 7.9% of cases requiring admission to intensive care units [1]. The case fatality rate of LD ranges from 5% to 30% during outbreaks but can reach up to 50% in nosocomial cases or if antibiotic treatment is delayed [2]. The European Legionnaires’ disease Surveillance Network (ELDSNet) has reported an increase in age-standardised LD notification rates in the period 2011 to 2017 [3]. Infection occurs through inhalation of aerosols produced by contaminated water systems [3]. Outbreaks have been linked to a variety of aerosol-producing devices, such as cooling towers, evaporative condensers and spa pools [5,6]
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