Evidence of repeated zoonotic pathogen spillover events at ecological boundaries

Human encroachment in natural habitats poses a significant threat to humans and wildlife but remains drastically understudied. By forming new ecological boundaries at the human/wildlife interface, anthropogenic modifications to the landscape have altered several ecological processes and disturbed ecological integrity and resilience worldwide. Outbreaks of zoonotic pathogens often occur in human populations at these ecological boundaries, but the mechanisms behind these new emergences remain under investigation. Here, we aim to unravel the roles of both landscape and biotic communities by comparing the characteristics of presence and pseudo-absence localities to provide a better mechanistic understanding of zoonotic disease emergence. Using the Ebola virus as a model, we link pathogen reservoirs and accidental host ranges with human land use using a machine learning framework. Our results show that species range edges and conversion from wildlands to agricultural areas increase Ebola outbreak risk. Moreover, we show clear evidence that species range edges are predominantly composed of agricultural landscape, possibly amplifying pathogen outbreaks. Given the current rate of landscape modification worldwide, we provide novel ecological and evolutionary insights to our understanding of zoonotic pathogen emergence and highlight the risk of aggressively developing ecological boundaries.

Methods
We compiled all reported outbreaks with human cases from Kuhn (2008), the Centers for Disease Control (CDC 2023), and ProMED (Yu and Lawrence 2004). We then pruned the dataset to include only outbreaks confirmed to be Ebola virus (including both Sudan ebolavirus and Zaire ebolavirus) by laboratory testing and that originated from a wild source (e.g., we excluded data points from accidental laboratory spillover events). For each outbreak, we compiled the date of outbreak, its geographical origin, and the number of human cases, allowing us to work with 44 outbreaks ranging from 1976 to 2020, inclusively (and excluding outbreaks reported after we initiated our work).