ECOLOGY OF HOST-PARASITE INTERACTIONS BETWEEN ECHINOSTOMES AND THEIR SNAIL INTERMEDIATE HOSTS

Trematodes in family Echinostomatidae (Looss 1899) are common complex life cycle parasites of both humans and wildlife. The work in this thesis aimed to further evince the diversity of these parasites, in addition to exploring the effects of co-exposure on infection outcomes and investigating the host-seeking behavior of echinostome larvae. These aspects of diversity, co-exposure, and behavior can all have important impacts on parasite transmission. In chapter 1, three genetic markers, ND1, ITS2, and 28S, were utilized to further elucidate the diversity of echinostomes in the Midwestern United States. A subset of snail first intermediate hosts were also sequenced offering insight into parasite host range and specificity. Results indicate that there is a greater diversity of echinostomes in the Midwest than previously appreciated and we suggest that echinostome evolution is often driven by the first intermediate host. In chapter 2, the snail Planorbella trivolvis was exposed to two of its echinostome parasites, Echinostoma trivolvis and Echinoparyphium lineage 3, to evaluate the relative competitive dominance of these two parasites, and to investigate the effects of co-exposure timing on infection outcomes. These parasites exhibit strong priority effects and snails are often infected with the echinostome they were first exposed to, even though Echinostoma trivolvis exhibited competitive dominance to Echinoparyphium. The study in chapter 3 was aimed at determining whether Echinostoma caproni cercariae exposure timing would alter infection outcomes of the human parasite, Schistosoma mansoni. Subtle effects of Echinostoma caproni cercariae exposure timing on Schistosoma mansoni infection prevalence and cercarial shedding were identified. Chapter 4 investigated whether the echinostomes, Echinostoma trivolvis and Echinoparyphium lineage 3, chose their Planorbella trivolvis snail hosts based on echinostome infection status. While Echinostoma trivolvis and Echinoparyphium lineage 3 were both able to detect Planorbella trivolvis, we found no evidence that either parasite chose their host based on echinostome infection status. Overall, this thesis offers novel insights into the diversity and ecology of this medically and ecologically important family of trematode parasites.