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Last updated April 30, 2018

CDC/Alexander J. da Silva, PhD/Melanie Moser

This illustration depicts the life cycle of A. simplex and P. decipiens, the causal agents of Anisakiasis.

What are the other Names for this Condition? (Also known as/Synonyms)

  • Anisakidosis
  • Herring Worm Disease

What is Anisakiasis? (Definition/Background Information)

  • Anisakiasis, a disease infecting the human digestive tract, is caused by consuming raw fish or squid that contain parasitic worms (called nematodes) of the anisakid species. Examples of these worms include:
    • Anisakis simplex (herring worm)
    • Pseudoterranova decipiens (cod or seal worm)
    • Anisakis physeteris
    • Contracaecum species
  • Anisakis simplex is the main worm that causes infection; most other cases of infection, are due to Pseudoterranova decipiens
  • The ingested worms then invade the lining of the esophagus, intestine, or most commonly, the stomach, leading to symptoms of the disease
  • Although symptoms can differ between infected individuals, typically, individuals present with symptoms similar to appendicitis and other gastrointestinal diseases
  • The prognosis is good, as the infection typically resolves on its own. However, endoscopic or surgical removal of the nematode larvae can alleviate symptoms more quickly

Who gets Anisakiasis? (Age and Sex Distribution)

  • Individuals of all ages, races, ethnic groups, and of both genders are susceptible to Anisakiasis upon consumption of raw or undercooked, infected fish or squid
  • Anisakiasis shows the highest prevalence in countries where raw fish is part of the typical diet, such as in Japan and the Netherlands

The consumption of raw and undercooked fish is becoming more common with the increasing worldwide popularity of sushi and sashimi. Thus, Anisakiasis can also occur in other parts of the world, such as in the United States, Europe, and South Africa.

What are the Risk Factors of Anisakiasis? (Predisposing Factors)

  • The main risk factor for Anisakiasis is the consumption of raw or undercooked fish or squid

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases one’s chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Anisakiasis? (Etiology)

The cause of Anisakiasis is ingestion of raw fish or squid that are infected with the parasitic larvae, which can progressively invade the human digestive system. Even the ingestion of a single worm can cause infection.

  • Fish and squid become infected through the following:
    • Marine mammals (dolphins, seals, sea lions, whales, etc.), aquatic birds, and turtles that are infected with the parasite defecate into the ocean, releasing eggs into the water where they become larvae
    • Crustaceans and mollusks can ingest these larvae, infecting the fish and squid, which eat these crustaceans
  • Anisakiasis is not contagious and human-human transmission is not reported to take place

What are the Signs and Symptoms of Anisakiasis?

The signs and symptoms of Anisakiasis may vary between the affected individuals.

  • Symptoms typically appear within the first 24 hours after consuming the infected fish. However, symptoms may be delayed up to 2 weeks in some cases
  • Some individuals may experience a tingling sensation occurring in the mouth or throat due to the worm moving around shortly after consumption of infected raw fish

If the worm attaches to the lining of the stomach or intestine, then the following symptoms may ensue due to an inflammatory response (similar to appendicitis and other gastrointestinal diseases):

  • Severe abdominal pain and distention, due to a granuloma (nodule) that forms at the point of worm attachment
  • Nausea and vomiting
  • Diarrhea
  • Blood and mucus in stool
  • Mild fever
  • Occasionally, allergic reactions of itchy rashes and anaphylaxis occur: This may occur without fish consumption, resulting simply from inhalation or direct contact while preparing infected fish
  • In rare cases, a small bowel obstruction may occur

Symptoms typically fade almost immediately with early removal of the worm. However, abdominal pain may persist for weeks to months after the worm dies.

How is Anisakiasis Diagnosed?

The diagnosis of Anisakiasis may be difficult, as symptoms caused by other microbes may present with similar symptoms. The diagnosis of the infection is made by the following tests and exams:

  • A thorough physical examination and an assessment of symptoms
  • An evaluation of medical history; history of raw fish consumption
  • Morphological examination and DNA sequencing of a coughed-up worm
  • A blood test to detect increased levels of eosinophils (a special type of white blood cell) during the early inflammatory response
  • An Anisakis skin prick test and measurement of Anisakis-specific IgE, but these tests are limited due to cross-reactivity of antigens of similar nematodes
  • Clinical tests such as upper endoscopy, radiography, or surgery (if the worm has embedded in the tissue lining of the intestine) 

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Anisakiasis?

The potential complications from Anisakiasis include the following:

  • Complete penetration of the intestinal wall caused by worms moving into the abdominal cavity, requiring surgical removal of larvae
  • Persistent abdominal pain in the weeks following removal of the worm from the body

How is Anisakiasis Treated?

In some cases, treatment of Anisakiasis is not always necessary. The worms typically die and are spontaneously expelled from the body after approximately 3 weeks, since humans are not suitable hosts for the nematodes to complete their parasitic life cycle.

However, the following common treatments can alleviate symptoms more quickly:

  • Endoscopic removal of the worm from the body, to help alleviate intense abdominal pain
  • Surgery for removing worms embedded in the lining of the stomach or intestine

Less common treatment options include the following:

  • Steroids (although this is controversial)
  • Antibiotics
  • Isotonic glucose solution

How can Anisakiasis be Prevented?

Anisakiasis can be prevented with the following measures:

  • Abstaining from eating raw fish or squid
  • Adequately cooking or freezing fish before consumption (however, this does not necessarily prevent an allergic response): Proper cooking procedure recommends reaching an internal temperature of 60°C for 10 minutes
  • Coughing-up the worm if a tingling sensation occurs in the mouth or throat soon after consuming raw fish or squid
  • Candling (examining fish on a light table), which is used commercially to reduce the number of visible worms in fish that are known to be infected frequently. Some individuals can manually remove the worm from their mouth, thus preventing infection

What is the Prognosis of Anisakiasis? (Outcomes/Resolutions)

  • Typically, the prognosis for Anisakiasis is good with or without treatment, since the worms generally die and are naturally removed from the body after a couple of weeks
  • However, to alleviate symptoms in a shorter amount of time, the nematodes can be successfully removed via endoscopy or surgery. The removal of the worm is curative
  • No cases of mortality from Anisakiasis have been reported

Additional and Relevant Useful Information for Anisakiasis:

Anisakiasis is also known by the following names - Anisakidosis and Herring Worm Disease.

What are some Useful Resources for Additional Information?

References and Information Sources used for the Article:

Helpful Peer-Reviewed Medical Articles:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: May 30, 2017
Last updated: April 30, 2018

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