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. 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  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.”