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

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