| Recent research in disease ecology has made great strides in understanding what influences parasite dynamics and host-parasite interactions. However, while there is evidence that hosts become infected and transmit pathogens differentially, this factor is rarely included in disease models. In my dissertation research, I studied ticks, rodent hosts, bacterial communities in ticks and rodent blood, as well as host immune function, to assess what host characteristics are associated with higher tick burdens and how this may affect host infection with vector-borne bacteria. Interspecific differences in tick burden, immune function, and bacterial infection between two host species, Peromyscus leucopus (white-footed mouse) and Microtus ochrogaster (prairie vole), were studied at eight sites across southern Indiana, and intraspecific differences within Peromyscus populations were investigated using repeated sampling of two sites during peak tick season. I used a combination of established and novel laboratory methods to rigorously measure the microbiomes of two tick species, Dermacentor variabilis and Ixodes scapularis, and blood from rodent hosts as well as multiple components of the host immune system. Ticks were overdispersed on hosts, where most hosts carried few or no ticks while a minority of hosts carried high numbers of ticks. Peromyscus had higher tick burdens and had higher innate immune function than Microtus. High-throughput sequencing revealed diverse bacterial communities in ticks and hosts. While there was considerable overlap in the distribution of taxa, bacterial communities were distinct between tick species and host blood. Further, hosts with higher tick burdens also had high bacterial sequence abundance, suggesting higher bacterial infection results from their greater exposure to pathogen vectors. There was a negative relationship between innate immune function and tick burden, which may be the result of immunomodulation by ticks or indicate individuals with lower ability to clear tick-borne bacterial infections. Adults, males, and individuals with high tick burdens had elevated levels of pro-inflammatory cytokines, describing immune profiles responding to challenges from tick bites and bacterial infections. In conclusion, the results of my dissertation research identify potential mechanisms influencing overdispersed patterns of tick distribution and skewed bacterial infection that can improve disease risk assessment and predictions of disease dynamics. |
