Legionellosis Cases Continue to Increase in Switzerland


The number of legionellosis cases in Switzerland has increased five-fold over the past 20 years. A study by the Swiss Tropical and Public Health Institute (Swiss TPH) published today in the International Journal of Hygiene and Environmental Health analysed case numbers from 2000 to 2020 and determined the potential impact of the COVID-19 pandemic on reporting. The researchers also found that the summer peaks of legionellosis have become more pronounced in recent years. Switzerland has one of the highest legionellosis incidence in Europe.

Legionella pneumophila bacteria

3D image of Legionella pneumophila bacteria in the human lung. (Photo: 123rf)

Legionellosis is caused by the Legionella bacteria and has two different forms, the more severe and most widespread one being called Legionnaires’ disease. The bacteria are transmitted from water or soil and can lead to a severe form of pneumonia. While the disease can be treated with antibiotics, it still proofs fatal in approximately 10% of cases.[1] In Switzerland, Legionnaires’ disease is a notifiable disease and case numbers are monitored by the Federal Office of Public Health (FOPH). Cases need to be reported both by the diagnostic laboratory and the physician.

Since 2000, the legionellosis notification rate has steadily increased in Switzerland, from approximately 140 cases per year in the early 2000s to about 500 per year in the period between 2016 and 2020. The upwards trend peaked in 2018 and plateaued thereafter, while the year 2020 saw a decline in cases, likely linked to the COVID-19 pandemic. These are the findings of a study published today by Swiss TPH researchers in the International Journal of Hygiene and Environmental Health.

Impact of COVID-19 on legionellosis reporting

The researchers investigated the impact of the COVID-19 containment measures during the first wave of the pandemic. “We expected a certain increase in cases after the lockdown due to stagnant water in unused buildings such as hotels or fitness centres – this however could not be observed,” said Fabienne Fischer, PhD student at Swiss TPH and first author of the paper. The authors also concluded that diagnostic laboratories reported in similar frequency during the pandemic despite the obvious focus on COVID-19 suggesting a robust surveillance system.

Seasonal peaks more pronounced

The study highlights how the summer peaks of legionellosis become ever more pronounced over the years (see figure 1). “This is particularly worrisome as we can expect climate change to further exacerbate these summer peaks,” said Daniel Mäusezahl, principal investigator of the study and head of the Household Health Systems group at Swiss TPH.

Another recent study on legionellosis by Swiss TPH published in Swiss Medical Weekly [2] suggests that case numbers may be underestimated as diagnostic tests are currently done in hospitals and not by family doctors, resulting in mild cases being treated but not diagnosed and thereby largely going unnoticed. “We need to do more research to better understand the prevalence of the disease in Switzerland, but also on how this bacterium behaves in the environment and by which mechanisms it is transmitted to humans,” said Mäusezahl. “This is particularly important as Switzerland continues to have one of the highest rates of legionellosis in Europe. The latest data from 2021 also seems to confirm the upward trend.”

Figure 1 shows the notification rate of legionellosis per 100,000 population in Switzerland for all calendar months for the years 2000 to 2020. Throughout all years, cases occur more often in the summer months. In recent years (2016-2020 in orange), this effect has become much more visible.

Comprehensive research underway

Swiss TPH is involved in several research projects on legionellosis in Switzerland. Researchers investigate the impact of weather conditions and air pollution through spatial analysis (GeoLEGIO project) and analyse risk factors and exposure sources for Legionella infections (SwissLEGIO). The SwissLEGIO project is a nationwide case-control and molecular source attribution study; recruitment of study participants starts in summer 2022. Together with the Eawag-led consortium on “Legionella Control in Buildings (LeCo)”, Swiss TPH analyses the genome of the Legionella bacteria (using “whole genome sequencing”) from the environment and from lung secretion of patients with Legionnaires’ disease. This allows comparing the bacteria found in the environment with bacteria found in the patients’ lung.

Funders and partners include the Federal Office of Public Health (FOPH), the Federal Food Safety and Veterinary Office, the Federal Office of Energy, Eawag, the University Hospital Basel, the Swiss National Reference Centre for Legionella, the Lucerne University of Applied Sciences and Arts, and the Cantonal Laboratory of Zurich and a network of 16 other hospitals.

About legionellosis

The Legionella bacterium became known as the cause of an outbreak of severe pneumonia at a convention of the American Legion in Philadelphia, US in 1976 – hence the name “Legionnaires’ disease” “and legionellosis”.

The most common form of transmission of Legionella is the inhalation of contaminated aerosols produced, for example, in conjunction with water sprays, jets or mists. Infection can also occur by aspiration of contaminated water, particularly in susceptible hospital patients. The disease is not transmitted from humans to humans.

About the publication

The study was conducted by Swiss TPH in collaboration with the FOPH, funded by the FOPH.

Fischer, FB, Mäusezahl D, Wymann MN (2022). Temporal trends in legionellosis national notification data and the effect of COVID-19, Switzerland, 2000-2020. International Journal of Hygiene and Environmental Health. DOI: 10.1016/j.ijheh.2022.113970