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Borrelia burgdorferi sensu lato (s.l.) is maintained in nature by complex zoonotic transmission cycles, involving a large variety of vertebrates as hosts and hard ticks of the genus Ixodes as vectors. Recent studies suggest that the genospecies of B. burgdorferi s.l. and sometimes their subtypes are propagated by different spectra of hosts, mainly birds and rodents. In order to test the concept of host-association, we analysed the relationships between Borrelia genospecies, rodent hosts and I. ricinus ticks in an endemic focus of Lyme borreliosis in western Slovakia. Rodents and questing ticks were collected at a forested low land locality near Bratislava. Tick infestation levels on rodents were determined, and spirochaete infections in ticks and in ear punch biopsies were analysed by PCR followed by genotyping. Mice were more heavily infested with ticks than bank voles, and a higher proportion of mice was infected with spirochactes than voles. However, the infectivity of soles was much higher than that of mice. The vast majority of infections detected in the skin and in ticks feeding on the rodents represented B. afzelii. In contrast, more than half of all infections in questing ticks collected in the same region of Slovakia were identified as B. valaisiana and B. garinii. In conclusion, whilst the study reveals that mice and voles play different quantitative roles in the ecology of Lyme borreliosis, it demonstrates that B. afzelii is specifically maintained by European rodents, validating the concept of host-association of B. burgdorferi s.l.

The genetic diversity of Borrelia burgdorferi sensu lato was assessed in individual adult Ixodes ricinus ticks from Europe by direct PCR amplification of spirochetal DNA followed by genospecies-specific hybridization. Analysis of mixed infections in the ticks showed that B. garinii and B. valaisiana segregate from B. afzelii. This and previous findings suggest that host complement interacts with spirochetes in the tick, thereby playing an important role in the ecology of Lyme borreliosis.

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