Nature provides many benefits for human health. However, they also harbour zoonoses, diseases that usually circulate in animals and may be transmitted to humans. Many tick-borne pathogens, such as the causative agent of Lyme borreliosis, are transmitted by the bite of an infected tick. They constitute a complex system involving multiple agents that interact in the environment: pathogens, hosts, ticks, and humans. The interconnectedness between humans, animals and the environments, and their influence on health calls for an integrated collaborative framework such as One Health.

The general objective of the thesis was to assess tick-borne disease risk in Belgium using a One Health approach. To account for the complexity of tick-borne disease ecology, we used datasets covering all components of the system, and methods ranging from mixed-effect regressions to multi-criteria decision analyses and Bayesian spatial models. We studied environmental determinants of tick abundance and infection at the forest scale in the Bois de Lauzelle (Louvain-la-Neuve). Our tick collections investigated spatial and temporal variability in tick abundance and tick infection prevalence, and identified tick-associated micro-bacterial communities. We also used surveillance serological data for Walloon cattle to create a spatial index identifying the risk of bovine ehrlichiosis infection in pastures. Finally, by combining environmental predictors of tick bite risk, we created a risk map for Belgian municipalities and compared the results with crowd-sourced tick bite incidence data. This dissertation underlined the advantages of using an integrated multi-level One Health approach to assess the risk of tick-borne disease.