The interactions between parasites, hosts, and the ecosystems they live in are complex and diverse. This can make predicting how they will respond to pressures like nutrient enrichment difficult. However, this endeavor is important, due to the far-reaching effects it has on ecosystems functioning, industries like aquaculture, and even human health. Even with the significance of this topic, there are key parts of the picture that have received little research attention. For this reason, I chose to study the effects of nutrient enrichment on parasite-host interactions at two different scales, first investigating the interaction directly between parasites and their host, and then manipulating experimental ponds to look at trophic effects. Using the theory of ecological stoichiometry as a conceptual framework allowed for comparison across the two investigations. In both studies, I used largemouth bass and the macro parasites of their digestive tract as my study system, due to the importance of largemouth bass as sport fish, their high trophic status, and diverse parasite assemblages. My first study examined the effects of eutrophication on parasite-host interactions occurring within the host. It centered on four hypotheses: 1) the nutrient content of different host tissues (infection sites) varies within and across hosts, 2) the nutrient contents of parasite genera differ from that of their host tissue(s), 3) the nutrient contents of parasite genera differ from one another, and 4) nutrient availability within specific host tissues is related to the nutrient content and abundance of parasite genera within tissues. I quantified the nutrient content (carbon (C), nitrogen (N), and phosphorous (P) and their ratios) of parasitic infracommunities in the tissues of the liver, stomach, pyloric caeca, intestines, and visceral mesentery of 30 largemouth bass (Micropterus salmoides). I found stoichiometric variation across host tissues and that fish collection site explained variation in pyloric caeca N:P ratios. Parasitic genera differed in their nutrient content with actively feeding parasite forms having higher %N and lower C:N ratios than encysted/non-reproductive forms. In addition, the %C of both actively feeding parasites varied across organs, and, for one genus, this variation reflected differences in host tissue %C. Finally, I found that the total number of actively feeding parasites in the pyloric caeca increased with that tissue’s N:P ratio. My results suggest that parasites encounter significant variation in nutrient availability within and across hosts, and that this variation can influence the nutrient content and abundance of some actively feeding parasites within specific tissues. To determine if the effects from my first study were a result of nutrient changes to host tissues or if they were an artifact of changes occurring at other trophic levels caused by nutrient addition, I conducted an experimental manipulation of a short trophic system. I applied two treatments, a fertilized treatment and a control, to 14 ponds stocked with largemouth bass. I hypothesized that adding a low N:P fertilizer to the ponds would result in fewer parasites in the pyloric caeca of the bass in accordance with findings from my previous work. Alternatively, I hypothesized that indirect changes to intermediate hosts would be responsible for changes in parasite abundance. To measure the effects of nutrient additions on multiple trophic levels, samples of fine particulate organic matter (FPOM), benthic invertebrates, and vertebrate prey were taken from each pond. Stocked bass were retrieved and dissected to obtain samples for nutrient analysis from their pyloric caeca, and to quantify parasite abundances. My treatment was able to affect the %P, C:N, C:P, and N:P of the FPOM in the ponds. Benthic invertebrate biomass was negatively correlated with FPOM %P. I did not find a direct effect of benthic invertebrates on vertebrate prey species, but fertilized ponds had a smaller decrease in the number of vertebrate prey over the course of the experiment. Counterintuitively, the %N of the pyloric caeca of fish in the fertilized ponds decreased, and the %C and C:N increased in relation to the number of vertebrate prey, but %P did not change. The treatment status of the pond and the nutrients of the pyloric caeca did not affect the number of parasites that were found, but parasite numbers were positively associated with prey fish. This result would suggest the number of intermediate hosts, in this case, prey fish, is the determining factor for parasitic loads in the bass I collected. The indirect effects of trophic interaction and the direct effects of changes to host and parasite dynamics both play important roles in predicting the outcome of eutrophication on disease. The conflicting results of these two studies highlight the need for in-depth knowledge of the systems being affected by nutrient additions.

