Can You Get an Infection From Eating Raw Fish? (2024)

Scared of sushi? The Anisakis worm is a parasite that can cause nasty infections in humans who enjoy eating raw fish. It is 2 cm long and is sometimes coughed up after ingesting it in raw or undercooked fish.

Species Name: Anisakis simplex

Type of Microbe: Parasitic nematode (roundworm)

How It Spreads

Anisakis simplex has a complex life cycle in which humans are an incidental host. Adult worms are found in the stomach of marine mammals, and their eggs are passed in the feces. After the larvae are hatched, they are ingested by shellfish. Infected shellfish get eaten by fish and squid, where the larvae make their way into the muscle tissues. Ingestion of infected fish or squid by other fish allows the spread of the infection. Ingestion by marine mammals is necessary for the larvae to develop into adult worms. However, accidental human consumption of raw or undercooked marine fish that harbor the infected larvae can result in an allergic reaction, sometimes causing the individual to cough up the worms if swallowed. Penetration of the worms into the intestinal tissue causes anisakiasis.

You're at risk if you eat raw or undercooked seafood. The parasite is found frequently in cod, haddock, fluke, Pacific salmon, herring, flounder, and monkfish.

The US reports fewer than 10 diagnosed cases each year. In Japan, where raw fish is an integral part of the Japanese diet, more than 1000 cases have been reported each year.

Symptoms and Diagnosis

Penetration of the worm into intestinal tissues causes a local inflammatory immune response that results in the symptoms of anisakiasis. Common symptoms include:

• Violent abdominal pain
• Nausea
• Vomiting

After penetrating the intestine, the worm attracts white blood cells called eosinophils and macrophages and forms a mass of immune tissues called a granuloma.

Diagnosis is made by gastroscopic or histopathologic examination of biopsy tissue.

Human infection is a dead end in the parasite’s life cycle. The worms are usually eliminated or expelled from the intestines within 3 weeks of infection.

Treatment and Prevention

Treatment is surgical or endoscopic removal of the parasites.

The FDA has provided several guidelines for retailers who sell fish intended to be eaten raw. These guidelines include freezing the fish to -31°F for 15 hours or -4°F for 7 days to kill parasites and physical examination known as “candling” for the presence of worms.

Complications

If the worm is not coughed up or vomited out, it may penetrate the intestinal tissues, causing a severe immune response that resembles appendicitis orCrohn’s disease.

By Ingrid Koo, PhD
Ingrid Koo, PhD, is a medical and science writer who specializes in clinical trial reporting