Can parasites use predators to spread between primary hosts?

Parasites typically have low reproductive fitness on paratenic hosts. Such hosts offer other significant inclusive fitness benefits to parasites, however, such as increased mobility and migration potential. The parasite fauna of the guppy (Poecilia reticulata) is dominated by the directly transmitted ectoparasites Gyrodactylus bullatarudis and Gyrodactylus turnbulli. In the wild, close predatory and competitive interactions occur between the guppy and the killifish Rivulus hartii. Previous observations suggest that these fish can share gyrodactylids, so we tested experimentally whether these parasites can use R. hartii as an alternative host. In aquaria, G. bullatarudis was the only species able to transmit from prey to predator. Both parasite species transferred equally well to prey when the predator was experimentally infected. However, in semi-natural conditions, G. bullatarudis transmitted more successfully to the prey fish. Importantly, G. bullatarudis also survived significantly longer on R. hartii out of water. As R. hartii can migrate overland between isolated guppy populations, G. bullatarudis may have an enhanced ability to disperse and colonize new host populations, consistent with its wider distribution in the wild. To our knowledge, this is the first empirical study demonstrating a predator acting as a paratenic host for the parasites of its prey.