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We investigated to what extent a European songbird ( P arus major) selectively transmits and amplifies B orrelia burgdorferi s.l. bacteria. B orrelia-naïve birds were recurrently exposed to Ixodes ricinus nymphs carrying a community of more than 34 5 S-23 S genotypes belonging to five genospecies ( B orrelia garinii, B orrelia valaisiana, B orrelia afzelii, B. burgdorferi s.s. and B orrelia spielmanii). Fed ticks were screened for B orrelia after moulting. We found evidence for co-feeding transmission of avian and possibly also mammalian genotypes. Throughout the course of infestations, the infection rate of B . garinii and B . valaisiana increased, indicating successful amplification and transmission, while the infection rate for B . afzelii, B . burgdorferi s.s and B . spielmanii tended to decrease. Within the B . garinii and B . valaisiana genotype communities, certain genotypes were transmitted more than others. Moreover, birds were able to host mixed infections of B . garinii and B . valaisiana, as well as mixed infections of genotypes of the same genospecies. We experimentally show that resident songbirds transmit a broad range of B orrelia genotypes, but selectively amplify certain genotypes, and that one bird can transmit simultaneously several genotypes. Our results highlight the need to explicitly consider the association between genotypes and hosts, which may offer opportunities to point out which hosts are most responsible for the B orrelia presence in questing ticks. [ABSTRACT FROM AUTHOR]

Our study tested whether two European bird-specialized ticks, Ixodes arboricola and I . frontalis, can act as vectors in the transmission cycles of B orrelia burgdorferi s.l. The ticks have contrasting ecologies but share songbird hosts (such as the great tit, P arus major) with the generalist I . ricinus which may therefore act as a bridging vector. In the first phase of the experiment, we obtained B orrelia-infected ornithophilic nymphs by exposing larvae to great tits that had previously been exposed to I . ricinus nymphs carrying a community of genospecies ( B orrelia garinii, valaisiana, afzelii, burgdorferi s.s., spielmanii). Skin samples showed that birds selectively amplified B . garinii and B . valaisiana. The spirochetes were transmitted to the ornithophilic ticks and survived moulting, leading to infection rates of 16% and 27% in nymphs of I . arboricola and I . frontalis respectively. In the second phase, pathogen-free great tits were exposed to the B orrelia-infected ornithophilic nymphs. None of these ticks were able to infect the birds, as indicated by the tissue samples. Analysis of xenodiagnostic I . ricinus larvae found no evidence for co-feeding or systemic transmission of B . burgdorferi s.l. These outcomes do not support the occurrence of enzootic cycles of B orrelia burgdorferi s.l. involving songbirds and their specialized ornithophilic ticks. [ABSTRACT FROM AUTHOR]