Uncharted Waters: Projecting the European Emergence of Naegleria fowleri, Colloquially Known as the 'Brain-Eating Amoeba', Under Climate Warming

Abstract

Background. Naegleria fowleri, the causative agent of primary amoebic meningoencephalitis (PAM), is a thermophilic free-living amoeba historically endemic to warm freshwater environments, particularly in the southern United States. While human infections remain exceedingly rare, they are almost uniformly fatal. Global climate change is hypothesized to expand the geographic range of thermotolerant pathogens, potentially introducing them into previously unaffected regions. Objective. This study aims to quantify the current and future habitat suitability for N. fowleri within the contiguous United States (US) and, critically, to project these models onto Europe, a continent traditionally considered low-risk, to identify regions of potential emergence under climate change scenarios. Methods. We employed a modeling approach using the machine learning algorithm Maxent, implemented via the `flexsdm` R package. We utilized 204 occurrence records from the contiguous US. We evaluated three environmental datasets (ENVIREM, Global Cloud Dynamics, Worldclim v.2), selecting Worldclim v.2 for its superior predictive performance. Future projections were generated for the 2041-2060 period under the Shared Socioeconomic Pathway (SSP) 2-4.5 (intermediate scenario). Variable importance was assessed using Shapley additive explanations (SHAP) to decouple the contribution of each Bioclimatic predictor and identify the key drivers of range expansion. Results. The Worldclim v.2 dataset provided the most robust model. The model accurately identified the southeastern US as the core current habitat, consistent with historical PAM case distribution. Crucially, when projected onto Europe, the model reveals extensive regions of currently low suitability that transition to moderate or high suitability by 2050, particularly across the Mediterranean basin, parts of Central Europe, and the Atlantic coastal fringes. SHAP analysis identified temperature-related variables, specifically Annual Mean Temperature, Mean Temperature of the Coldest Quarter, and Mean Diurnal Range, as the most influential predictors, followed closely by precipitation variables (Precipitation of Driest Quarter/Month and Precipitation Seasonality). Under the SSP2-4.5 scenario, the model projects a distinct northward and eastward expansion of suitable habitat, with Ukraine emerging as a region of particular concern, especially in light of recent anthropogenic hydrological changes to the Dnipro River basin. Conclusion. Our findings support the hypothesis that global warming will facilitate the geographic spread of N. fowleri, carrying this pathogen into uncharted European waters. The projections for Europe, and for Ukraine, in particular, suggest a non-negligible risk of future autochthonous PAM cases. These results warrant the proactive inclusion of this pathogen in public health surveillance programs and climate change adaptation strategies across the continent.