The importance of ticks as disease vectors in North America is well recognized. In Iowa, and in the United States generally, ticks are responsible for transmission of the majority of arthropod-borne disease agents that impact human health. The studies contained in this dissertation explored the spatial distribution of ticks in Iowa, particularly Ixodes scapularis, and the infection of this species with Borrelia burgdorferi, the causative agent of Lyme disease. Further sections focus on a more specific aspect of tick biology regarding the capacity of the tick innate immune responses to phagocytose invading microorganisms. We postulated that the phagocytic capacity plays an important role in determining the pathogens which survive to be transmitted by ticks. I examined variation in cellular immune responses between tick species in order to illuminate the potential of this mechanism to serve as a determinant of vector competence. The studies described within yielded a holistic understanding of the relationship between ticks and the microorganisms they carry in the context of the three most commonly encountered ticks in the state of Iowa.

The three most common species of human-biting ticks in Iowa are Amblyomma americanum, Dermacentor variabilis, and I. scapularis. Detailed microscopic and flow cytometric studies were performed examining hemocyte phagocytosis in these species toward a variety of injected model insults including Gram-positive and -negative bacteria, and fluorescent beads. Comparison of the phagocytic responses between tick species to the injected insults was performed and flow cytometrically-defined hemocyte populations were evaluated for variation in phagocytic response. Results highlighted several differences between tick species and hemocyte types. Additionally, a flow cytometry-based technique was developed for quantifying hemocyte phagocytosis and applied to a variety of arthropod taxa. In the tick D. variabilis, further studies were performed exploring the effect of multiple bacterial injections upon the phagocytic immune response. To isolate hemocyte phagocytosis from the humoral aspect of tick innate immunity, the most prevalent antimicrobial peptide in tick hemolymph, defensin 1, was suppressed at the transcript level using RNA interference. Defensin 1 transcript was successfully suppressed, allowing for the independent operation and observation of the roles of this antimicrobial peptide and the phagocytic response.

These studies provide a perspective regarding the distribution of human-biting ticks in Iowa, their infection with a significant disease organism, and the internal interactions of the innate immune response with microorganisms. In particular it is clear that phagocytosis is a critical facet of the tick innate immune response, and contributes significantly to defining the role of ticks as disease vectors.