As sushi and other raw seafood dishes become more popular in Western countries, doctors are warning about a dangerous parasite that could be lurking in undercooked fish and squid. Writing in BMJ Case Reports, gastroenterologists in Portugal recount the tale of a 32-year-old patient who was admitted to the hospital with severe stomach pain, vomiting, and a week-long fever. When he mentioned that he’d recently eaten sushi, his doctors suspected anisakiasis—a parasitic disease caused by tiny worms, called nematodes, that attach to the wall of the stomach, esophagus, or intestine.

Sure enough, using an endoscope (a long tube with a with a camera on the end), doctors were able to spot an anisakis larva attached to the patient’s swollen and inflamed gut lining. After the worm was removed using a tiny net, the man’s symptoms cleared up and he was released from the hospital.

More than 90% of reported anisakiasis cases have occurred in Japan, where raw fish is a large part of most people’s diets. But because of changes in food habits around the world—and the increasing prevalence of sushi—it has been increasingly recognized in Western countries, co-author Joana Carmo, MD, a gastroenterologist at the Hospital Egas Moniz in Lisbon, told Health via email.

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Because of this, the authors say, doctors should consider anisakiasis as a possible diagnosis for patients who have symptoms such as abdominal pain, nausea, vomiting, digestive bleeding, bowel obstruction, perforation, or abdominal inflammation and have recently eaten raw or undercooked fish. An infection can also cause allergic reactions such as rash, itching, and anaphylaxis.

According to the Centers for Disease Control and Prevention (CDC), some people can actually feel a tingling sensation after or while eating seafood contaminated with anisakis larva.

“This is actually the worm moving in the mouth or throat,” the CDC website states. “These people can often extract the worm manually from their mouth or cough up the worm and prevent infection.” Ingesting the parasite can also sometimes cause vomiting, which can expel the worm from the body.

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If that's not gross enough, anisakiasis isn’t the only unwanted critter that raw-fish lovers should know about. In February, researchers confirmed that another parasite, the Japanese broad tapeworm, has been found in wild pink salmon from the Alaskan Pacific.

The best way to avoid parasites like these is to cut out raw fish entirely, according to the CDC. Cooking fish to 158 degrees Fahrenheit (70 degrees Celsius), or freezing raw fish to -4 degrees Fahrenheit (-20 degrees Celsius) for a minimum of 72 hours, can also kill any parasites that are present, Dr. Carmo says. 

But based on the number of reported cases, anisakiasis is still a rare disease—and Dr. Carmo says that properly trained sushi chefs can detect Anisakis larvae. "They are grossly visible in the fish," she says. Food experts say the risk of getting sick from sushi is low, especially if you’re buying it from a trusted restaurant or an FDA-approved distributor and eating it right away.

And, of course, there are still lots of benefits to including fish—raw or otherwise—in your diet. As long as your doctor hasn't warned you against it for specific reasons, most experts say that sushi is generally a healthy choice.

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Cynthia Sass, RD, Health's contributing nutrition editor, recommends that clients who are pregnant, are trying to get pregnant, or have a weakened immune system avoid raw fish. "For these people, the risks of contracting a parasite can be far more serious," she says. "For healthy adults, I don't think you need to nix sushi or raw fish completely, but be very choosy about the source—don't be afraid to talk to a chef to ask questions about how raw seafood was sourced, handled, and prepared."  

As with any raw food, it’s important to know the risks and benefits before digging in. And if you do start to feel ill afterward, be sure to mention your recent menu choices to your doctor.