The socio-economic and geographical distribution of malaria overlaps with that of many parasitic helminths and in these areas co-infections are common. Co-infection with helminths can influence disease outcome causing either exacerbation or amelioration of malaria. Understanding the complex host-parasite interactions that lead to these different disease outcomes is important for the success of control programmes aimed at these parasites. The immune system has evolved diverse types of response (e.g. T-helper 1 (Th1) and T-helper 2 (Th2)) to efficiently combat infection with ‘microparasites’ and helminths respectively. When faced with co-infection however, the need for the host to multitask means it must manage these counter-regulatory responses. In this study a murine model of malaria-hookworm (Plasmodium chabaudi- Nippostrongylus brasiliensis) co-infection was utilised to investigate how changes in T-helper bias affect malaria disease outcome. Antibody isotypes were used as indicators of Th1/Th2 bias and revealed that helminth co-infection reduced the malaria-specific Th1 response. Counter-intuitively this resulted in ‘protection’ from malaria with co-infected mice having reduced peak P. chabaudi parasitaemia and suffering less severe anaemia. In addition to providing a measure of Th1/Th2 bias, analysis of antibody responses revealed the occurrence of cross-reactive antibodies. The potential for these crossreactive antibodies to influence disease outcome was investigated but in this murine model resource-mediated mechanisms of parasite regulation appear to be responsible for the ‘protection’ that co-infection affords. The question of why cross-reactive antibodies are produced has important immunological and ecological implications. Cross-reactive responses may arise through some physiological constraint on the immune mechanisms that usually result in antibody-specificity. However experiments designed to investigate if the specificity of antibodies is constrained by availability of antigen suggest that this is not the case in the model system used here. There is also the possibility that production of cross-reactive antibodies represents an evolutionary optimal strategy for a host faced with unpredictable exposure to a variety of parasites. However a major finding of this study indicates these two taxonomically distinct parasite species share antigens, which in itself is crucial to understanding host-parasite interactions in a co-infection setting. The main findings of this thesis are relevant to co-infection studies in general and the implications for both evolutionary and applied biology are discussed.

De nombreux parasites circulent dans les populations naturelles. Au sein d’un hôte, souvent pluri-infecté, ils peuvent interagir, augmentant ou réduisant le risque d’infection et les symptômes d’autres pathogènes. L’étude de ces interactions commence seulement dans les populations naturelles. Les enjeux sont cruciaux : détecter les interactions d’intérêt, estimer la probabilité de coinfection, comprendre la cocirculation des parasites. La détection des interactions sur le terrain est compliquée par la nature des données (e.g., présence-absence) et les facteurs confondants créant des associations statistiques (fausses interactions). Ce travail visait à mener une réflexion transversale sur ces interactions et le multiparasitisme, avec des applications à des données sérologiques pour 4 virus félins suivis dans des populations rurales de chats domestiques. De nouvelles méthodes de modélisation dynamique et statistique ont été développées pour prendre en compte les facteurs générant de fausses interactions (effet cumulatif de l’âge, facteurs de risque communs) et évaluer le biais des méthodes classiques. Des synergies entre 3 couples de virus félins ont été révélées. On a aussi identifié des caractéristiques comportementales et physiologiques (modes de vie, niveau de testostérone), qui, en modulant l’exposition et la sensibilité aux pathogènes, génèrent une forte hétérogénéité entre les hôtes. Enfin, une vision intégrative des systèmes hôte-parasites est indispensable pour appréhender la complexité des communautés et évaluer l’impact de la multitude d’hôtes, de parasites et d’interactions sur la coévolution, la conservation des espèces et la gestion des maladies infectieuses / Numerous parasites circulate within natural host populations. Within a host, often pluri-infected, parasites can interact, increasing or decreasing the infection risk and/or symptoms’ severity of other pathogens. Studies of such interactions only start in natural populations. Their stakes are high: detecting interactions of interest, estimating coinfection probabilities and understanding the cocirculation of parasites. The detection of interactions in the field is however complicated by the nature of data (often presence-absence) and the existence of confounding factors that can create statistical associations (false interactions). This work aimed at having a cross-cutting reflection on those interactions and on multiparasitism, with applications on a rich dataset of four feline viruses followed in rural populations of domestic cats. New dynamical and statistical modeling methods were developed to take into account factors generating false interactions (cumulative effect of age, shared risk factors) and evaluate the biases of classical methods. Synergies between three pairs of feline viruses were revealed. In addition, we identified behavioral and physiological factors (e.g., way of life, testosterone levels) that, by modulating exposition and/or susceptibility to pathogens, generate strong heterogeneity between hosts. Finally, a more integrative approach to host-parasites systems is proposed. It now appears necessary if one wants to deal with communities’ complexity and further evaluate the impact of multiple hosts, multiple parasites and their interactions on their coevolution, species conservation and infectious diseases management

The original aim of my diploma thesis was examination of fixed samples of cladocerans from the Daphnia longispina species complex from reservoirs Vír and Vranov for the presence of 4 groups of microparasites: microsporidia, oomycetes, the protozoan Caullerya mesnili, and the yeast Metschnikowia bicuspidata. The next aim was to determine the effect of parasites on Daphnia fecundity, and determination of their spatial and temporal distribution within reservoirs. From the total number 4452 of examined Daphnia females, however, only 56 individuals were infected. This number was not sufficient for the planned analyses. As I recorded for all examined individuals the clutch size, I thus dealt to a large extent with an alternative issue: the temporal and spatial variation of, and the influence of environmental factors on Daphnia fecundity. Both studied reservoirs are characteristic by canyon-shaped profile that allows formation of environmental gradients on the horizontal as well as vertical axis. The results confirm that reservoir identity, season, and location within the reservoir (or gradient of food supply) have all significant effects on fecundity. During my work I have encountered difficulties associated with determining microparazites from fixed zooplankton samples. The appendix section of my thesis...