The eye fluke Diplostomum pseudospathaceum, a common parasite, pulls strings to put its host in the path of a predator.
In the interest of self-preservation and survival, the eye fluke has to make sure that it serves its host up to predatory birds at the right time. Eye flukes live in the eyeballs of fish and alters their host's movements to their own benefits. The demise of the host fish is necessary for the eye fluke to continue its life cycle.
Thus, either the fish dies, or the parasite fails to reproduce. Unfortunately for the fish, it may not be able to put up much of a fight against the eye fluke's manipulations.
The eye fluke's life cycle needs to take place in three different animals. These parasites mate in the digestive tracts of birds, and their eggs are excreted along with the bird's feces. When the larvae hatch, they infect freshwater snails. They stay in the snails for a while as they grow, then they seek out fish to infect.
When the larvae mature, they need to be in a bird's digestive tract in order to mate and reproduce. A bird therefore has to eat the fish that the parasite has infected. Thus, the cycle begins anew.
Of course, while the parasites weren't ready to mate yet, their host had to stay safe. The fluke larvae manipulate the host fish's movements to make sure that the fish isn't easily visible to predators. An earlier study found that fish that had fluke larvae in their eyes were less active than their counterparts who didn't have the parasites.
When the larvae have matured and are ready to mate, they make their hosts more active and thus more conspicuous to predators. Newer research tested the effects of mature eye flukes on rainbow trouts. The researchers found that trouts with mature eye flukes were more active and even swam closer to the surface of the water. This made them more accessible to hungry birds looking for prey.
These studies offer an interesting insight into the disconcerting manipulations and life cycle of this parasite. Eye flukes, however, aren't the only parasites that put their hosts right in the paths of their predators. The Toxoplasma gondii infects mice and makes the mice braver than they usually are. Therefore, the mice are more likely to fall prey to cats. This parasite takes control of a mouse's immune system and facilitates [the production of a chemical](http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1003051) that reduces fear in the mouse.
The Toxoplasma gondii spreads through a cat's feces, and is the reason that pregnant women shouldn't go near litter boxes. The parasite can infect and harm a fetus, which can then lead to a lifelong infection that has psychological impacts.
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