Impacts of nutrition quality on host-parasite dynamics in wild wood mice

Abstract

Factors from the environmental, host, and parasite community levels can all determine helminth burden in natural populations. In particular, the nutritional resources available to the host have long been associated with helminths; a large body of work in the laboratory has shown that both macro-and micro-nutrients play an important role in host response to infection. However, the relationship among nutrition, immunity, and helminth infection can depend upon several factors in the wild including season, host condition, and co-infecting parasites. Co-infection is the norm in natural populations, and the many parasites present may each have unique and contradictory relationships with nutrition quality. Recent increase in anthropogenic influences to the food available to wild animals –either accidental through urban waste or intentional through supplemental feeders—has therefore generated a crucial need for understanding the short- and long-term effects of changes to nutrition quality on disease outcome in natural host-parasite systems. To date, however, experimental, empirical data is still lacking in these areas particularly in regards to naturally co-infected populations. This thesis comprises a combination of statistical analysis and experimental work in the field and laboratory in a wood mouse (A. sylvaticus) system. I carried out diet supplementation manipulations for one laboratory and two field experiments designed to investigate how experimental perturbation to host environment in the context of resource availability influence the dynamics of both a highly prevalent nematode, Heligmosomoides polygyrus, and co-infecting parasites within the system. Making use of historical wood mouse trapping data, I further designed statistical approaches to determine how much the natural variation in environmental context affects host-parasite relationships Using experimental diet supplementation in both a wild and a captive population of A.sylvaticus, I found that supplemented nutrition quality increased both natural resistance to H. polygyrus and the efficacy of anthelminthic treatment via increased host condition and both general and H. polygyrus-specific immune investment. These results have important consequences for the control of disease and transmission of helminth infections in natural populations. I screened wood mouse populations in the wild following diet supplementation for an additional >10 parasite species including several other gastrointestinal helminths, gastrointestinal protozoans, ectoparasites, and blood-borne protozoans, bacteria, and viruses. I show that although supplemented nutrition decreased infection with helminths and ectoparasites via increased investment in immunity and condition, it unexpectedly increased infection risk and burden of some blood-borne and intestinal microparasites. This gives important insight into how nutrition may shape parasite communities and host fitness in wild populations where co-infection is the norm. I carried out a long-term field experiment with ongoing nutrition supplementation to investigate the effects of nutrition supplementation for host infection, reproduction, and survival over multiple seasons. I found that beyond short-term effects on parasite infection dynamics, supplemented nutrition drastically alters population dynamics for wood mouse populations, and the effects of nutrition on immunity within the population were both season- and cohort- dependent. Finally, through statistical analysis of six years of trapping data across multiple sites and seasons, I first show that there were significant drivers of helminth infection intensity at both the environment and host level. However, by accounting for spatiotemporal variation, I show further that these drivers varied significantly in magnitude and direction according to environmental context (i.e. across-years), and that sampling regime is key for the estimation of biological variation in H. polygyrus dynamics in a natural population. These results represent important experimental and statistical insights into the role of resource availability and environmental context for host-parasite dynamics in the wild. I discuss these findings and their implications for the study of nutrition quality and infection dynamics in disease ecology. I also present several avenues of ongoing and future work to complement insights provided by these experiments.