The benefits of coinfection: Trematode infections alter pathogen interactions and disease outcomes in hosts

Emerging infectious diseases are affecting host populations of humans, wildlife, and plants. Historically, epidemiological research has focused on singular hosts and singular pathogens. However, growing evidence suggests that many pathogens co-occur in wild populations, increasing the likelihood of coinfection within hosts. Pathogen interactions within hosts are likely to change disease dynamics, which has important implications for host pathology and pathogen transmission within communities. Incorporating principles from community ecology are important for understanding the consequences of coinfection on disease outcomes within natural systems. Amphibians are a useful study system for exploring the influence of coinfection on both within- and between-host interactions. Two amphibian pathogens that naturally occur with one another are ranaviruses and echinostomes. Here, I examined the influence of pathogen interactions on disease outcomes at both the individual and community level. My objectives were to: 1) examine how the sequence and timing of pathogen exposure influenced infection success and disease outcomes at the individual level, and 2) determine how pathogen interactions influence between-host processes and disease outcomes. In the laboratory experiment using larval gray treefrogs, I found that interactions between ranaviruses and echinostomes were asymmetric. Prior echinostome infection influenced ranavirus infection but there was no effect of ranavirus on echinostomes. The prior infection of echinostomes reduced viral loads across different exposure times by 9%. Additionally, host survival rates in individuals exposed to echinostomes 10 days prior to virus exposure significantly increased by 25% when compared to virus only treatments. These results led me to conduct an experiment to determine if these pathogen interactions could be scaled up to more natural conditions within a community. I found that with the prior infection of echinostomes, viral load decreased by 19%, 27%, and 28% in gray treefrogs, leopard frogs, and spring peepers respectively, with no effect on American toads. These results suggest that macroparasite infection can reduce microparasite replication rates across multiple amphibian species, possibly through cross-reactive immunity. While this is the first experimental study to examine the influence of coinfection on disease dynamics within amphibian communities, my results demonstrate that laboratory patterns of coinfection dynamics within amphibian hosts are broadly applicable to more natural settings and host communities