Direct and Indirect Effects of Abiotic and Biotic Variables on Human Incidence of Tick-Borne Diseases
Amissah, Carlos
Amissah, Carlos
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Abstract
Tick-borne disease incidence is increasing globally and is driven by both biotic and abiotic factors. These factors do not act independently but interact to influence disease incidence in humans. The aim of this study was to determine the direct and indirect effects of abiotic factors on human disease incidence in Vermont. We used annual data (2015�2023) on reported human cases of Lyme disease, anaplasmosis, and three additional tick-borne diseases (babesiosis, relapsing fever, and Powassan virus), along with data on tick density, pathogen prevalence, and remotely sensed climatic and vegetation variables. We fitted piecewise structural equation models (SEMs) to the human disease incidence data, using climate, habitat, tick density, and pathogen prevalence in ticks as potential causal variables. Separate SEMs were developed for Lyme disease, anaplasmosis, and the combined incidence of all five diseases (total disease burden). Temperature, tick density, and pathogen prevalence were directly associated with Lyme disease incidence, whereas NDVI, relative humidity, and elevation influenced Lyme disease indirectly through their effects on tick density. Temperature and elevation directly influenced anaplasmosis incidence and also had indirect effects mediated by tick density and pathogen prevalence. For total disease burden, climate variables (PC1) directly influenced tick abundance and overall pathogen prevalence in ticks, whereas habitat variables (PC2) affected human disease incidence indirectly through tick density. Overall, our results demonstrate that human disease risk is structured by a partial mediation framework, in which climate and local habitat variables exert their effects through distinct pathways.
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Date
01/01/2026
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Graduate Student
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Poster
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Ecology and Evolution
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College of Arts and Sciences
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Life Science