Distribution and Genetic Diversity of Borrelia from Lyme Borreliosis and Relapsing Fever Groups in Ixodidae Ticks in the Russian Far East

Ixodidae ticks are vectors of pathogenic spirochetes from the Borrelia burgdorferi sensu lato (s.l.) complex and the relapsing fever (RF) group. The Russian Far East is endemic for borrelioses, yet data on the distribution and genetic diversity of Borrelia spp. in this region remain scarce. We examined 3107 Ixodidae ticks (Ixodes spp., Haemaphysalis spp., and Dermacentor silvarum), collected from two mainland provinces and five islands of the Russian Far East in 2005–2023. A total of 86 Borrelia afzelii, 297 Borrelia bavariensis, 81 Borrelia garinii, 44 Borrelia miyamotoi, and 33 unclassified RF borreliae were identified. Borrelia burgdorferi s.l. and B. miyamotoi were strongly associated with Ixodes spp., whereas unclassified RF borreliae were associated with Haemaphysalis spp. Among B. burgdorferi s.l. isolates, B. afzelii and B. bavariensis were found significantly more frequently in I. persulcatus compared to I. pavlovskyi, whereas B. garinii was significantly more common in I. pavlovskyi. Genetic characterization of B. burgdorferi s.l. revealed high haplotypic diversity, particularly within B. bavariensis and B. garinii. Among RF borreliae, all B. miyamotoi isolates were highly conserved and belonged to the Asian type. In contrast, unclassified RF borreliae were genetically variable and clustered with RF borreliae previously found in Haemaphysalis and Rhipicephalus ticks. The obtained results expand the understanding of the distribution and genetic diversity of Borrelia spp. in the Russian Far East.