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Abstract
Most animals are concurrently infected with multiple parasite species, and interactions among co-occurring parasites can influence disease dynamics and host fitness. I used a cross-sectional field study, a parasite removal experiment, a laboratory experiment, and a longitudinal field study to investigate the causes and consequences of parasite community composition. The four-year field study presented a unique opportunity to investigate parasite communities in a wild population of African buffalo (Syncerus caffer) with repeated sampling of condition, immune function, and coinfection status by multiple parasitic worms. My goals were to understand the drivers of individual variation in infection and to measure the costs of coinfection for hosts and parasites. To test the utility of a community ecology trophic framework for studying of parasite coinfection, I conducted a factorial experiment using laboratory mice as mesocosms where two helminths (Nippostrongylus brasiliensis and Heligmosomoides bakeri) and a microparasite (Mycobacterium bovis) could interact. Resources altered the strength of interactions between coinfecting parasites and had opposing effects on the two helminths. Immune-mediated effects of coinfection were stronger than resource-mediated effects on parasite reproduction. Importantly, I documented stronger immune-mediated facilitation between micro- and macroparasites during resource limitation that may have implications for susceptibility to and severity of major human pathogens because resource scarcity and helminth infection frequently co-occur. Examining the costs of coinfection in wild African buffalo populations, I found that one helminth species may decrease host survival via impacts on host condition, while, surprisingly, a closely related species was positively associated with host condition. This study is one of the first to empirically link coinfection in a wild population to changes in condition and survival. Together, the field and laboratory components of my dissertation highlight how different types of parasites interact with each other and with their hosts, and underscore the context-dependency of these interactions. More broadly, my research highlights that measuring the intermediate mechanisms by which parasites and hosts interact, namely via resources and immune function, can provide insight into the variability in coinfection outcomes for hosts and parasites.