The ecological and evolutionary relationships of ticks and the bacterial pathogens they transmit: With special emphasis on ticks in the Ixodes ricinus group and the bacterium Anaplasma phagocytophilum

This dissertation consists of three chapters, each dealing with a different aspect of the ecology of tick borne disease. The first chapter investigates the molecular ecology of the argasid tick, Ornithodoros coriaceus (Koch), the only confirmed vector of epizootic bovine abortion (EBA) in the United States. While the disease and its vector have historically been reported in the mountainous regions of California, within the past two decades the range of EBA has apparently expanded into southern Oregon and northern Nevada. Overall, very little evidence of gene flow among tick populations was detected, making it unlikely that widespread tick movement had introduced O. coriaceus and the EBA agent into new regions. The second chapter describes the ecology of tick borne disease in Guatemalan horses and cattle. Data on host condition and tick infestation were recorded for each animal in the study. The seroprevalence of important tick-borne pathogens including Babesia spp. and Anaplasma spp. were determined and the first report of A. phagocytophilum infection from this region was made. The prevalence of infection, tick infestation levels, host factors and environmental data were analyzed for association. The third chapter describes a study investigating whether the transmissibility of Anaplasma phagocytophilum strains is influenced by the geographic origin of its tick vector. We tested our hypothesis by performing a reciprocal cross-transmission experiment using an eastern and a western North American strain of A. phagocytophilum (Webster and MRK, respectively) and the two tick species, I. scapularis and I. pacificus , which serve as the bacterial strain's natural vectors respectively. The western tick, I. pacificus, showed a significantly higher vector competence for A. phagocytophilum than I. scapularis and the eastern strain, Webster, was more transmissible than its western counterpart, MRK. These results indicate that geographic variation in host susceptibility to A. phagocytophilum strains may play a more important role in the epidemiology of granulocytic anaplasmosis than does the competence of its tick vectors to transmit the pathogen.