Susceptibility of ixodid ticks to Lyme disease borreliae : a search of the control molecules

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The ixodid ticks of the Ixodes ricinus species complex serve as vectors for Lyme disease spirochetes belonging to the genus Borrelia. The three genospecies, Borrelia burgdorferi, Borrelia garinii, and Borrelia afzelii, are pathogenic for humans ; however, there are several closely related microbes which are transmitted by other ixodid ticks. In general, a tick species serves as a vector for a certain Borrelia species. In order to investigate the specific relationship between ticks and borreliae, the following experiments were conducted. (1) Experimental infection of borreliae to several unsuitable tick species revealed that the susceptibility of ticks involves two phases (expulsion and permission of borreliae) through the molting process of ticks. The experiments also demonstrated that molted ticks retaining borreliae do not always serve as a vector for transmitting borreliae to vertebrates. (2) The quantitative and qualitative dynamics of borreliae were sequentially observed in a suit … More able tick species. Borreliae proliferated in diapausing ticks after taking blood meals ; however, the number of borreliae was dramatically decreased after molting. I found that excess borreliae in tick midgut discharged in feces of newly molted ticks. The remaining borreliae within the molted ticks did not proliferate through the period of starvation and failed to maintain the infectivity to vertebrates. However, the infectivity revived when the ticks were allowed to feed on new vertebrate host. During the feeding process, a part of outer surface proteins (osp) was expressed on borreliae. (3) I postulated that the control molecules for the susceptibility of ticks are the osp molecules on borreliae. Although the experiment for transfecting the osp plasmid to heterogenous Borrelia species was carried out, the trial to find an appropriate condition for transfection was failed. (4) The comparative genetic study on ticks and borreliae was conducted in two Japanese regions, Hokkaido and Nagano. The characteristic base substitution in DNAs of ticks and borreliae suggests that geographical isolation is important for convolution of ticks and borreliae. Less