Climate-driven expansion of climatic suitability for Phlebotomus mascittii at its northern distribution margin in Western Europe under future socioeconomic scenarios.

Climate change is expected to drive the northward expansion of phlebotomine sand flies in Europe, increasing the potential for transmission of vector-borne pathogens such as Leishmania infantum. This study assessed the historical baseline and future climatic suitability for Phlebotomus mascittii, the northernmost distributed sand fly species in Europe, across Western Europe. A mixed-effects logistic regression model was developed using summer climatic variables and land cover data, based on standardised field surveys conducted in 2023-2024 across France, Germany, Luxembourg, Belgium, and the Netherlands. The model was applied to project future suitability under three Shared Socioeconomic Pathways (SSP2-4.5, SSP3-7.0, SSP5-8.5) for the periods 2041-2060 and 2081-2100. Results indicate an increase of 20-30 percentage points in predicted climatic suitability relative to the historical baseline across Luxembourg, Belgium, western Germany, and the southern Netherlands by 2041-2060 under all SSP scenarios. By 2081-2100, this expansion intensifies, reaching projected average increases in climatic suitability across the study area of approximately 28 percentage points under SSP2-4.5, 45 percentage points under SSP3-7.0, and 57 percentage points under SSP5-8.5, with the largest increases occurring in the northernmost regions. The model showed moderate discrimination (AUC = 0.74, 95% CI: 0.68-0.80), with grouped 5-fold cross-validation yielding a mean AUC of 0.71. At the optimal threshold (0.164), apparent sensitivity was 0.55 and specificity 0.83 (accuracy = 0.80; TSS = 0.39). Accordingly, the projections should be interpreted as indicators of areas with increasing climatic suitability rather than precise predictions of future species occurrence, establishment, or pathogen transmission. Overall, the findings suggest that climate change may substantially expand climatically suitable areas for Ph. mascittii in the northern distribution margin in Western Europe, highlighting the need for adaptive surveillance and providing a spatial framework to support targeted vector surveillance and future One Health preparedness.