- The last century has experienced a marked increase in emerging infectious disease (EID, hereafter) – jeopardizing human, domestic animal, and wildlife health. EIDs are commonly associated with spillover from one host species into a novel host species, with many destructive diseases, for both livestock and wildlife, emerging at the wildlife-livestock interface. As global change continues to erode the boundaries between human and wildlife systems, it will become increasingly more important to understand the key components influencing host susceptibility as well as pathogen/parasite spread and persistence.
However, understanding disease systems, especially within wildlife, is complex, as processes at multiple scales of biological organization are relevant to pathogen/parasite dynamics. At the within-host scale, pathogens interact with host cells and co-infecting pathogens, and these within-host dynamics affect host susceptibility, infectious period, and pathogen transmission potential. At the host population-level scale, heterogeneity across hosts as well as pathogen dispersal between hosts interacts with within-host processes to ultimately influence the distribution of infectious agents within-hosts, across hosts, and over time. Studying disease in natural systems enables researchers to observe the outcome of interactions of numerous multi-scale sources of variation and predict realistic parasite/pathogen dynamics. Ultimately, this work should enable the development of adaptive disease management.
For my PhD dissertation, I explored how within-host patterns and processes inform population-level patterns in African buffalo (Syncerus caffer) of Kruger National Park (KNP), South Africa. Specifically, I studied infectious agents associated with two diseases that infect cattle and buffalo at the South African wildlife-livestock interface: the bovine respiratory disease complex and theileriosis. In Chapter 2, I found that evolutionarily conserved immune responses (i.e., non-specific inflammatory response) can be used to detect disease exposure without a priori knowledge of pathogen identity – a tool than can be further developed for EID surveillance. In Chapter 3, I weighed the effect of host traits, pathogen co-occurrence and environmental variability on probability of infection by viral and bacterial pathogens within the bovine respiratory disease complex as well as characterized temporal trends in pathogen incidence. I found that the importance of each factor was inconsistent across pathogens – co-occurrence was the best indicator of virus occurrence whereas host ID was the best indicator of bacterial infection. Importantly, I found that within-host dynamics only partially elucidated seasonal cycling in population-level disease dynamics. In Chapter 4, I developed molecular methods to quantify cryptic spatio-temporal variation in vector-borne, hemoparasite (Theileria: the etiological agent of theileriosis) assemblages of African buffalo. In Chapter 5, I used the high resolution data from Chapter 4 to describe the structure of Theileria assemblages within and across hosts, in both space and time. Chapter 5 uses novel analytical approaches to distill complex Theileria assemblages into functional groups based upon their life-history patterns. This characterization enabled me to estimate the relative importance of dispersal and host heterogeneity on distribution of these parasites thereby enabling me to predict efficacy and side-effects of vector-borne disease management tools.