Anisakiasis is a human parasitic infection of the gastrointestinal tract caused by the consumption of raw or undercooked seafood containing larvae of the nematode Anisakis simplex (herring worm). Fish, squid, and crustaceans often act as transport host in which larvae grow but do not mature into adults. When larger marine mammal such as seals, sea lions, dolphins and whales consume these smaller fish and crustaceans, the larvae develop into adult worms. In humans, larvae are unable to complete their development and cause infection. In some cases, antigens present in Anisakis simplex can produce allergic reactions and hypersensitivity.
Characteristics of Anisakiasis
The characteristics of anisakids include a long, cylindrical body shape (what is called vermiform, or worm-like). It does not contain segments. The posterior part narrows to a cavity (pseudocoel), with the anus somewhat off-centred. The mouth is encircled by projections, which are used for sensing and feeding.
Transmission of Anisakiasis
- The transmission of anisakiasis begins when infected marine mammals defecate into seawater and release eggs of the parasite.
- These mature into larvae and are eaten by crustaceans.
- In turn, the crustaceans are consumed by fish or squid, so that human who eats raw fish risk ingesting the larvae.
- In the human host, the larvae pass into the gastrointestinal tract, eventually dying and forming a mass there.
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 molluscs 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
- The main risk factor for Anisakiasis is the consumption of raw or undercooked fish or squid
Anisakiasis has been reported worldwide, with the higher incidence in areas where raw fish are eaten such as Japan, the Netherlands, France, Spain, Germany, and California. Approximately 14,000 cases have been reported since 2000. Given the widespread consumption of raw fish in the form of sushi and sashimi, Japan is the country with the highest prevalence of gastric anisakiasis.
Many infections have also been reported in the Netherlands from the consumption of pickled herring. Over 150 cases involving the allergic reaction to anisakis have been reported and investigated in Spain. Less than ten cases occur annually in the United States. Development of better diagnostic tools and greater awareness has led to more frequent reporting of anisakiasis.
- 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
- If the worm is not coughed up or vomited out, it may penetrate the intestinal tissues, causing a severe immune response that resembles appendicitis Crohn’s disease.
Symptoms of Anisakiasis
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.
- Tingling sensation occurring in the mouth or throat due to the worm moving around shortly after consumption of infected raw fish
Because of the invasion of the gut wall, it shows the following symptoms.
- Abdominal pain
- Vomiting within hours of ingesting the larvae
- Small bowel obstruction
- Diarrhea with signs of peritoneal irritation
- Blood and mucus in stool
- Mild fever
- Occasionally, allergic reactions of itchy rashes and anaphylaxis occur
Diagnosis and Test
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)
Treatment and Medications
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: Albendazole
- Isotonic glucose solution
Prevention of Anisakiasis
- Anisakiasis infection can be prevented by heating the seafood to a temperature higher than 122°F (50°C), or freezing it to at least -4°F(-20°C) for at least 24 hours. Such actions kill the larvae.
- If fish or shellfish is to be consumed raw or semi-raw, the FDA recommends that the food is blast frozen to -31°F (-35°C) or below for 15 hours, or regularly frozen to -4°F (-20°C) for seven days.
- Abstaining from eating 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.
- Avoid raw, lightly salted, home pickled, lightly smoked, dried, cured and salted fish unless it was frozen first because salting, cooking and/or marinating will not necessarily kill the parasites