Disentangling complex disease ecology networks: Using structural equation modelling to quantify the direct and indirect effects of deer on Lyme borreliosis hazard

Abstract

Quantifying the strengths of interactions in complex vector-borne disease ecological systems is challenging. Yet, overcoming this challenge is fundamental for understanding the ecological mechanisms shaping disease hazard. Here we quantified the strengths of the hypothesised direct and indirect mechanistic pathways through which deer affect ticks and one of the Lyme borreliosis pathogens, Borrelia afzelii, by conducting a combined analysis of three previously published datasets from 39 sites across Scotland. Structural equation modelling revealed that, as predicted, deer had a strong positive overall effect (direct and indirect pathways combined) on questing Ixodes ricinus nymph density and a weak, non-significant, negative overall effect on B. afzelii prevalence. This resulted in an overall weak, non-significant, positive effect of deer on B. afzelii hazard (the density of infected nymphs), indicating that their negative effect on B. afzelii prevalence was weaker than their positive effect on questing nymph density. A key novelty of this study was being able to tease apart the direct and indirect pathways for each of these overall effects and demonstrating that they were primarily driven by direct mechanisms, such as deer driving nymph density. Although deer negatively affected rodent abundance, the hypothesised indirect pathways from deer to ticks and pathogen, acting through vegetation and/or rodents, were weak. This could result from low densities of rodents relative to deer in Scotland, consistent with Scotland having among the lowest nymphal Lyme borreliosis pathogen prevalences in Europe. Applying the methodological framework used in this study would be useful for teasing apart complex interactions in other vector-borne disease systems.

Dr Sara Gandy

Research Associate

My research primarily focuses on understanding the ecological drivers of tick-borne diseases to provide insights on the mechanisms involved in transmission cycles, especially on the interactions between ticks, hosts and pathogens. My research includes investigating the impacts of environmental changes (woodland restoration, rewilding) and host community composition (deer, rodent and birds) on tick-borne diseases risks in the UK. Some of my findings uncovered an opposing effect of deer densities on Lyme disease hazard through their positive effects on tick density and negative effects on infection prevalence. I also published work looking at spatiotemporal changes in tick distribution using passive surveillance data and I have been leading the National Tick Survey, which involves collaborating with stakeholders in thirty National Parks and recreational areas to collect ticks and test them for various tick-transmitted pathogens between 2021 and 2024. The goal is to understand disease hazard and temporal variation in areas visited by members of the public and deliver tick awareness messages.