Parasite-modified behaviour in non-trophic transmission: Trematode parasitism increases the attraction between snail intermediate hosts

Abstract

Many parasites with complex life cycles cause host behavioral changes that increase the likelihood of transmission to the next host. Parasite modification is often found in trophic transmission, but its influence on non-trophic transmission is unclear. In trematodes, transmission from the first to second intermediate host is non-trophic suggesting that free-swimming larvae (cercariae) emerging in closer proximity to the next host would have higher transmission success. We performed a series of behavioral experiments with echinostome trematodes and their snail hosts to determine if potential second hosts (Planorbella sp.) were more attracted to parasitized first hosts (Lymnaea elodes Say, 1821). In a Y-maze, a responding snail (Planorbella Haldeman, 1842 sp.) was placed in the base and its response to five treatments was assessed: no stimulus, duckweed (a food item, Lemna turionifera Landolt), non-parasitized L. elodes, parasitized L. elodes, and finally parasitized versus non-parasitized L. elodes. Snails showed some attraction to uninfected snails, but had a stronger response to infected first host snails. These results indicate that potential second host snails were more attracted to parasitized, heterospecific first host snails over non-parasitized heterospecific snails. This study demonstrates that echinostome trematodes alter snail behaviour by changing navigational choices in uninfected potential hosts through a chemical communication mechanism.