Print PageParasites and hosts interact with a diverse community of organisms and those interactions can influence parasite transmission. The dilution effect hypothesis predicts a reduction in disease risk in more diverse communities through a variety of mechanisms, but current investigations have focused on encounter reduction by alternative hosts on vector-borne diseases. However, parasites with free-living stages, such as trematode cercariae, provide a good model to examine other mechanisms. In addition to the presence of alternative hosts, predators can lead directly to increased mortality of parasite free-living stages thereby controlling transmission. To evaluate the effects of both predation and alternative hosts on parasite transmission, we assessed the likelihood of aquatic invertebrate and vertebrate species to consume or serve as alternative hosts for Ribeiroia ondatrae in the laboratory. To determine whether consumption or infection of alternative hosts (decrease in encounter rate) influenced the transmission of R. ondatrae to Pacific Chorus Frog (Pseudacris regilla) tadpoles, we conducted a second experiment in which tadpoles were exposed to R. ondatrae cercariae alone, in the presence of predators/alternative hosts, or predator/alternative host scent (to simulate presence but without consumption or infection).
Results/Conclusions
Species of aquatic vertebrates and invertebrates showed differential removal of cercariae. Specifically, mosquitofish (Gambusia affinis), California clam shrimp (Cyzicus californicus) and damselfly nymphs (Lestes sp. and Enallagma sp.) reduced cercariae by >90% and ~60% respectively. Only 4% of cercariae encysted in mosquitofish, suggesting that fish consumed the majority of cercariae removed. California newt larvae (Taricha torosa) served as an alternative host and removed 85% of cercariae while P. regilla tadpoles removed 75%. Infection was similar between newts and tadpoles (65% and 75%, respectively). In the transmission experiment, damselfly nymphs and newts individually reduced transmission of R. ondatrae to tadpoles by approximately 50% compared to controls, where they were absent. Mosquitofish did not significantly influence transmission. Damselflies generally do not become infected by R. ondatrae and influenced transmission directly by predation. However, when newts were present with tadpoles, they experience similar infection levels, further supporting their role as alternative hosts. Collectively, these results show that members of the broader community can significantly influence parasite transmission through different mechanisms. Community composition is therefore an important factor when considering the effects of biodiversity loss and species invasions on transmission of infectious parasites.
