Integrated landscape genetics of the Lyme disease vector «Ixodes scapularis» and its host «Peromyscus leucopus» in southern Québec

Understanding how interactions between vectors, hosts, and pathogens, as well as their responses to climate and landscape conditions, influence disease emergence events is crucial for planning effective disease monitoring and control strategies. In southern Québec, the emergence of Lyme disease has been attributed to the northward range expansion and increase in abundance of both the disease vector - the black-legged ticks (Ixodes scapularis Say), and the reservoir host – the white-footed mice (Peromyscus leucopus Rafinesque). The goal of my thesis is to quantify interactions between populations of black-legged ticks and white-footed mice in southern Québec, as well as their individual responses to landscape heterogeneity using the multi-taxa integrated landscape genetics approach (MTILG). MTILG can provide valuable insights on biotic and abiotic variables that influence zoonotic disease emergence in a region by revealing correspondences in spatial genetic patterns that distinguish possible cases of species-coupled dispersal or shared spatial dependence, and identifying landscape variables and the extent to which they influence species movement. When I investigated white-footed mouse population structure relative to the landscape and examined spatial morphological variation among populations, I found that white-footed mouse populations in southern Québec exhibited significant genetic differences among populations that could be explained by the presence of barriers to gene flow. There was also evidence of adaptations for dispersal and range expansion in the form of a significant and positive correlation between mouse foot length and latitude. I found evidence of weak genetic differentiation among populations of black-legged ticks, as well as multiple genetic signatures of frequent long-distance dispersal events, supporting the hypothesis that migratory passerine birds are the primary transport of black-legged ticks into southern Québec. Furthermore, I found that tick genetic diversity exhibited greater temporal variation than spatial variation, a result that I was able to utilize to determine stages of establishment for each tick population. Finally, using structural equation modeling that incorporated insights from the previous chapters, along with climate and landscape variables, I was able to produce a model that successfully projected current Lyme disease risk observed in the field. The model revealed that Lyme disease infection risk (defined here as the prevalence of Borrelia burgdorferi at a site) was significantly dependent on climate-based disease vector occurrence probability. This model can be a useful tool for projecting future Lyme disease infection risk in response to changing climate and landscape conditions, and can also be used to assess the potential effectiveness of different disease intervention strategies (e.g. land management policies vs. host culling).The research in this thesis reveal the importance of an integrated approach for understanding some of the underlying variables influencing disease emergence events, and for more accurate and reliable predictions of disease risk that are essential for disease control and monitoring efforts. Insights obtained from my work will prove valuable in future efforts to control and monitor Lyme disease emergence in southern Québec, and at the same time serve as a useful source for raising both public and physician awareness of the emerging Lyme disease threat in Québec and the rest of Canada.

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