Parasites become a concern when consumers eat raw or lightly preserved fish such as sashimi, sushi, ceviche, and gravlax. When preparing these products, use commercially frozen fish. Alternatively, freeze the fish to an internal temperature of -4°F for at least 7 days to kill any parasites that may be present. Home freezers may not be cold enough to kill the parasites.
Roundworms called nematodes are the most common parasite found in marine fishes. Some people call these nematodes herring worms or cod worms. Actually, several different species exist and it is hard to distinguish between them. All are in the family Anisakidae and are anisakid nematodes.
Freshwater fish like trout and fish that spend part of their life in freshwater such as salmon may carry Diphyllobothrium tapeworm larvae. These small, whitish, and somewhat flabby worms are common in salmon from some areas of Alaska.
Nematodes rarely cause health problems because they are uncommon in fish fillets and normal cooking easily destroys them. In most cases, swallowing a live nematode is harmless. The nematode passes through the intestine without causing problems.
In rare cases, swallowing a live nematode larva can cause severe gastric upset called anisakiasis. This happens when the nematode attaches to or penetrates the intestinal lining. Nematodes do not find humans to be suitable hosts and will not live longer than 7-10 days in human digestive tracts.
Swallowing live tapeworm larvae can cause a tapeworm infestation. The tapeworms may live in the human intestinal tract for several years. Symptoms can include abdominal pain, weakness, weight loss and anemia. Doctors successfully treat tapeworm infections with medicines.
Canada's Health Protection Branch recommends using only commercially frozen fish in raw fish dishes because home freezers will not kill the parasites. The U.S. Food and Drug Administration recommends using fish frozen commercially for 7 days at -10°F or 15 hours at -31°F for raw fish dishes.
http://seafood.ucdavis.edu/pubs/parasite.htm