We investigated isolated ecosystems to determine if
Lyme borreliosis is maintained in the absence of reservoir-competent small mammals. Human cases of
Lyme disease have been reported on the isolated islands of Gotska Sandön and Stora Karlsö in the Baltic Sea. The varying hare, Lepus timidus, is the only terrestrial vertebrate species capable of serving as a host for all stages of Ixodes ricinus (L.) on these islands. In August, mean larval infestation on 5 hares from each island was 548 with a maximum of 2,276 larvae on 1 hare. Smaller numbers of nymphal and female ticks were also engorging on the hares. During August-September, B. burgdorferi s.l. was detected in 11-24% of nymphal I. ricinus fed as larvae on hares and in 8-19% of host-seeking nymphal I. ricinus collected from the vegetation. We conclude that L. timidus serves as a maintenance host for B. burgdorferi s.l. and its vector, I. ricinus, on both islands.