Eutrophication does not alter survival of Rana sylvatica tadpoles infected with Echinostoma trivolvis trematode cercariae

Environmental context can influence the outcome of host-parasite interactions.  One important change that is occurring in some freshwater system is eutrophication due to the input of nitrogen and phosphorous.  Utilizing tadpole infection with trematode cercariae as a host-parasite system, this study examined (1) growth, development and the maintenance of trematode, Echinostoma trivolvis, infection levels in second intermediate host larval wood frogs, Rana sylvatica, reared outdoors in mixed infection groups and (2) post-infection impacts of eutrophication on R. sylvatica tadpoles infected to varying degrees with E. trivolvis cercariae and reared under eutrophic or control conditions in mixed infection groups in outdoor mesocosms.  Results from the growth and development experiment suggest no impact of infection with 50 cercariae on R. sylvatica growth and development, as compared with uninfected controls.  Results from the second experiment, investigating the impact of eutrophication on infected tadpoles, found that survival to metamorphosis of the individuals in the highest infection treatment (80 cercariae) was reduced regardless of eutrophication treatment.  However, for individuals surviving infection with 80 cercariae, and for individuals infected with only 20 cercariae, no impact on mass at metamorphosis was documented, although individuals were larger at metamorphosis in the eutrophic tanks.  These data demonstrate that infection with E. trivolvis can impact R. sylvatica survivorship, at least above some threshold infection level, and that once infection has occurred, eutrophication may have minimal impacts on tadpole hosts.  Future studies will address how the alteration of host densities associated with eutrophication can alter the flow of parasites through aquatic systems and also whether host susceptibility to infection is altered by eutrophication.