To determine whether the pathogenic variant of Lyme disease spirochetes, isolate A14S, is perpetuated in a particular reservoir-vector relationship, we screened vector ticks in various Central European sites for a related spirochete and determined its host association. A14S-like spirochetes infect numerous questing ticks in the Petite Camargue Alsacienne (PC). They frequently infect dormice, but no mice or voles. Garden dormice appear to be better reservoir hosts for A14S-like spirochetes than for Borrelia afzelii, because these spirochetes are retained longer and infect ticks more readily. Spirochetes associated with garden dormice in the PC site form a homologous entity with those isolated from a human patient in The Netherlands. Its unique biological relationship together with previous genetic characterization justifies designating this dormouse-associated genospecies as a distinct entity. Garden dormice serve as the main reservoir hosts of a novel genospecies, Borrelia spielmani sp. nov., one of several that cause Lyme disease in people.

To determine whether relapsing fever–like spirochetes associated with hard ticks may infect Ixodes ricinus ticks in central Europe, we screened questing ticks for 16S rDNA similar to that of Asian and American relapsing fever–like spirochetes. We compared the prevalence of these spirochetes to that of Lyme disease spirochetes transmitted by the same vector. Relapsing fever-like spirochetes infect 3.5% of questing vector ticks in our three central European sites near the Rhein Valley. These spirochetes differ genetically from their American and Asian analogs while being relatively homogeneous in the region we sampled. The Lyme disease genospecies most commonly detected in central Europe are distributed broadly, whereas those that are less frequently found appear to be place-specific. The absence of co-infected ticks suggests that relapsing fever–like and Lyme disease spirochetes may not share hosts. Exposure risk for relapsing fever–like spirochetes is similar to that of certain Lyme disease genospecies. Although many persons may be bitten by ticks infected by relapsing fever–like spirochetes, health implications remain unknown.

To determine whether the pathogenic Lyme disease spirochete Borrelia spielmanii is adapted exclusively to garden dormice, we compared the reservoir competence of various rodent species for this spirochete, including sympatric and peridomestic rodents. The different kinds of rodents varied in their attractiveness to nymphal ticks and their level of susceptibility to tick-borne B. spielmanii infection, but only the edible dormouse appeared to be refractory. Although hazel dormice and Norway rats became infectious to ticks somewhat later than did garden dormice, they remained infectious for a longer period of time. During the course of a tick season, garden and hazel dormice contributed theoretically more than twice as many B. spielmanii-infected ticks than the somewhat less susceptible Norway rats and wood or yellow-necked mice. Hazel dormice appeared to be extraordinarily competent as reservoir hosts for B. spielmanii. Because peridomestic rodents proved to be reservoir competent, urban foci of transmission of B. spielmanii may affect the health of townspeople.