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Anisakis simplex and related worms

Anisakis simplex (herring worm), Pseudoterranova (Phocanema, Terranova) decipiens (cod or seal worm), Contracaecum spp., and Hysterothylacium (Thynnascaris) spp. are anisakid nematodes (roundworms) that have been implicated in human infections caused by the consumption of raw or undercooked seafood.

 

Anisakiasis is generally used when referring to the acute disease in humans.

Anisakiasis is diagnosed when the affected individual feels a tingling or tickling sensation in the throat and coughs up or manually extracts a nematode worm. With severe cases there is acute abdominal pain, much like acute appendicitis accompanied by nausea. Symptoms occur from an hour to about 2 weeks after eating raw or undercooked seafood.

Only one nematode is usually recovered from a patient.

 

In cases where the patient vomits or coughs up the worm, the disease may be diagnosed by looking at the worm. Other cases may a scope that allows the physician to examine the inside of the stomach and the first part of the small intestine and use a mechanical forceps to remove the worm.

Other cases are diagnosed upon finding a granulomatous lesion with a worm on surgery.

Seafoods are the principal sources of human infections with these larval worms. The adults of A. simplex are found in the stomachs of whales and dolphins. Fertilized eggs from the female parasite pass out of the host with the host's feces. In seawater, the eggs embryonate, developing into larvae that hatch in seawater. These larvae are infective to minute crustaceans and other small invertebrates. The larvae grow in the invertebrate and then become infective for the next host, a fish or larger invertebrate host such as a squid. The larvae may penetrate through the digestive tract into the muscle of the second host. The nematode larvae move from the viscera to the flesh f fish caught by fishermen so it is important to gut the fish quickly after catching. These parasites are known to occur frequently in the flesh of cod, haddock, fluke, pacific salmon, herring, flounder, and monkfish.

Fewer than 10 cases are diagnosed in the U.S. annually. Raw, undercooked or insufficiently frozen fish and shellfish transmit the disease.

 

Severe cases of anisakiasis are extremely painful and require surgical intervention. Physical removal of the nematode(s) from the lesion is the only known method of reducing the pain and eliminating the cause (other than waiting for the worms to die). The symptoms may persist after the worm as irritation may also be caused by nematode remnants.

 

Commercial processors use candling or examining fish on a light table. This method is not totally effective.

Japan has the most reported cases because of all the raw fish consumed there.