They are very common in saltwater fish. My parents live on Pine Island on the gulf in Florida, and my dad catches a lot of trout (sea) there. It is not at all uncommon to find a small worm/larva in them. He holds them up to the sunlight and they are visible. He just cut them out. He said he's probably eaten a thousand of them, and not known.

Not sure if it's the same with freshwater though.