Diagnostic testing for Legionnaires' disease in the Netherlands between 2007 and 2009: a possible cause for the decline in reported Legionnaires' disease patients.

Legionnaires' disease (LD) is an acute pneumonia caused by the inhalation or aspiration of aerosols contaminated with the Legionella bacteria. In the Netherlands, around 300 LD cases per year were reported between 2000 and 2008, but in 2009, the number dropped to 251, which was the lowest number in the previous 5 years of surveillance. We investigated if this decrease could be explained by the number of performed Legionella diagnostic tests in this year. We analyzed the number of tests performed between 2007 and 2009 in three large microbiological laboratories in different geographical regions in the Netherlands. Our data showed that there was no decrease in the number of patients for whom a diagnostic test for Legionella was performed in this period. These results are not in line with our hypothesis that the decrease in reported Legionella pneumonia patients in 2009 would be due to a decrease in patients for whom a diagnostic test was performed. We conclude that it is more likely that other factors such as the influence of weather patterns might explain the sudden drop in reported Legionella pneumonia patients in 2009 compared to the previous years, and it would be interesting to investigate this for the period described.