Multiple environmental conditions precede Ebola spillovers in Central Africa

Many emerging viruses of public health concern originate in animals and have the potential to transmit to humans or cause spillover. Orthoebolavirus zairense (EBOV) is one example, circulating in wildlife in Central and West Africa, with high mortality in humans and animals. Previous studies have linked Ebola outbreaks to transitions in rainfall seasonality, proximity to deforestation and areas with high human population density. However, a universal driver or mechanism has not been described and spillovers remain unpredictable. To assess potential environmental determinants of EBOV spillovers, we analysed time series of vegetation health, rainfall, temperature, forest loss and human population size surrounding Central African spillover locations from 1990 to 2022. We evaluated whether environmental conditions before spillover were atypical for each location by quantifying the similarities in environmental time series between spillover years and non-spillover years. Contrary to past work, we found no single environmental trigger that universally prompts spillover. While some outbreaks were preceded by atypical environmental conditions, others occurred when conditions were strongly similar to non-spillover years. We also modelled the relationship between anthropogenic factors and the occurrence of spillover and found no association. We find EBOV spillovers occur following various environmental conditions and no consistent anthropogenic associations.