Supplemented nutrition decreases helminth burden and increases drug efficacy in a natural host–helminth system

Gastrointestinal helminths are common parasites of humans, wildlife, and livestock, causing chronic infections. In humans and wildlife, poor nutrition or limited resources can compromise individuals’ immune response, predisposing them to higher helminth burdens. This relationship has been tested in laboratory models by investigating infection outcomes following reductions of specific nutrients. However, much less is known about how diet supplementation can impact susceptibility to infection, acquisition of immunity, and drug efficacy in natural host-helminth systems. We experimentally supplemented the diet of wood mice Apodemus sylvaticus) with high quality nutrition and measured resistance to the common gastrointestinal nematode Heligmosomoides polygyrus. To test whether diet can enhance immunity to reinfection, we also administered anthelmintic treatment at random in both natural and captive populations. Supplemented wood mice were more resistant to H. polygyrus infection, cleared worms more efficiently after treatment, avoided a post-treatment infection rebound, produced stronger general and parasite-specific antibody responses, and maintained better body condition. In addition, when applied in conjunction with anthelmintic treatment, supplemented nutrition significantly reduced H. polygyrus transmission potential. These results show the rapid and extensive benefits of a well-balanced diet and have important implications for both disease control and wildlife health under changing environmental conditions

Synchronous seasonality in the gut microbiota of wild mouse populations

The gut microbiome performs many important functions in mammalian hosts, with community composition shaping its functional role. However, the factors that drive individual microbiota variation in wild animals and to what extent these are predictable or idiosyncratic across populations remains poorly understood. Here, we use a multi-population dataset from a common rodent species (the wood mouse, Apodemus sylvaticus), to test whether a consistent “core” gut microbiota is identifiable in this species, and to what extent the predictors of microbiota variation are consistent across populations. Between 2014 and 2018 we used capture-mark-recapture and 16S rRNA profiling to intensively monitor two wild wood mouse populations and their gut microbiota, as well as characterising the microbiota from a laboratory-housed colony of the same species. Although the microbiota was broadly similar at high taxonomic levels, the two wild populations did not share a single bacterial amplicon sequence variant (ASV), despite being only 50km apart. Meanwhile, the laboratory-housed colony shared many ASVs with one of the wild populations from which it is thought to have been founded decades ago. Despite not sharing any ASVs, the two wild populations shared a phylogenetically more similar microbiota than either did with the colony, and the factors predicting compositional variation in each wild population were remarkably similar. We identified a strong and consistent pattern of seasonal microbiota restructuring that occurred at both sites, in all years, and within individual mice. While the microbiota was highly individualised, some seasonal convergence occurred in late winter/early spring. These findings reveal highly repeatable seasonal gut microbiota dynamics in multiple populations of this species, despite different taxa being involved. This provides a platform for future work to understand the drivers and functional implications of such predictable seasonal microbiome restructuring, including whether it might provide the host with adaptive seasonal phenotypic plasticity

Sex-differences in parasite infection and immunity in wild wood mice

Within mammals and other vertebrates, males are often found to be the more infected sex and are more likely to suffer from mortality or adverse effects due to parasite infections compared to females. This is certainly true in humans, as mortality from infectious diseases is heavily male-biased. Sex-differences in both susceptibility and parasite exposure are two suggested mechanisms underlying the observed male bias. Studies from wild vertebrates that report sex-biased parasitism however, are usually cross-sectional and conducted in a single parasite-host system at a specific time point or even within a specific population. There is increasing evidence to suggest however, that sex-differences in the wild might be more dynamic and context-dependent. Understanding the contexts within which sex-differences are observed might not only provide a better understanding of the causes and consequences underlying sex-biased parasitism, but might also provide better insight into the role of host sex in host-parasite dynamics and epidemiology. This thesis aims to study sex-differences in parasite infection at the individual as well as a population level using the wild system of the wood mouse (Apodemus sylvaticus) and its natural parasite community. To address this key aim, I conducted paired field and laboratory experiments to investigate sex-differences in susceptibility, infection risk and immune responses in the wood mice system. Through experimental immunisations and diet manipulations, I addressed how the sexes differ in their vaccine-induced immune responses and the role of nutrition in driving these sex differences. I then investigated sex-differences in parasite infection at the population level by using a 6 year-long longitudinal dataset of multiple populations of wood mice and a community of ~14 parasite and pathogen species. Specifically, I tested whether there were consistent sex-differences in parasite prevalence and intensity across different parasite species, years, woodland field sites, and host condition. Finally, using the same long-term dataset, I addressed sex-differences in the fitness and survival consequences of infection and treatment. I analysed data from large-scale, multi-year helminth removal experiments to test whether anthelmintic treatment reduces helminths in wild wood mice, if this reduction impacts mouse condition and survival, and importantly, if this effect is sex-specific. To conclude, I discuss my findings and their implications both for applied translational research as well as fundamental research in disease ecology