This thesis represents two lines of investigation that as a whole integrate physiology and ecology of infectious diseases. While each chapter is a distinct body of work, these studies are linked in that both focus on extrinsic or intrinsic factors influencing hosts’ susceptibility to infection, as well as hosts’ potential to transmit disease. In chapter one, I used a meta-analysis as a tool to review what is known about the stress-linked susceptibility hypothesis, which poses that persistent activation of the stress axis might result in increased disease susceptibility in wild animals, mainly as a result of allostatic overload and the immunosuppressive actions of stress hormones. My goal in evaluating the strength of association between distinct stressors and health indices in wildlife populations was manifold. First, I wanted to investigate which specific stressors are more likely to increase disease susceptibility in wild animals. Second, I was interested in the reliability of stress biomarkers as indicators of stress-induced immunosuppression or disease susceptibility. Finally, I used this chapter as a bridge for my second chapter by addressing how “fast-paced” and “slow-paced” phenotypes within wild populations cope differently with stressors, thus also differentially altering epizootic risk. My meta-analysis indicated that the highly variable outcomes of ecological studies can be attributed to several factors, ranging from possible erroneous classification of stimuli as physiological stressors to a lack of consensus on the endocrine profiles of stressed animals. The second chapter of this work consists of an empirical investigation of the pace-of-life (POL) hypothesis in the context of host disease susceptibility and transmissibility. Individuals with a fast-paced life history often exhibit relatively high metabolic rates and investment in growth, development, and reproduction. To support these faster rates, they often exhibit decreased investment in immunity, as well as associated bold behaviors for increased foraging and competiveness to ensure access to resources. These associated functional physiological and behavioral traits likely also influence exposure and susceptibility to pathogens, and infectiousness; factors central to disease dynamics. Through transmission trials using ranavirus and larval amphibians as a model system, I found that repeatable latency-to-food profiles of larval hosts, which characterize a POL axis associated with development and metabolic rates, were predictive of individual susceptibility and infectiousness. Faster-paced larval amphibians had greater exposure to pathogens (contacts), higher risk of infection (susceptibility), and shed greater pathogen populations when infected. Through these findings, I argue that The POL framework can allow for a priori identification of individual hosts that are more likely to spread infectious disease and may provide insight into understanding and potentially managing disease outbreaks that threaten wildlife and humans alike.

Updated: Current version includes the name of Supervisor, and Co-supervisor. Error corrected in preliminary pages. / Background: Emerging evidence suggests that there is both within-disorder heterogeneity and across-disorder overlap in the clinical presentation of children with ASD, ADHD, and OCD. Two prevalent dimensional phenotypes in children with these NDDs that warrant close attention, and are suitable for cross-disorder investigation, are internalizing and externalizing problems. Objectives: The current study uses a data-driven, diagnosis-agnostic approach to identify homogenous clusters that describe behavioural profiles of internalizing and externalizing problems within and across ASD, ADHD, and OCD. Methods: Data on 1565 children (M = 10.76 years) were drawn from the Province of Ontario Neurodevelopmental Disorder (POND) Network. Non-hierarchical clustering approaches were used to empirically derive, distinct behavioural profiles of internalizing and externalizing problems indexed by the Child Behavior Checklist (CBCL). Empirically derived groups were characterized using measures of adaptive functioning indexed by the Adaptive Behaviour Assessment (ABAS-II), and interpreted in relation to original diagnoses. Results: Cluster analyses identified four distinct behavioural profiles that cut across all diagnostic groups: High Internalizing Externalizing (HIE; 15%), High Externalizing (HE; 21%), Low Internalizing Externalizing (LIE; 38%), and Low Externalizing (LE; 26%). Derived clusters had variable levels of adaptive behaviours and reflected different behavioural profiles than the ones defined by the original diagnostic category groups of ASD, ADHD, and OCD. Conclusion: Data-driven, diagnosis-agnostic approaches can inform our understanding of the between and within phenotypic heterogeneity seen in ASD, ADHD, and OCD. Empirical ways of classifying children with homogeneous behavioural profiles may complement existing diagnostic models in our efforts to develop cross-disorder, more personalized interventions for children with neurodevelopmental disorders. / Thesis / Master of Science (MSc) / ASD, ADHD, and OCD are heterogenous neurodevelopmental disorders (NDDs) with some overlapping clinical characteristics and etiological factors. Internalizing and externalizing behavioural problems persist across these three NDDs, and in this study, are used to identify unique behavioural profiles. Study findings reveal four groups with distinct behavioural profiles of internalizing and externalizing problems that are not identified by the original diagnostic groups. This empirical way of classifying children with similar behavioural profiles can be used in combination with diagnostic labels to enhance transdiagnostic interventions that can be tailored to each child’s needs.