Birds are the reservoir hosts of Borrelia garinii, the primary causative agent of neurological Lyme disease. In 1991 it was also discovered in the seabird tick, Ixodes uriae, in a seabird colony in Sweden, and subsequently has been found in seabird ticks globally. In 2005, the bacterium was found in seabird colonies in Newfoundland and Labrador (NL); representing its first documentation in the western Atlantic and North America. In this thesis, aspects of enzootic B. garinii transmission cycles were studied at five seabird colonies in NL. First, seasonality of I. uriae ticks in seabird colonies observed from 2011 to 2015 was elucidated using qualitative model-based statistics. All instars were found throughout the June-August study period, although larvae had one peak in June, and adults had two peaks (in June and August). Tick numbers varied across sites, year, and with climate. Second, Borrelia transmission cycles were explored by polymerase chain reaction (PCR) to assess Borrelia spp. infection prevalence in the ticks and by serological methods to assess evidence of infection in seabirds. Of the ticks, 7.5% were PCR-positive for B. garinii, and 78.8% of seabirds were sero-positive, indicating that B. garinii transmission cycles are occurring in the colonies studied. Five I. uriae from two seabird colonies were positive for the Asian strain of the Lyme disease-causing species, B. bavariensis, which has previously only been described from the terrestrial realm associated with rodent reservoirs. The complete microbiome of ticks from two seabird colonies was also explored, which was consistent with PCR-based estimates of Borrelia spp. prevalence and identified infections with Coxiella and Ehrlichia spp., which may also be tick-borne. Third, the phylogenetic relationships of B. garinii found in the study samples, with B. garinii from elsewhere in the world, were explored using concatenated multi-locus sequence typing (MLST) gene sequences. This revealed close relationships between B. garinii in Eurasia and seabird colonies in NL. These results add to our knowledge of all levels of this complex, under-studied system, and help to inform us on how seabirds facilitate the global dispersion of B. garinii and other Borrelia species
