The wild-caught fish you're eating could have worms in it

Not to open a can of worms, but actually, there could be worms in your wild-caught fish.

Tiny worms can make it all the way from the ocean to a dinner plate. One food blogger found three "squirming, blood-red" worms in her leftovers from one of Portland's best-reviewed restaurants, accord to Vice's Munchies.

All living organisms, including fish, can have parasites. Roundworms, called nematodes, are the most common parasite found in saltwater fish, such as cod, plaice, halibut, rockfish, herring, pollock, sea bass and flounder, according to Seafood Health Facts, an online resource about seafood products operated by the Delaware Sea Grant.

These nematodes are sometimes called "herring worms" or "cod worms." Experts say there are actually several different species of nematodes. All are in the Anisakidae family, which can cause the parasitic disease anisakiasis when ingested by humans in raw or insufficiently cooked fish, Seafood Health Facts says.

Once inside the human body, the worms can invade the gastrointestinal tract, according to the U.S. Centers for Disease Control and Prevention. Eventually, the parasite dies and causes an inflamed mass in the esophagus, stomach or intestine.

Symptoms of anisakiasis are abdominal pain, nausea, vomiting, abdominal distention, diarrhea, blood and mucus in stool, and mild fever, the CDC says.

The life cycle of an anisakid nematode begins when seals or sea lions eat infected fish, according to Seafood Health Facts. The larval nematodes grow to maturity, and the marine mammal then excretes the nematode eggs into the sea where they hatch. Shrimp-like animals eat the larvae, and fish eat the shrimp. The larvae then develop into the form we see in fish.

Fish like trout and salmon that spend all or part of their life in freshwater might carry diphyllobothrium tapeworm larvae. These "small, whitish, and somewhat flabby" worms are common in salmon from some areas of Alaska, Seafood Health Facts states.

The life cycle for a tapeworm is similar. Mammals or birds eat infected fish, and the eggs hatch in freshwater. Crustaceans eat the eggs, freshwater and anadromous fish eat the crustaceans, and then humans eat the fish.

"Parasites are a natural occurrence, not contamination," Seafood Health Facts says. "They are as common in fish as insects are in fruits and vegetables. Parasites do not present a health concern in thoroughly cooked fish."

These parasites become a cause for concern when diners eat raw, undercooked or lightly preserved fish such as sashimi, sushi, ceviche and gravlax, health experts say.

There are precautions seafood processors can take to make sure the fish is free of worms, however. Many inspect seafood with a technique called "candling," which involves examining fish fillets over lights so shadows from the worms become visible.

If diners choose to eat raw fish anyway, the Food and Drug Administration says a rule of thumb is to eat fish that has been previously frozen.

Freezing fish to an internal temperature of -4 degrees Fahrenheit for at least seven days will kill parasites. Cooking seafood to an internal temperature of at least 145 degrees Fahrenheit for 15 seconds also kills parasites, the CDC says.

The problem, however, is that some foodies prefer the delicate flavor and texture of uncooked fish, and many chefs are reluctant to overcook fish as it can become dry.

"Most fish shrink at 120 degrees Fahrenheit and begin to become dry around 140 degrees Fahrenheit," food science writer Harold McGee explains in his book "On Food And Cooking: The Science And Lore Of The Kitchen."

In general, McGee says the ideal temperature for most cooked fish and shellfish is between 130 to 140 degrees.

"Some dense-fleshed fish, including tuna and salmon, are especially succulent at 120 degrees, when still slightly translucent and jelly-like," McGee writes. That temperature, however, falls short of the heat needed to completely kill the larvae.

While the foodie trend is to eat wild-caught seafood, the best way to avoid the risk might be to avoid eating raw, wild fish altogether.

The good news is that nematode infections appear to be rare. Of the roughly 20,000 cases of seafood-associated parasite infections reported worldwide, more than 90 percent were found to occur in Japan.

"Although it has likely been greatly underdiagnosed and underreported, (approximately) 60 cases of anisakidosis have been described in the United States," a 2010 study published in Clinical Infectious Diseases states.

So at the end of the day, it might be about weighing the risk versus reward when ordering raw fish during your next sushi outing.