Helminth parasitism of rodents in Algonquin Provincial Park, Ontario, Canada

Abstract

Parasites are a major force shaping the ecology, physiology, and evolution of wildlife populations. This thesis investigates the complex interactions between gastrointestinal (GI) helminths and rodents in Algonquin Provincial Park, Ontario, focusing on how parasite infections are influenced by host traits such as age, sex, body mass, and reproductive condition. It also explores how parasitism alters host immune responses and body condition. While ectoparasites are widely studied, GI helminths remain underexplored despite their significant impacts on host health and fitness. The introductory material reviews the ecological relevance of helminth infections, detailing their effects on host survival, reproduction, and immune investment. Emphasis is placed on the role of host life history and behaviour in mediating infection risk, alongside sex-specific reproductive strategies. Two helminth species, Capillaria hepatica and Hymenolepis spp., are highlighted as common parasites infecting deer mice (Peromyscus maniculatus), red-backed voles (Clethrionomys gapperi), and woodland jumping mice (Napaeozapus insignis). Anthelmintic experiments and immune biomarkers such as neutrophil-to-lymphocyte ratios are introduced as tools for assessing host–parasite dynamics. Chapter 2 examines infection patterns across the three rodent species, revealing species-specific effects of host traits on parasite richness and burden. Notably, females and reproductively active individuals exhibited elevated infection intensities, supporting the reproduction–immunity trade-off. Chapter 3 assesses immune responses using neutrophil-to-lymphocyte ratios and an antiparasitic treatment. Results show that parasite diversity, rather than intensity, best predicted immune activation in deer mice, while host sex and reproductive status were key predictors in red-backed voles. Overall, this thesis highlights the role of host biology, immunity, and parasite diversity in shaping infection outcomes in wild rodents.