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Die Epithelzellen des Dünndarms bilden die Hauptinfektionsroute für viele Protozoen wie zum Beispiel Toxoplasma gondii und Giardia duodenalis. Jedoch sind die Mechanismen dieser Infektionswege unbekannt, da geeignete Modelle fehlen, welche das Darmepithel nachbilden. In der folgenden Studie wurde eine in-vitro Plattform mit Darmorganoiden (organoid derived monolayers oder ODMs) etabliert, welche man für vergleichende Studien zu Parasit-Wirt-Interaktionen anwenden kann. Das ODM-System wurde angewendet, um die Anfangsphase einer T. gondii-Infektion zu modellieren, wobei der Schwerpunkt auf die Rolle von Interferon gamma (IFNγ) und immunitätsbezogenen GTPasen (Irgs) lag. Es wurde gezeigt, dass sich die Irg-abhängige Kontrolle virulenter Toxoplasma-Stämme zwischen dem Labormausmodell und anderen wildlebenden Nagetierarten unterscheidet. Aus diesem Grund wurden Vergleiche mit Organoiden verschiedener Labormausstämme und der Rötelmaus Myodes glareolus durchgeführt. Myodes glareolus ist eine Nagetierart, von der angenommen wird, dass sie eine höhere Resistenz gegen T. gondii aufweist. Basierend auf die Resultate der quantitativen Immunofluoreszentests und qPCR dieser These, führt die Stimulation mit IFNγ zu einer tendenziell verringerten Replikation der Parasitenstämme RH und Prugniaus in M. glareolus ODMs im Vergleich zu Mus ODMs. In dieser Studie, wurde zum ersten Mal die Rolle von Irgs bei intestinalen T. gondii-Infektionen identifiziert. Zu diesem Zweck wurden Organoide von M. glareolus mit einem fluoreszierend markierten Irgb10-Protein transfiziert, wodurch gezeigt werden konnte, dass Irgb10 T. gondii-Vakuolen dekoriert, was auf eine Beteiligung des Irg-Systems hindeutet. Schließlich wurde ein Koinfektionsmodell für T. gondii und/oder G. Duodenalis in Maus-ODMs etabliert. In diesem Modell wurde gezeigt, dass T. gondii weder die Induktion der Barrierestörung durch G. duodenalis noch die Replikation von T. gondii durch G. duodenalis beeinflusst. / The small intestinal epithelium is the primary route of infection for many protozoan parasites such as Toxoplasma gondii and Giardia duodenalis. Understanding the mechanisms of infection with such parasites, has been hindered due to the lack of appropriate models mimicking the complexity of the intestinal epithelium. Here, an in vitro platform was established, using intestinal organoids (organoid derived monolayers or ODMs) for comparative studies on parasite-host interactions. The ODM system was used to model the events during the early phase of a T. gondii infection, focusing on the role of Interferon gamma (IFNγ) and Immunity Related GTPases (Irgs). Irg dependent control of virulent Toxoplasma strains has been shown to differ between the laboratory mouse model and other wild-derived rodent strains. How these responses occur in rodent species that do not belong to the murine family, is yet to be determined. For this reason, comparisons were made with organoids from different laboratory mice strains and the bank vole Myodes glareolus, a non-muridae rodent species assumed to be more resistant to T. gondii. Based on this thesis, stimulation with IFNγ leads to a trend of reduced replication in M. glareolus ODMs compared to Mus ODMs for both Type I parasite strain RH and for Type II strain Prugniaud, based on quantitative immunofluorescence assays and qPCR. Analysis of the role of Irgs in intestinal T. gondii infections was performed, by transfecting organoids from M. glareolus with a fluorescently labelled Irgb10 protein, showing that Irgb10 decorates T. gondii vacuoles, suggesting Irg-system involvement. Finally, a co-infection model in murine ODMs was established for T. gondii and/or G. Duodenalis. Here, it could be observed that T. gondii did not influence G. duodenalis induction of barrier breakdown nor did G. duodenalis influence T. gondii replication.