Legionnaires' Disease in Germany: trends and seasonal variations by exposure category; 2001 – 2016

Introduction:

The differentiation of Legionella infections over time by exposure categories (community-acquired [CALD], travel-associated LD [TALD] and nosocomial LD [NLD]) provides information about seasonal effects of Legionnaire's Disease (LD) and enables tailoring targeted prevention strategies.

Methods:

We analyzed data of reported LD infections in Germany between 2001 and 2016 (n = 9,969). Further, we analyzed data from 4,297 reported LD cases from 2007 – 2016 with known exposure category. We calculated (1) the pooled incidence over time by month, stratified for the three exposure categories and (2) the incidence rate ratio (IRR) for February to December using January as the reference.

Results:

Since 2001, the annual number of reported cases increased. Older people, men, and people living in urban areas were more affected. LD showed a seasonality with a higher incidence in the summer/early autumn months of the year. Stratifying for exposure categories, the pooled incidence over time by month showed that CALD followed a curve similar to all cases. TALD had a unimodal curve among adults < 60 years with a broad peak from June-September and among adults aged at least 60 years a bimodal curve with two peaks in June/July and September/October. IRR of TALD reached 3.0 – 3.5 compared to CALD with 1.8. NLD showed no seasonality.

Conclusions:

Seasonality among the three exposure categories showed strong differences suggesting different approaches for prevention and/or clinical management. The stronger and differential seasonality among TALD may be influenced by travel behavior and age. Physicians need to be vigilant for LD among patients with nosocomial pneumonias year-round.