Project Summary/Abstract While both ticks and tick-borne disease agents have rapidly become a serious and growing threat to human and veterinary public health, mounting evidence indicates that many tick-borne diseases due to rickettsial infection may be under-reported in Northern America because of their uncharacteristic symptoms in humans and domestic animals. Ixodes pacificus is a principal vector for pathogenic and endosymbiotic bacteria on the Pacific West Coast of the United States. Compared to pathogenic rickettsiae identified in other tick species in California, the ecology and biology of pathogenic rickettsiae in I. pacificus remains largely unexplored. Our lab detected a novel Rickettsia species, Rickettsia species phylotype G022, in questing adult ticks of I. pacificus in three counties of Northern California in 2011, 2013, and 2020. Due to phylotype G022’s low prevalence (2-4.5%) in adult I. pacificus and published phylogenetic trees placing the bacterium within other pathogenic spotted fever group rickettsiae, our hypothesis is that phylotype G022 is a pathogenic Rickettsia. The proposed study ultimately serves as a continuation of our previous findings regarding the ecology and pathogenesis of Rickettsia species in I. pacificus. Our research pursues the following specific aims: 1) We will conduct a survey to estimate the prevalence of Rickettsia species phylotype G022 in nymphal ticks of I. pacificus by collecting the nymphs from the field and detecting the bacterium by real-time PCR; 2) We will study transmission routes of Rickettsia species phylotype G022 in I. pacificus by collecting all life stages of the ticks via feeding on New Zealand white rabbits and detecting the bacterium by real-time PCR; 3) We will perform the propagation and isolation of the Rickettsia phylotype G022 isolate from I. pacificus using tick embryonic cell lines and limiting dilution assay; 4) We will perform the molecular typing of the Rickettsia species phylotype G022 isolate by Multilocus Sequence Typing; 5) We will investigate the presence of Rickettsia species phylotype G022 infection in domestic dogs from Northern California. This study will not only result in the official naming of phylotype G022 by typing the pure bacterial isolate, but also enable us to start gathering data on transmission dynamics of the bacterium as well as its potential pathogenicity, the long-term research goal of our lab, to humans and animal hosts in Northern California.