We examined the Borrelia burgdorferi s.l. circulation in a tick community consisting of three species (Ixodes ricinus, I. frontalis, I. arboricola) with contrasting ecologies, but sharing a common host: the great tit (Parus major), one of the most common birds of European gardens and woodlands.
Field data show that the birds hosted Borrelia-infected larvae of both I. frontalis and I. ricinus, indicating the facilitation of Borrelia transmission. The low, but significant numbers of Borrelia in unfed I. arboricola ticks collected from bird nest boxes, provide the first evidence that it is competent in maintaining Borrelia over long periods of time. Aside from the known avian genospecies (B. garinii and B. valaisiana), several less dominant genospecies were observed in the three ticks, including B. turdi and some mammalian genospecies.
In laboratory experiments, we imitated the natural situation during the bird’s post-fledging period, in which Borrelia-naïve juvenile birds are repeatedly exposed to infected I. ricinus nymphs. Birds developed systemic infections of the avian genospecies. Although birds showed a very low competence to facilitate the transmission of mammalian genospecies, a low number of birds remained permissive for B. afzelii. Infected birds were able to transmit Borrelia to naïve I. frontalis and I. arboricola individuals, however, latter tick species were not able to transmit the bacteria to a new host.
When using the great tit as a host, transmission cycles are driven by I. ricinus, and are not maintained by the ornithophilic ticks (I. frontalis and I. arboricola). Still, spill-over of the bacteria from I. ricinus to the ornithophilic tick species often occurs in the wild. The use of bird species, other than the great tits (e.g. thrushes and finches), may result in different transmission outcomes than reported here.