Climate change and parasite transmission: how temperature affects parasite infectivity via predation on infective stages

Climate change is expected to affect disease risk in many parasite-host systems, e.g., via an effect of temperature on infectivity (temperature effects). However, recent studies indicate that ambient communities can lower disease risk for hosts, for instance via predation on free-living stages of parasites (predation effect). Since general physiological theory suggests predation effects to be temperature-dependent, we hypothesized that increases in temperature may lead to reduced parasite infectivity via elevated consumption rates of free-living parasite stages (temperature-predation interaction). We experimentally investigated such interactions in three marine predators of infective parasite stages. Two species (the oyster Crassostrea gigas, and the barnacle Austrominius modestus) significantly reduced cercarial stages of the trematode Renicola roscovita in mussel hosts (Mytilus edulis), while the third (the crab Hemigrapsus takanoi) did not show a reduction of infective stages at all. In barnacles, cercarial consumption significantly interacted with temperature, with lowest infectivity at highest temperatures. Since these patterns reflected the known thermal responses of the three cercarial predators' feeding rates, parasite consumption rates may be predictable from temperature dependent feeding rates. Our results suggest that integrating temperature-predation interactions into studies on parasite transmission and on climate change effects is essential and that predators of free-living stages of parasites may play an important role in indirectly mediating disease risk under climate change.