Merck Manual

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(Herring Worm Disease; Cod Worm Disease; Seal Worm Disease)


Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Sep 2022
Topic Resources

Anisakiasis is infection with larvae of worms of the Anisakis simplex complex and the other anisakid species, Pseudoterranova decipiens complex and Contracecum osculatum complex. Infection is acquired by eating raw or poorly cooked saltwater fish or squid; larvae burrow into the mucosa of the gastrointestinal (GI) tract, causing abdominal pain and sometimes vomiting. Endoscopy is used for diagnosis and treatment (ie, removal of larvae).

Anisakis is a parasite that resides in the GI tract of marine mammals. Excreted eggs hatch into free-swimming larvae, which are ingested by fish and squid; human infection is acquired by ingestion of these intermediate hosts in a raw or undercooked state. Thus, infection is particularly common in locations such as Japan, Korea, and other areas where raw fish is traditionally consumed. Larvae burrow into the stomach and small bowel of humans. Commercial sushi prepared in the United States and elsewhere where fish is frozen under conditions that kill anisakid larvae is safe.

Symptoms and Signs of Anisakiasis

Symptoms of gastric anisakiasis typically include abdominal pain, nausea, and vomiting within hours of ingesting the larvae. Involvement of the small intestine is less common and may result in an inflammatory mass and subacute symptoms resembling Crohn disease Symptoms and Signs Crohn disease is a chronic transmural inflammatory bowel disease that usually affects the distal ileum and colon but may occur in any part of the gastrointestinal tract. Symptoms include diarrhea... read more Symptoms and Signs that develop 1 to 2 weeks later. Ectopic infections outside the lumen of the gastrointestinal tract may rarely occur.

Anisakiasis typically resolves spontaneously after several weeks; rarely, it persists for months.

Diagnosis of Anisakiasis

  • Upper endoscopy

Anisakiasis can be diagnosed by seeing the parasite during upper endoscopy, and patients may cough up larvae and bring them in for analysis. Stool examination is unhelpful. Serologic testing is available in some countries.

Treatment of Anisakiasis

  • Endoscopic removal of the larvae

  • Possibly albendazole

Endoscopic removal of the larvae is curative.

Treatment of presumptive anisakiasis with albendazole 400 mg orally twice a day for 6 to 21 days may be effective, but data are limited.

Prevention of Anisakiasis

Proper freezing conditions are key to preventing anisakiasis in sushi. Anisakis larvae are destroyed by

  • Cooking to > 63° C (> 145° F)

  • Freezing at −20° C (−4° F) or below for 7 days

  • Freezing at −35° C (−31° F) or below until solid, then storing at that temperature for ≥ 15 hours, or at −20° C (−4° F ) for 24 hours

Larvae may resist pickling, salting, and smoking.

Key Points

  • Humans acquire Anisakis when they consume the intermediate hosts (fish or squid) that are raw or undercooked; thus, anisakiasis is common in Japan and other cultures where raw fish is traditionally consumed.

  • Anisakiasis typically causes abdominal pain, nausea, and vomiting within hours of ingesting the larvae; an inflammatory mass may form in the small intestine and symptoms may resemble Crohn disease.

  • Anisakiasis typically resolves spontaneously after several weeks.

  • Do upper endoscopy to diagnose anisakiasis.

  • Endoscopic removal of the larvae is curative.

  • Proper freezing conditions prevent anisakiasis in sushi.

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