Sickened by the Weather: Exploring the Climatic Impact on West Nile Virus (WNV) and Legionella pneumophila in Piedmont-A Retrospective Observational Study (2021-2024)

Background: Climate change represents a major global health challenge, with rising temperatures and altered precipitation patterns influencing the spread of infectious diseases. This study investigated the association between climatic factors (average temperature and precipitation) and the monthly proportion of laboratory-confirmed Legionella pneumophila serogroup 1 and West Nile Virus infections among clinically suspected patients in a large teaching hospital in Northern Italy. Methods: We retrospectively analyzed data from 2021 to 2024. The primary outcome was the monthly proportion of positive tests (standardized per 1000 clinically suspected patients) for Legionella pneumophila serogroup 1 (urinary antigen) and West Nile Virus (serology). Associations with climatic variables were assessed using linear and multivariate regression models, as well as Generalized Additive Models (GAMs). Seasonal effects were evaluated through ANOVA. Results: For Legionella pneumophila, precipitation was not significantly associated with the proportion of positive tests (p = 0.1438; R2 = 0.049). In contrast, average temperature was a significant predictor: each 1 °C increase was associated with +0.52 positive cases per 1000 tested patients (p = 0.000283; R2 = 0.267). Multivariate models confirmed temperature as the dominant factor. For West Nile Virus, precipitation showed no meaningful effect (p = 0.914). However, average temperature demonstrated a significant positive association with the proportion of positive cases (p = 0.00293; coefficient = 9.33), with seasonal analysis highlighting a marked summer peak (mean = 399.68 positive cases per 1000 tested; p = 0.00653). Conclusions: Our findings underline the predominant role of temperature over precipitation in driving the burden of both Legionella pneumophila and West Nile Virus infections among hospitalized patients. These results strengthen the evidence that the life cycles of these pathogens are tightly climate-dependent. Developing effective adaptation strategies is essential to mitigate climate-related health risks.