Clethrionomys glareolus, but not Apodemus flavicollis, acquires resistance to Ixodes ricinus L., the main European vector of Borrelia burgdorferi.

The European rodents Clethrionomys glareolus (bank vole) and Apodemus flavicollis (yellow-necked-mouse) are important hosts of the tick species Ixodes ricinus, the main European vector of Borrelia burgdorferi. We have addressed the question whether or not these tick hosts develop resistance to I. ricinus larvae. C. glareolus and A. flavicollis were exposed to 40 I. ricinus larvae for five consecutive times at two week intervals. Resistance was tested by the following parameters: percentage of ticks fully engorged, time of attachment, engorgement index, percentage of recovered ticks and proportion of larvae moulting to nymphs. Repeated infestation of C. glareolus resulted in progressive and significant reductions in the percentage of fully engorged ticks, the time of attachment of partially engorged ticks, the scutal index of partially engorged ticks and the moulting success. In contrast, repeatedly infested A. flavicollis did not acquire resistance to larval I. ricinus. Effects of resistance in C. glareolus could be partially disrupted by treatment with the immunosuppressive agent, cyclosporin A (CsA), indicating that T helper cells participate in the immune responses to tick bites. The data suggest that acquired immunity to I. ricinus larvae in C. glareolus is a density-dependent factor regulating natural tick burdens and that it may have an impact on the transmission cycle of B. burgdorferi in Central Europe.