In large woodlands, ticks are more numerous, and birds and roe deer play a role in harbouring Borrelia - BIODILUTIQUE

The links between biodiversity and health represent a frontier in science where the concept of the dilution effect plays a central yet still debated role. Tick-borne diseases, which are often zoonotic, provide a particularly relevant pathosystem for exploring this concept. The Biodilutique project explored the links between the biodiversity of host and tick communities and the diversity of pathogens transmitted by these ticks, with a particular focus on bacteria of the genus Borrelia responsible for Lyme disease. 

Approaches

BIODILUTIQUE investigated the roles of woodland, grassland and cropland on the diversity of vertebrates and ticks, as well as on the prevalence of the pathogens they carry. Tick collection and camera monitoring were carried out at 62 landscape plots distributed across two landscape gradients of woodland and grassland proportions within the Plaine & Val de Sèvre Study Area.

In the springs of 2022 and 2023, 4,546 ticks were collected, of which 4,386 were found on vegetation, 71 on small mammals and 89 on birds.

In total, seven tick species were identified, of which 90.1% were Ixodes ricinus, a generalist species capable of biting humans, and 9.2% were Ixodes frontalis, a species specialising in birds (passerines – Turdidae and Corvidae – and Columbidae). 

Captures of small mammals (n=271) revealed the presence of six species, the three most abundant being the common shrew (50.2%), the common vole (34.2%) and the field vole (10%). Analysis of data from camera traps (n=723 videos) placed in the windows revealed the presence of 10 species of wild mammals and various species of birds.

Results

The highest tick densities were observed in the largest wooded areas, whereas smaller wooded areas, particularly those located in less wooded patches and less connected to other wooded areas, had lower or even zero densities.

Analysis of tick DNA revealed an overall prevalence of Borrelia of 8.2%, of which 3.2% corresponded to two species of Borrelia whose main reservoirs are birds, and 2.2% to a species of Borrelia whose main reservoirs are small mammals. Furthermore, these ticks collected from vegetation were also carriers of zoonotic pathogenic bacteria and protozoa.

Analysis of DNA extracted from biopsies of small mammals (n=80) revealed the presence of bacteria of the genera Bartonella (37.5%) and Ehrlichia (1.2%), Neoehrlichia mikurensis (1.2%) and the apicomplexan protozoan Hepatozoon sp. (33.7%). Among the I. ricinus ticks collected from birds (n=66), 51.5% were infected with B. garinii or valaisiana.

A multi-host, multi-pathogen mechanistic model was developed. A sensitivity analysis highlights the very significant influence of roe deer density; although roe deer are not suitable hosts for the multiplication of Borrelia, they play an important role in tick population dynamics. Birds appear to play a more significant role than small mammals as reservoirs for these pathogens.

A PhD thesis co-funded by Biosefair is continuing this research.

Participants

INRAE units involved

• UMR BIOEPAR - Biologie Epidémiologie et Analyse de Risque en santé animale - Oniris
• USC CEBC – Résilience - Centre d'Etudes Biologiques de Chizé