Anisakis simplex and Anisakiasis
Parasites of Ocean Mammals and Fish that Infect People by Accident
Aug 20, 2009
The nematode parasite Anisakis simplex and its close relatives (anisakid worms) have been around for a long time, but it is only in the last fifty years that we have become aware of the dangers these worms of whales, seals and other sea mammals pose to people. Infection in humans is usually caused by herring worm or cod worm and results from eating raw fish. An infected human takes the place of a sea mammal in the parasite’s life cycle.
When anisakids cause health problems in people, the disease is called anisakidosis, though anisakiasis is commonly used, and some prefer anisakiosis. Four different medical problems are caused by these worms.
Gastric Anisakidosis
When an anisakid larva is consumed whole and alive in raw fish the flesh of the fish is digested away, or the larva frees itself by working its way out of the fish with a boring tooth located near its mouth. Once free in the stomach, the larva often tries to bore into the human stomach wall to continue its life cycle.
Symptoms of gastric anisakidosis appear quickly — often within a few hours of eating raw fish, and tend to persist unless the worm is removed, vomited up, or carried into the intestine. Vomiting and sever stomach pain are typical.
Today, the best way to confirm a suspicion of gastric anisakidosis is to insert an optical device down the throat into the stomach to view the worms. With care and skill, they can be removed with forceps attached to the optical device.
Intestinal Anisakidosis
Sometimes an ingested anisakid larva is swept out of the stomach into the intestine during the digestive process. Undeterred, the larva may penetrate the intestinal wall with its boring tooth causing bleeding and abdominal pain, and sometimes diarrhea. The condition may mimic appendicitis, duodenal ulcer, or cancer.
Symptoms of intestinal anisakidosis take longer to appear — sometimes days — and the illness is harder to diagnose. A thorough history of what the patient has eaten, and blood tests for antibodies to the parasite are often helpful.
Ectopic Anisakidosis
Anisakid larvae, like other worms, can migrate away from the human gastrointestinal tract and invade other tissues and organs. When this occurs, the symptoms are likely to reflect the part of the body where the larva is located.
Because it is not a typical scenario, ectopic anisakidosis can be very difficult to diagnose. If the parasite is suspected, blood tests for antibodies are helpful, but ultimately, surgery may be necessary.
Allergic Anisakidosis
In the last couple of decades, the role that the anisakid worms play in human allergies has come to light. Not only do people who ingest live larvae in raw fish sometimes develop allergy, but even the ingestion of dead larvae can trigger it. As well, those who handle or clean infected fish and come in contact with the larvae or their waste products can become sensitized and develop an allergy.
Allergic reactions to anisakid worms include respiratory symptoms, skin rashes, red teary eyes, and even the more severe life threatening anaphylactic response. The more sensitive person will even have a reaction to chicken meat if the chicken was fed larval remains in fish meal.
Allergic anisakidosis may turn out to be the most common form of the disease and constitute a significant source of food borne illness.
How to Avoid Anisakidosis
Because one must eat live parasitic larvae in raw fish to acquire gastric, intestinal, and ectopic anisakidosis, in theory the disease is very easy to prevent:
- Clean fish as quickly as possible: larvae in the gut area of fish quickly move into the muscle when the fish dies, especially if the fish is roughly handled.
- Avoid raw, lightly salted, home pickled, lightly smoked, dried, cured, and salted fish unless it was frozen first.
- Kill larvae by thorough cooking. Heating at 60°C for one minute, or 74°C for 15 seconds in the microwave is sufficient.
- Light freezing won’t kill larvae, but freezing at -20°C for one week, or flash freezing for 15 hours at 35°C or colder, kills all anisakid species.
Avoiding allergic anisakidosis completely is not possible because the larvae are more difficult to see in cooked fish, and one may have no way of knowing that larval waste products or remains are present in food. Those who do not eat fish at all are at lowest risk, while workers in fish processing operations face a high risk of exposure.
Anisakidosis will remain with us for the foreseeable future because the number of larvae encapsulated in ocean fish has increased in recent years, along with an increased tendency to eat fish raw or lightly cooked.
Sources
“Anisakis simplex: From Obscure Infectious Worm to Inducer of Immune Hypersensitivity.” Audicana, M. Teresa and Malcolm W. Kennedy.Clinical Microbiology Reviews 21(2), 2008. pp. 360-79. doi: 10.1128/CMR.00012-07
“Anisakis spp.” Berland, Bjorn. In: Parasites of the Colder Climates. Akuffo, Hannah, Ewert Linder, Inger Ljungström, and Mats Wahlgren, eds. London: Taylor and Francis, 2003. pp161-8.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
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