If you are a squeamish person, new to cooking fish, I suggest that you don't read any further. I don't want to be responsible for people stopping to eat fish purely because of squeamishness. If you are cooking the fish (with heat), no harm will come to you. I promise.
No matter how hard I looked, it was hard to find solid information on this topic. What I needed to find was someone passionate about parasites. Yes, there are people who care about parasites as much as I care about food (and fish). They are called parasitologists.
Let me say that parasitologists are some of the kindest and most patient people. They were willing to explain fish parasites in terms that even you could understand and painstakingly answered all questions. I learned that there are over a hundred species of parasites that can be found in fish, but only 3 of them are potentially harmful to humans: Pseudoterranova decipiens (a.k.a. “cod worm” or “seal worm”), Anisakis Simplex, and Tapeworm.
Today, our special guest will be cod worm. Have you ever unwrapped a cod fillet, and was greeted by a little worm squirming out of the fish? I have. Good thing I heard about these worms before, or I would have thrown the fish in the trash and never set foot into that fish market. In spite of my initial disgust (I am the kind of person who screams at the sight of an itsy-bitsy spider or snake), I inspected the fillet, removed the worms and cooked the fish. It tasted just fine, yummy-yummy. These little worms are the pain of any fishmonger's existence because they freak the hell out of consumers. They are particularly common in white fish (cod, haddock, flounder, sole, and halibut), but I've also seen them in swordfish and monkfish. How often do you see them as a consumer? I cook fish at least 3 times a week and I'll see them a few times a year.
To prevent us, consumers, from having to look at these unsightly animals, the fish processors put all white fish through a process called "candling." They put the fillets on glass sitting over a lamp. This allows them to see through the fillet and remove any visible parasites. Think about this process as an airport inspection -- it makes everyone feel better, but it's not full-proof. On occasion, a few worms can escape the inspection and travel from the fish processing facility to your fishmonger and then to your kitchen. If this happens to you, don't panic. Remove the worms, and cook your fish the usual way. If you don't want to cook your fish after seeing the worms, I quite understand. Just don't go out of your way to ruin the fishmonger's reputation. The presence of worms has nothing to do with the freshness of the fish and I assure you that your fishmonger tried his or her hardest to protect you from this terrible experience. Last thing they want to happen is for you to find worms in your fish, but unfortunately this does happen sometimes.
So what do happens if you eat a cod worm? If it's dead, which it's bound to be if you cooked your fish to opaque state (or 140F), nothing at all happens. Even if you prefer your fish cooked a little less (120-130F) like I do, the odds of you eating a live worm are very slim. It would have to be a really hardy worm to survive those temperatures. If you are serving fish raw, and one of those guys manages to stay intact after you sliced the fish, and makes it all the way to your tummy intact, you are in trouble. Your stomach will eventually kill them, but since they originate in seals, they can get quite comfy in any mammal including us humans making the experience extremely unpleasant. As parasitologist puts it, “It is better not to eat them alive.”