Anisakis simplex (herring worm) is a worm which has a life cycle involving fish and marine mammals. The larvae of Anisakis are common parasites of marine mammals, fish and shellfish and can also be found in squid and cuttlefish. They are usually found on the mesenteries and internals organs and they have been implicated in human infections caused by the consumption of raw seafood.
These parasites are frequently found in the flesh of cod, haddock, fluke, pacific salmon, herring, flounder, and monkfish.
It is frequently reported in areas of the world where fish is consumed raw, lightly pickled or salted. Its incidence is expected to increase with the increasing popularity of sushi and sashimi bars. The areas of highest prevalence are Scandinavia, from cod livers; Japan, after eating sushi and sashimi (raw fish) and sunomono (pickled fish); the Netherlands, by eating Maatjes or green herring which are fermented, raw or slightly salted herrings, along the Pacific coast of South America, from eating ceviche (lightly marinated, raw fish) and in Spain due to marinated anchovies.
Anisakids pose a risk to human health in two ways:
- Through intestinal infection with worms from the eating of under-processed fish which is known as anisakiasis. This disease is often misdiagnosed as acute appendicitis, acute abdomen, stomach ulcers or ileitis. The time to onset of clinical symptoms with intestinal anisakiasis in most cases is within 48 hours and the duration of abdominal pain, nausea and/or vomiting and diarrhea is about 1 to 5 days. It is usually self-limited, requiring no specific treatment. Sometimes an endoscopy to diagnose and remove the parasite may be needed; only very rarely a surgical procedure may have to be performed.
- Through allergic reactions to anisakis molecules. Acute allergic manifestations such as urticaria, respiratory symptoms, skin rashes, read teary eyes and even the more severe life threatening response: an anaphylaxis may occur. Allergic reactions can occur 1 to 12 hours after the infected meal, with most occurring within 6 hours. Usual medical treatment of the symptoms (antihistamines, corticosteroids or adrenaline, in case of anaphylaxis) may be required.
Sensitization against Anisakis is determined by skin-prick test and detection of specific IgE antibodies in blood tests.
How can we prevent anisakiasis?
- 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.
- 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 because salting, cooking and/or marinating will not necessarily kill the parasites.
- Those who do not eat fish are at lowest risk, while workers in fish processing operations face a high risk of exposure.
- Humans are thought to be more at risk of anisakiasis from eating wild fish than farmed fish as anisakis are absent from fish in waters of low salinity.