The First Study of Borrelia burgdorferi Sensu Lato Persistence in Small Mammals Captured in the Ixodes persulcatus Distribution Area in Western Siberia

Borrelia burgdorferi sensu lato (s.l.) persistence in reservoir hosts is essential for the maintenance of the spirochaetes in the enzootic cycle. In this study, we investigated the persistence of Siberian B. burgdorferi s.l. strains in naturally infected voles and their transmission to Ixodes ticks. A long-term study conducted in 2013–2024 demonstrated the presence of Borrelia afzelii, Borrelia bavariensis, and, rarely, “Candidatus Borrelia sibirica” DNA in blood samples of small mammals. Among these, B. bavariensis exhibited the highest genetic diversity. All identified Borrelia species persisted in naturally infected Clethrionomys spp. voles throughout their lifespan (up to 50 weeks), providing the first evidence of long-term persistence of B. bavariensis and “Candidatus B. sibirica” in these hosts. Notably, the persistence of two Borrelia genospecies or several genovariants of a single genospecies within the same vole was common. Xenodiagnosis with laboratory-reared Ixodes spp. confirmed efficient transmission of all identified Borrelia genospecies to Ixodes persulcatus after 35–42 weeks of B. burgdorferi s.l. persistence. Moreover, B. bavariensis was transmitted to Ixodes pavlovskyi and I. persulcatus/I. pavlovskyi interspecies hybrids after at least 23 weeks of pathogen persistence. These findings demonstrate the reservoir competence of Clethrionomys spp. for B. afzelii, B. bavariensis, and “Candidatus B. sibirica”.