Legionnaires’ disease on the rise in Switzerland: A denominator-based analysis of national diagnostic data, 2007-2016

Purpose The risk of Legionnaires’ disease (LD) is suggested to increase, but the global burden of disease is unknown due to lack of appropriate diagnosis and surveillance systems. In Switzerland, the number of LD cases, captured by the National Notification System for Infectious Diseases, has more than doubled since 2008. The aim of this study is to evaluate disease surveillance data using denominator data i.e. the number of tests performed for Legionella . spp. Methods We collected the testing data for Legionella spp. of 14 Swiss diagnostic laboratories and calculated the positivity rate, which is the rate of the number of positive tests to the number of tests performed. Results The number of positive tests increased proportionally to the number of tests performed; hence the positivity remained stable. However, the cause of the increase in test volume is unclear and has a large impact on the interpretation of the positivity curve. Further, the test outcome was found to be dependent on regional determinants and the diagnostic method applied. Conclusion In concert with the lack of understanding if and at which stage LD is considered in current case management of pneumonia patients, it is difficult to interpret heterogeneities in incidence or underestimation in Switzerland.