California Roll, Dragon Roll, Nematode Roll?

Your tuna sashimi looks pink and innocent fanned out next to a cloud of grated daikon radish. It’s fresh, it’s all-natural … and at some point in its life cycle it may have harbored parasites.

Take a big slug of sake and read on.

Most varieties of fish, both wild and farmed, are prone to parasites. Farmed fish may have fewer of them because they live in a controlled environment and are commonly treated with parasiticides such as formalin, an FDA approved substance that contains formaldehyde. Wild fish can become infected by eating parasite larvae excreted by larger fish or sea animals. (Fish from off the U.S. Pacific coast and the northeastern coasts of Canada and the United States are particularly likely to pick up parasites from the seals and other mammals living there.)

But any fish can potentially yield a surprise.

After fish are caught, they’re taken to a processor to be gutted and filleted. It’s at this point that processors—or you, if you’re cutting up your own fish—most commonly encounter three kinds of parasites: nematodes, tapeworms, and sea lice.

Nematodes typically start their life cycle in a fish’s belly and move outward into the flesh as they grow. Under half an inch long, these worms are difficult to remove from fish. Processors use a labor-intensive system called candling, in which fillets are laid on a kind of light table, and parasites are removed with tweezers. But many fish are too thick for a candling light to shine through, causing processors to miss some of the nematodes.

If they’re consumed, they may simply die; in rare cases they can cause severe stomach upset, though most people infected with nematodes won’t even know it. Nematodes don’t find humans to be suitable hosts and can’t live in our digestive tracts for more than ten days, even in the worst cases.

Cooking and Freezing Guidelines

Cooking: Fish should be cooked so that all parts of it reach an internal temperature of 145°F. This will kill any parasites. If you don’t have a thermometer, you can tell that the fish is properly cooked when it flakes easily with a fork and loses its translucency.

Freezing: If you plan to eat fish raw or in ceviche, you can kill any parasites by freezing it ahead of time. Fish needs to be frozen at –4°F (–20°C) or below for seven days, or at –31°F (–35°C) or below for at least 15 hours after it’s frozen solid. These conditions may not be suitable for freezing large fish over six inches thick.

Tapeworms are most commonly found in trout and salmon. They live in the stomach of a fish, feeding off what it eats, but do not move into the flesh. Tapeworms can grow to be very large, up to 32 feet long, so they’re easy to see and are generally discarded along with the fish’s guts during processing. The FDA considers tapeworm infestation to be very rare now, though in times past, Jewish and Scandinavian women who tasted the raw mixture as they prepared traditional gefilte fish, or fish balls, occasionally contracted tapeworms. An infestation can cause stomachache, diarrhea, and fatigue, and a long-term infection with a fish tapeworm can interfere with vitamin B12 absorption, causing anemia and, in severe cases, such neurological symptoms as confusion and loss of balance. Without treatment, a tapeworm can live up to 20 years, but there are a number of drugs, such as Niclocide and Biltricide, that kill tapeworms in a single dose.

Sea lice are external parasites, some microscopic and some nearly half an inch long, that feed on the mucus secreted through a fish’s skin. Shaped like little suction cups, they cling to the sides of fish, though they fall off and die in fresh water. Processors simply wash them off; if any are missed, they don’t affect human health.

Eating raw fish in restaurants is always a gamble, but the risk of being infected by parasites is low—the FDA says there are fewer than ten cases of nematode infection reported a year. And freezing or properly cooking fish drops the risk to almost zero (see the sidebar).

So don’t turn up your nose if the sushi chef tells you that the tuna was previously frozen … and consider a nice tempura roll.