Rickettsial tick-borne pathogens exhibit a wide range of prevalence in their tick vectors, yet the drivers of this spatial and temporal heterogeneity are unknown. Through a multidisciplinary study of two parallel systems, we will tease apart influences of environmental, physiological, and microbial factors on human risk of tick-borne rickettsial infection. The model tick-borne disease (TBD) systems for this work target Amblyomma maculatum and Rickettsia parkeri in the US, and Amblyomma hebraeum and Rickettsia africae in Africa; both R. parkeri and R. africae are spotted fever group Rickettsia, known to cause disease in humans. R. parkeri has been found at high prevalence in A. maculatum at the leading edge of this tick's rapidly expanding range, and R. africae is endemic in southern Africa with few reported studies. TBD risk is occurring in locations where environmental conditions that affect pathogen-tick-host interactions or tick life history and behavior create new opportunities for pathogen exposure in humans. The US and African Amblyomma/Rickettsia systems will comparatively examine different aspects of the same phenomenon: the spatiotemporal heterogeneity in pathogen prevalence in ticks in different parts of the vector's distribution. In both systems, we will focus our study at sites at the edge and core of the ranges of A. maculatum and A. hebraeum. This proposal details a comprehensive approach to this question using a suite of targeted studies to inform mathematical models. Laboratory-based empirical determination of transmission rates will provide important life history information and determine model structure. Molecular studies of both host ticks and rickettsia will generate information on tick movements and population dynamics on bacterial strain variation and tick microbiomes. Field ecology studies will examine natural temporal and spatial variability of ticks and rickettsial pathogens, as well as influences of putative host community composition. Geospatial analyses will reveal the contribution of environmental factors to prevalence gradients and genetic structuring of ticks and rickettsia. Finally, all of this information will be synthesized in mathematical models to weigh relative importance of different drivers in these TBD systems and of TBD risk in general. Spatial heterogeneity in occurrence of disease vectors and vector-borne pathogens is fundamentally important to TBD risk, and this study gives us the unique opportunity to use a comparative approach to explore TBD occurrence in a number of theoretical contexts including dilution effects of host communities, core-margin effects on pathogen prevalence and vector abundance, and center of origin impacts on vector and pathogen population genetic structure. The proposed work will develop advanced mathematical models of tick-borne rickettsial infection dynamics that will be informed by transmission studies, microbiome analyses, population genetics, GIS, and remote sensing, and will provide a framework by which similar studies may be performed for other pathogens. Ultimately, these efforts will aid in risk assessment, control measures, and education efforts, in expanding tick ranges and areas of changing land use.