Tick-borne diseases (TBD) represent a major public health threat in North America, particularly for military personnel training on Department of Defense (DoD) installations. Ecological theory predicts that climate change will likely alter vector-borne disease transmission by a variety of direct and indirect pathways. This project explored several of the predicted consequences of climate change, including altered fire regimes and plant communities, and their interactions with wildlife, for human risk of exposure to TBDs in the southeastern U.S. The specific project objectives were to: 1) Evaluate the interactions between fire and plant invasions spanning a gradient in fire management, invasive plant distribution and abundance, and climatic conditions across the southeastern U.S.; 2) Quantify the effects of fire and plant invasions, and their interactions, for variation in wildlife abundance, tick abundance, tick-borne pathogen infection rates, and TBD risk to humans; and 3) Calibrate a spatially explicit model of TBD risk in response to fire-invasion interactions and incorporate simulations of climate change scenarios to examine the responses of fire, plant invasions, wildlife, TBD risk, and their interactions.

This project sampled nine DoD installations to empirically examine the interactions between fires and plant invasions spanning a climate gradient across the southeastern U.S. Across these sites the project team utilized gradients in fire management and plant invasions to perform multiple surveys of vegetation, wildlife, tick abundance, and tick-borne pathogen prevalence. The project team also conducted mechanistic experiments to quantify plant-fire-tick interactions. Using these empirically derived estimates from the field and laboratory, the project team employed a structural equation modeling framework to test the direct and indirect effects of fire regimes, plant invasions, host abundance, tick abundance, and pathogen prevalence, on TBD exposure risk. Additionally, an ecosystem-based model capable of capturing the physiology of a focal invasive plant species was developed and used these relationships to develop a web-based Decision Support Tool that will allow site managers to integrate decisions concerning application of prescribed fire, control of invasive plant species, and mitigation of tick-borne disease risk.

The overall objective of this research was to investigate the consequences of climate change, including altered fire regimes and plant communities, and their interactions with wildlife, for human risk of exposure to TBDs in the southeastern U.S. Through field surveys conducted in 2017 and 2018 spanning nine DoD installations, the project team evaluated the direct and indirect effects of climate, recent weather, fire management, vegetation, and host abundance, on ticks. It was found that abiotic mechanisms are the major drivers of tick abundance, mediated by the indirect effects of fire on leaf litter accumulation and understory biomass. Litter cover and depth were positively related to tick abundance and time since fire, indicating that frequent fires lower tick abundance by removing litter. Interestingly, host abundance was unrelated to tick abundance, suggesting that abiotic effect of time since fire on litter cover and depth is a stronger determinant of tick abundance in longleaf pine forest than biotic pathway mediated by host abundance. Additionally, the results indicate that fire management (fire return interval) is primarily determined by characteristics of the installation (97% of variation explained by the random effect). Therefore, climate is not yet the most limiting factor for using prescribed fire as management tool at these installations but is predicted to be in the relatively near future.

Recent research has addressed only individual components for the effects of climate, fire, and plant invasions on TBD exposure pathways; no effort has evaluated the many potential interactions and feedbacks that may mediate the effects of any one component on the transmission of TBDs. This research will provide new understanding of the multitude of factors that determine TBD risk. The results of this study will be directly applicable to predicting TBD risk under non-stationary conditions, which can then be used to inform management decisions for mitigating disease risk currently and in the future. The project team created a web-based interface for forecasting TBD risk under alternative management scenarios accessible to land managers at DoD installations and other federally managed sites. Movements of military personnel could be adjusted to account for areas of high risk, and the strategic deployment of other management approaches will be enhanced. The combination of outputs and deliverables from these efforts will not only benefit personnel operating on military installations in the southeastern U.S. but potentially will offer widespread benefits to public health throughout the region.