Epidemiology and Transmission Dynamics of Environmentally Mediated Infectious Diseases at the Wildlife-Livestock-Human Interface

Abstract

Infectious disease transmission is increasingly recognized as an ecological process shaped by interactions among hosts, environmental reservoirs, and landscapes. Many pathogens persist outside their hosts and are transmitted through shared environments, creating opportunities for disease spread within species, among species, and ultimately across wildlife–livestock–human systems. Understanding these interconnected transmission pathways is essential for improving disease surveillance, risk assessment, and prevention. This dissertation investigates the epidemiology and transmission dynamics of environmentally mediated infectious diseases across a continuum of ecological complexity, progressing from within-species transmission to cross-species transmission, and ultimately to transmission within a One Health framework. Within-species transmission was examined using feral pigs (Sus scrofa) as a model system for environmentally mediated disease spread. High-resolution GPS telemetry data were used to quantify indirect contact opportunities under alternative assumptions regarding pathogen persistence, host behavior, and spatial use. Results demonstrated that transmission opportunities were highly heterogeneous and strongly influenced by behavioral and spatial constraints. Incorporating movement behavior into transmission models substantially altered contact network structure and epidemic projections, highlighting the importance of mechanistically defining indirect transmission pathways when evaluating disease dynamics in wildlife populations. Cross-species transmission was investigated through avian influenza dynamics at the wild bird–feral pig interface across the contiguous United States. Using a Bayesian multi-species occupancy framework, I evaluated how environmental conditions, hydrological connectivity, wetland characteristics, and waterfowl communities influenced the probability of cross-species avian influenza occupancy. Transmission probability was highest in environmentally connected wetland systems characterized by extensive wetlands, favorable moisture conditions, and abundant dabbling duck populations. Predicted hotspots were concentrated in Texas, the Gulf Coast, southern California, and portions of the Mississippi River Basin, demonstrating how landscape structure and environmental connectivity can facilitate pathogen exchange among host species. The broader One Health transmission system was examined using leptospirosis as a model zoonosis linking wildlife reservoirs, domestic animals, environmental contamination, and human populations in southern Chile. Machine-learning analyses identified distinct drivers of human exposure across urban slum, semi-rural, and agricultural communities. Environmental conditions, animal reservoirs, contaminated water sources, and socio-demographic factors all contributed to disease risk, although their relative importance varied among landscapes. These findings emphasize that zoonotic disease transmission emerges from the interaction of ecological, environmental, and social processes operating simultaneously within coupled human–animal–environment systems. Collectively, the findings of this dissertation demonstrate that environmentally mediated transmission operates across multiple ecological levels, from contacts among individuals within a host population, to pathogen exchange among species, and ultimately to human exposure within complex socio-ecological systems. Across all three disease systems, environmental conditions consistently shaped opportunities for pathogen persistence, transmission, and exposure. By integrating movement ecology, disease ecology, spatial epidemiology, and One Health approaches, this dissertation advances understanding of disease transmission at the wildlife–livestock–human interface and provides a framework for improving surveillance, identifying transmission hotspots, and informing disease prevention strategies in an increasingly interconnected world.