Bridging of cryptic Borrelia cycles in European songbirds

The principal European vector for Borrelia burgdorferi s.l., the causative agents of Lyme disease, is the host-generalist tick Ixodes ricinus. Almost all terrestrial host-specialist ticks have been supposed not to contribute to the terrestrial Borrelia transmission cycles. Through an experiment with blackbirds, we show successful transmission by the widespread I. frontalis, an abundant bird-specialized tick that infests a broad range of songbirds. In the first phase of the experiment, we obtained Borrelia-infected I. frontalis (infection rate: 19%) and I. ricinus (17%) nymphs by exposing larvae to wild blackbirds that carried several genospecies (Borrelia turdi, B. valaisiana, B. burgdorferi s.s.). In the second phase, pathogen-free blackbirds were exposed to these infected nymphs. Both tick species were able to infect the birds, as indicated by the analysis of xenodiagnostic I. ricinus larvae which provided evidence for both co-feeding and systemic transmission (infection rates: 10%-60%). Ixodes frontalis was shown to transmit B. turdi spirochetes, while I. ricinus transmitted both B. turdi and B. valaisiana. Neither species transmitted B. burgdorferi s.s. European enzootic cycles of Borrelia between songbirds and their ornithophilic ticks do exist, with I. ricinus potentially acting as a bridging vector towards mammals, including man.This research was supported by the Fund for Scientific Research - Flanders Belgium (grant G0.049.10) and the University of Antwerp (KP BOF UA 2015). The molecular work was done under the frame of COST action TD1303 EurNegVec. Dieter Heylen is a postdoctoral fellow at the Fund for Scientific Research - Flanders Belgium (FWO). This study received some financial support from Fundação para a Ciência e a Tecnologia by the strategic program of MARE (MARE - UID/MAR/04292/2013) and the fellowships to Ana Claudia Norte (SFRH/BPD/108197/2015 and SFRH/BPD/62898/2009).