TRANSMISSION OF BORRELIA-BURGDORFERI S1 FROM MAMMAL RESERVOIRS TO THEPRIMARY VECTOR OF LYME BORRELIOSIS, IXODES-RICINUS (ACARI, IXODIDAE),IN SWEDEN

Factors regulating prevalence of Borrelia burgdorferi s.l. Johnson Sch mid, Hyde, Steigerwalt & Brenner in Ixodes ricinus (L.) were examined during 1991-1992 at Bogesund, near Stockholm in south-central Sweden. Nine species of small and medium-sized mammals (Sorex araneus L., S. m inutus L., Neomys fodiens Pennant, Clethrionomys glareolus [Schreber], Microtus agrestis [L.], Apodemus sylvaticus [L.], A. flavicollis [Mel chior], Lepus europaeus Pallas, L. timidus L.) were found to infect fe edin tick larvae with B. burgdorferi s.l., whereas two species of larg e mammals (Capreolus capreolus L., Alces alces L.) failed to infect fe eding tick larvae with this spirochete. The most important mammalian r eservoirs at the study locality were S. araneus and rodents, accountin g for 91% of all I. ricinus larvae infected. In view of the great numb er of potentially effective reservoirs for B. burgdorferi s.l. in Swed en, control of Lyme disease by reduction of abundance of reservoir hos ts will be difficult to achieve. We also found that infectivity of a r odent species is related to the number of infesting, potentially infec tive nymphal I. ricinus. Insectivores and rodents were the most import ant hosts of larval I. ricinus, whereas most nymphal ticks fed on hare s and cervids. Adult I. ricinus were frequently found on all species o f hares and cervids examined but never on insectivores and rodents. No single species seemed to be of paramount importance as a source of bl ood for female ticks. Therefore, control of Lyme disease by reduction of abundance of mammal hosts available for female tick engorgement wil l probably require massive reductions of numbers of both C. capreolus and L. timidus.