This is how I imagine you’d feel if you were a fish infected by a parasitic worm called Euhaplorchis californiensis.

I first encountered these remarkable parasites about ten years ago, when I took a trip out to Santa Barbara. In the estuaries and salt marshes along the California coast, one of the most common fish is the California killifish, and many of them carry the parasites. The parasites get their start in horn snails, where they produce offspring that can swim around the water searching for killifish. In their next phase, they live as cysts on the brains of the killifish, but in order to reach the stage when they can reproduce, they must get inside the guts of shore birds.

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