Impact of climatic variables on West Nile Virus epidemiology in Lombardy, Italy

Abstract Background Climate Change is among the most urgent global health threats of the 21st century, driving changes in infectious disease patterns, including mosquito-borne viruses like West Nile Virus (WNV). Lombardy, in northern Italy, is an endemic area for WNV circulation. Despite the well-established impact of climate on mosquito population dynamics and WNV transmission, the current surveillance system does not incorporate climatic parameters to monitor WNV spread. This study describes WNV epidemiology in Lombardy (2013-2022) and assesses the relationship between temperature, precipitation trends, and WNV cases. Methods A retrospective observational study was conducted using human WNV case data from the national surveillance system ArboISS across six Lombardy provinces (2013-2022). Weekly cases were matched with meteorological data from the University of Milan's Department of Environmental Sciences. Descriptive analyses included choropleth maps and time series of cases, temperature, and precipitation. A hurdle model (logistic + zero-truncated Poisson) was used to assess associations, with lagged climate variables and fixed effects for province and year. Results A total of 311 cases were reported (2013-2022), mostly in males (74%) and individuals over 65 (58%), with peaks in August and in southern provinces. Since 2019, 53% of cases were WNND. Warmer years (2018, 2020, 2022) showed higher incidence. The hurdle model showed that a 1 °C increase (2-week lag) raised the odds of occurrence by 27% and case counts by 11%. Precipitation (1-week lag) increased case counts (IRR=1.13) but had no effect on occurrence. Provincial and yearly effects were significant for occurrence, but not for case counts. Conclusions Climatic factors strongly influenced WNV transmission dynamics in Lombardy. These findings reinforce the need to integrate environmental indicators into One Health surveillance systems to better predict and manage arbovirus-related health risks in the Climate Change context. Key messages • Temperature and precipitation strongly impact WNV transmission in Lombardy. • The results underline the need to integrate environmental data into arboviruses surveillance systems in the context of Climate Change, to better monitor and predict epidemiological trends.