Resource competition drives natural and rebound dynamics of snails and schistosomes

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RESOURCE COMPETITION DRIVES NATURAL AND REBOUND DYNAMICS OF SNAILS AND SCHISTOSOMES Project summary More than 250 million people are infected with schistosomes, flatworms in the genus Schistosoma, and 20 million humans suffer from severe morbidity due to schistosomiasis. Humans become infected after exposure to larval parasites (cercariae) that are produced by infected snails in freshwater habitats. Therefore, the production of cercariae by snail populations represents an important component of the human risk of exposure, infection, and disease. Schistosomiasis control incorporates many programs, including drug administration, behavioral intervention and snail control. Snail control programs reduce snail density by applying toxic chemical molluscicides or lethal predators. However, snails themselves do not directly infect humans. Instead, snails release free living cercariae that directly cause human infections following skin contact. This mismatch between the target of control (snails) and the proximate cause of human infections (cercariae) complicates schistosome control because the production of cercariae per snail is sensitive to ecological conditions, such as snail density. The vast majority of models and control trials examining the natural dynamics and control of schistosomes assume that snails are all equally infectious, leading to the assumption that snail density directly correlates with cercariae density, and therefore potential for human exposure. However, infected snails can produce >50-fold more cercariae when food is abundant, competitors are scarce, and physical conditions are otherwise benign. Thus, counter to conventional wisdom, cercarial densities, and human exposure potential, could be greatest when the density of snails is lower and growing. Therefore, studying the dynamic link between snail and cercarial density is critical to designing optimal snail control strategies, because these dynamics determine the timing and magnitude of human risk. This research will combine field and laboratory experiments to test novel hypotheses for the dynamics of cercariae in natural settings that arise from theory we developed to explicitly incorporate energy uptake and use by snails and schistosomes in dynamic scenarios. Specifically, we will test predictions that: (1) there are brief, intense peaks of cercarial density early in the season, when individual snails are large and highly reproductive, (2) the presence of other food sources, such as decaying plants can sustain cercarial production over longer periods, and (3) reducing, but not eliminating, snails from water bodies could backfire, causing little reduction or even an increase in cercariae, by relaxing competition for food. Ultimately, this work can improve the prediction and control of a parasite causing major global health burden.