Epidemiology, prevention and control of Legionnaires' disease in Europe

Abstract

Legionnaires’ disease (LD) is a water-borne infection cause by Gram-negative bacteria Legionella spp. with virtually no person-to-person transmission. The clinical presentation is a severe pneumonia with a case fatality of approximately 10%. Known risk factors include increasing age, chronic lung disease and various conditions associated with immunodeficiency. Most cases are community-acquired and sporadic. LD is notifiable in the European Union (EU) and European Economic Area (EEA). LD incidence is thought to be increasing in Europe and the USA for reasons not fully understood, including climate change, changing demographics and improved surveillance. The overarching aim of this thesis was to explore various aspects of LD epidemiology, prevention and control using surveillance data.

In study I, we retrieved travel-associated Legionnaire’s disease (TALD) surveillance data for 2009 from the European Surveillance System, and tourism denominator data from the Statistical Office of the European Union. We estimated the risk for TALD in several European countries and highlighted potential under-ascertainment of LD in some countries. To confirm and generalize findings of studies performed at regional or national level, we investigated the effect of temperature, rainfall, and atmospheric pressure on short-term variations in LD notification rate in Denmark, Germany, Italy, and the Netherlands in Study II. We fitted Poisson regression models to estimate the association between meteorological variables and the weekly number of community-acquired LD cases. We found that the higher risk was associated with simultaneous increase in temperature and rainfall. These findings contributed to the growing evidence supporting a possible impact of climate change on LD incidence. In Study III, we investigate the actors associated with LD recurrence in hotels. We conducted a retrospective cohort analysis and use survival analysis methods to estimate the association between hotels characteristics and the occurrence of a further case. We found that hotel size and previous association with multiple cases were predictors of the occurrence of a further case. This study also highlighted weaknesses in data collected in the surveillance scheme. In Study IV, we used a large sample of LD over a 10-year period to look more closely at healthcare-associated (HCA) LD. We found that HCA LD cases are responsible for a major part of LD and differ from community-acquired cases in many aspects, including demographics, causative strains and outcome.

Taken together, the findings support the use of surveillance data for research purposes. They shed light on some epidemiological aspects of LD and inform the surveillance system for possible improvements.