Patterns and drivers of the distribution of Leishmania infection in humans using environmental and socioeconomic factors: a modeling study in mainland Portugal

BACKGROUND: Leishmaniasis remains endemic in the Mediterranean region, where Leishmania infantum causes both visceral disease and widespread asymptomatic infection. In Portugal, the true burden and geographical distribution of human infection are incompletely understood and likely influenced by environmental, climatic, and socioeconomic factors. This study aimed to identify determinants of symptomatic and asymptomatic Leishmania infection, map current distribution, and explore spatial heterogeneity across mainland Portugal. METHODS: We performed a municipality-level ecological modeling study using two complementary datasets: notified visceral leishmaniasis (VL) cases reported between 2010 and 2020 and Leishmania seroprevalence among blood donors in 2022. Environmental, climatic, social, and economic covariates were extracted from national and international databases. Zero-inflated beta-regression models with random intercepts were fitted to assess associations while accounting for excess zeros and regional clustering. RESULTS: Marked spatial heterogeneity was observed for both VL incidence and Leishmania seroprevalence. Higher VL incidence was independently associated with greater migrant population share and a higher proportion of residents whose highest attained education was basic education or below, while greater forest cover and a proxy for stray animals were inversely associated. In contrast, seroprevalence was higher in municipalities with a greater proportion of residents living in small localities and lower altitude and precipitation and was inversely associated with lower educational attainment and stray-animal proxies. Climatic and socioeconomic factors showed differential associations with clinical disease versus asymptomatic infection. CONCLUSIONS: Distribution of human VL and Leishmania infection in Portugal reflects a complex interaction between environmental suitability, social vulnerability, and surveillance context. Integrating social determinants into One Health-based control strategies and strengthening surveillance are essential to address current transmission patterns and anticipate future changes under climate change.