The trematode infiltrates the killifish and sets up camp in the brain. But one fluke isn't enough to turn a killifish into a mindless zombie -- for that, it needs an army of thousands. Thousands of murderous, brain-altering parasites. It's like the headquarters of TMZ up in this poor fish's head.
As the flukes set up shop in the fish's brain-meat, they form cysts. This actually affects very little, since it's not like the fish was solving complex physics problems or anything. It's a damn fish. It continues on with life as usual, eating and reproducing, unaware that the trematodes have tweaked one vital behavior that will almost certainly end up killing it.
University of California, Santa Barbara
It hits the killifish kill switch, located just southwest of the worm detector.
See, the trematodes' first act of mind control is to regulate the fish's serotonin and dopamine levels, giving it no reason to swim away when there's danger afoot. Next up, the part of the brain responsible for locomotion is ramped up to an absurd degree, forcing the fish into wild herky-jerky movements that attract a lot of attention from a lot of predators. The killifish swim near the surface, rendered unaware of danger by the fluke's chemical cocktail, where they execute a series of attention-grabbing turns and are devoured by predatory birds. The trematodes get their new home, the bird gets an easy meal, and the fish gets to go fuck itself. Hey, nature is a competition. There's always a last place.