Seasonal and annual variation in Borrelia garinii infectiousness in a wild bird

Successful reproduction of zoonotic diseases depends, among others, on the reservoir host’s ability to maintain and transmit the infection. Temporal variation in host infectiousness, i.e. the capacity of the host to infect susceptible individuals, and contact rates with naïve hosts strongly influence transmission dynamics. The vector-borne Borrelia garinii , causative agent of Lyme neuroborreliosis, is maintained in the environment through transmission between birds and Ixodes ricinus ticks. Although temporal variation in avian infectiousness affects B. garinii spread, it remains poorly understood in wild populations. Here, we assessed year-round variation in infectiousness of wild great tits ( Parus major ) using xenodiagnosis. Over seven consecutive seasons (from the 2022 until the 2024 breeding season), we conducted 420 xenodiagnoses. Our results showed a higher prevalence of infectious individuals during breeding seasons, particularly in 2023 (49% of birds were infectious). This peak was followed by a sharp decline in autumn and winter, which could suggest transient infectiousness. Few birds were repeatedly infectious, and most birds transmitted a single B. garinii haplotype, further indicating that most infections were transient. Our findings highlight the importance of accounting for species-specific temporal variation in host infectiousness, in addition to spatiotemporal host-vector overlap, when estimating avian contributions to Lyme neuroborreliosis transmission.