Seasonality and effects of climatic exposures on community-acquired Legionnaires’ disease incidence, Italy, 2005 to 2023

BACKGROUND Community-acquired Legionnaires’ disease (LD) has increased globally, including Europe. Climatic exposures may contribute to this rise, yet evidence from high-incidence countries, such as Italy, is limited. AIM We aimed to assess LD seasonality at the national level and the effects of temperature, relative humidity and precipitation on LD incidence at the municipality level. METHODS We analysed data on notified community-acquired LD cases and national climatic data from 2005 to 2023. Seasonality was assessed (peak-to-trough ratio (PTR)). A case time-series design with distributed lag nonlinear models was applied to estimate 1–10 week lagged effects of climatic exposures on LD incidence at the municipality level, as incidence rate ratios (IRRs). RESULTS Of the 28,662 notified LD cases, 27,458 (95.8%) were included. A clear seasonality was observed in the incidence, with a trough in early spring and a peak in autumn (PTR = 3.62; 95% confidence interval (CI): 2.45–5.34). Higher temperature and relative humidity had the strongest effects 10–9 weeks before disease onset (for 5°C increase above 15°C: IRR = 1.45; 95% CI: 1.33–1.58 and for 5% increase above 60%: IRR = 1.19; 95% CI: 1.12–1.26). Precipitation showed more immediate effects, particularly with a 1-week lag (for 5 mm increase above 10 mm: IRR = 1.07; 95% CI: 1.06–1.09). Findings were consistent among individuals aged ≥ 60 years. CONCLUSION A clear seasonal pattern was seen in the LD incidence, peaking in summer and autumn. Temperature and humidity increased LD risk over longer lags, while precipitation had shorter-term effects, suggesting climatic exposures influence environmental proliferation and immediate transmission of Legionella .