In Switzerland, the number of Legionnaires’ disease cases more than doubled since 2008 to about 600 cases in 2019. The cause of this increase is not well understood. The impact of testing volumes and positivity rates on case numbers has been recognised well before the public discussion around the COVID-19 pandemic. Therefore, in 2017, the Federal Office of Public Health together with the Federal Food Safety and Veterinary Office had mandated a study on the testing volume for Legionnaires’ disease between 2007 and 2016.
The study showed that the testing volume increased greatly in the same period and in parallel to the increasing case numbers. Consequently, the positivity rate remained constant. Why we observe an increase in test volume remains unclear from the available data. This in turn hampers the interpretation of the positivity curve. Additionally, the test outcome was found to be dependent on regional determinants, and the diagnostic method applied.
Further research at the Swiss TPH deals with the spatial and temporal distribution of Legionnaires’ disease in Switzerland and the causes of its spread.
The analyses of the notification data provide an important basis for research on Legionnaires’ disease and the causative bacteria, which are now being intensively addressed in the LeCo project.