Drivers of Ixodes ricinus population dynamics in Northeast France: a Bayesian modelling approach

Abstract

Ixodes ricinus is the primary vector for Lyme disease and tick-borne encephalitis across Europe. Despite playing a critical role in disease transmission dynamics, the environmental drivers of its complex life cycle have not been quantified using real-world data. To address this gap, we fitted a unique mechanistic model to a detailed 10-years longitudinal dataset from four sites in Northern France, where I. ricinus is abundant and Lyme disease and tick-borne encephalitis have been reported for decades, within a Bayesian framework. By incorporating key demographic processes and the influence of environmental conditions on these processes, our model estimated oviposition, hatching, and moulting rates across a range of temperature or saturation deficit, as well as questing and vertebrate host contact rates. Notably, moulting peaked at 14.2{degrees}C (95%HDI: 12.5-16.1{degrees}C), substantially lower than commonly suggested by laboratory-based studies, whereas oviposition and hatching peaked at 24.4{degrees}C (95%HDI: 10.9- 27.2{degrees}C) and 24.7{degrees}C (95%HDI: 17.8-27.2{degrees}C), respectively. Furthermore, vertebrate host contact rates significantly varied between the four study sites, with one site presenting up to 2.90 (95%HDI: 2.15-3.86) times higher contact rates than the other three sites. Additionally, we showed the importance of diapause in reproducing the observed seasonal population dynamics. For ticks overwintering through diapause, moulting in spring more accurately matched the predominantly unimodal questing activity patterns observed, compared to moulting in summer. Finally, model projections under several climate change scenarios indicated decreasing tick abundance trends over the next two decades. This study provides a foundation for models of I. ricinus-borne pathogen transmission and can be adapted to other Ixodidae populations of public health significance.