Ixodes ricinus is an important vector of Borrelia burgdorferi in Europe, and small rodents (Apodemus flavicollis, A. sylvaticus and Clethrionomys glareolus) are important sources for infecting ticks. In this study, we examined their reservoir role by studying the duration of their infectivity for ticks. A. flavicollis and A. sylvaticus mice captured in nature were exposed to uninfected I. ricinus larvae at different times after their capture: 10 days, and 2, 7, 11, 14 and 40 months. Ticks were examined for spirochaetes after moulting using direct immunofluorescence. All animals remained infective for ticks their life long but the efficiency of transmission from hosts to ticks varied from one individual to the other, presenting a three-fold variation (26.5% to 81.4%). Rodents continously exposed to successive infestations by larval I. ricinus ticks over a period of one month showed an enhancement of infectivity for larval ticks during this period.

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SUMAIMARYBorrelia burgdorferi isolates were obtained from Ixodes ricinus from three sites in Switzerland. They were examined by SDS-PAGE and immunoblotting. The phenotypes, in respect of three outer surface proteins (Osp), differed between the sites of collection. In site 1, most isolates had an OspA of 31 kDa and an OspB of 34 kDa; in site 2, isolates presenting an OspA of 33 kDa dominated and in site 3, the isolates with an OspA of 32 kDa and an OspB of 35 kDa were most frequent. This distribution differed significantly. About half of the isolates from sites 1 and 3 reacted with anti-OspA monoclonal antibody H5332 compared to 29 % from site 2. Site 1 isolates reacted significantly more frequently (81 %) with another antiOspA monoclonal antibody LA-31 than isolates from site 3 (P < 0 0001). These findings have implications for the epidemiology of Lyme borreliosis, for the further development of serodiagnostic reagents and for the development of a vaccine.