Introduction and objectives. Relapsing fever spirochetes (RF, Borrelia) are known pathogens for humans, often transmitted by soft ticks. Compared to their sister group, the Lyme disease spirochetes (Borreliella), our understanding of their transmission cycle, natural reservoirs, spatial scale of circulation, wildlife role and impact, and vectors is less comprehensive. Our work focuses on the circulation of Borrelia in seabirds and their associated ticks. Sporadic studies have reported phylogenetically similar isolates in seabird species from South Africa and Eastern Asia, in association with an ill bird, and present in a soft tick. Its association with a human clinical case (then named Borrelia sp. K64) and the ecological proximity between seabirds and humans raise particular interest in the context of zoonotic disease emergence.

Material and methods. We investigated the distribution of Borrelia in ticks associated with seabirds. We screened 1,944 individual tick samples of the Ornithodoros capensis species complex, collected globally from nests of 22 seabird species, using PCR and qPCR assays. Additionally, we examined Borrelia circulation by testing avian blood samples in a colony where tick infestation is high. 

Results, discussion, and conclusion. Our findings confirmed the presence of Borrelia in 19 locations, spanning diverse geographic areas up to 7,600 km apart, with 62 positive samples (3.19%). No blood samples were positive. Further characterization was performed using a multi-locus typing approach. Most samples, associated with 5 seabird genera, clustered with the Borrelia sp. K64 isolate. Genetic distance analysis with known RF bacteria revealed that these isolates represented a distinct species, leading us to propose the species Candidatus Borrelia capensii. Studies are now needed to validate tick vector competence and to examine the impact of this Borrelia species on seabirds. Additional colony-level surveys are also called for to evaluate the risk of exposure for human populations that live near to nesting seabirds.