I live in New England and cod is pretty easy to come by around here. I've eaten a lot of it through the years. I'd notice that sometimes the cod fillets had dense veins in them but thought nothing of it and ate them just like with chicken veins.
In the last 5-6 years, I've started to do a lot of saltwater fishing. Last year I got invited on a cod trip. Steaming home, we're filleting the fish. The guy who invited me go, "When you get home, hold them up to the light and you'll be able to get the worms out easier." Turns out those "veins" were really parasitic worms that are in just about all the cod you can get up here. They are completely harmless to humans, but fuck that.
I've been at the fish market since and noticed fillets there with the parasites in them.
I don't eat that shit any more.
Grosssss..
Why didn't I know this earlier... YUCKKKKKK