Date of Award

Fall 12-2020

Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences


Biomedical Sciences

Committee Director

Wayne Hynes

Committee Member

Holly Gaff

Committee Member

David Gauthier

Committee Member

Christopher Paddock


Cases of spotted fever group rickettsiosis are becoming more prevalent in the United States. In Virginia, there are three human-biting ticks which are largely responsible for the spread of rickettsial pathogens and the increase in disease cases. These species include Dermacentor variabilis, Amblyomma americanum, and Amblyomma maculatum; all of which are vectors of rickettsial agents to vertebrate hosts. These species are sympatric as adults and have the potential to share large and small mammal hosts. Their interactions on and off host and their associated rickettsiae were the focus of this dissertation work. Amblyomma americanum is the vector of R. amblyommatis. Amblyomma maculatum is the vector of R. parkeri. Dermacentor variabilis is a vector of R. rickettsii. The purpose of this dissertation was to better understand and identify the potential drivers of rickettsial pathogen spillover and transmission between these species and in local host populations.

The first aim of this research was to determine the small mammals acting as hosts to immature A. maculatum, and to assess the role these hosts may play as potential reservoirs or amplifying hosts of R. parkeri. Six small mammal species were identified as hosts of immature A. maculatum; three of which potentially play a role in the enzootic cycle of R. parkeri as both carriers of infected ticks and systemically infected hosts.

The second aim of this research was to assess spillover of R. parkeri and R. amblyommatis into local D. variabilis populations. Wild-caught D. variabilis were found to harbor R. montanensis, R. parkeri, and R. amblyommatis at all life stages. Laboratory studies found horizontal and transstadial transmission of R. parkeri during the larval and nymphal life stages of D. variabilis when co-feeding with infected A. maculatum.

The third aim of this research was to determine the potential role of the microbiome in the high local prevalence of R. parkeri seen in A. maculatum populations from Virginia. Microbiome analyses were performed on lab-raised and wild-caught A. maculatum. Overall, site and host appear to play a role in the microbiota seen in both lab-raised and wild-caught individuals.


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