Small round worms, varying in color from creamy white to dark brown, are occasionally found in the flesh of fish, particularly cod. These seal worms or nematodes as they are called by scientists, also known as Terranova decipiens or Porrocaecum decipiens spend part of their life cycle in seals. Eggs passed into the water by seals hatch, are eaten by small fish and shrimp-like animals which are in turn eaten by larger fish such as cod. Only fish caught in areas frequented by seals are affected in this way and only a small portion of the total catch is taken from such area.
The occurrence of nematodes in fish is a natural phenomenon which cannot be prevented and is not indicative of mishandling or spoilage. Fillets from fish taken from areas where seals abound are carefully examined by passing them over an illuminated glass top table. The light shining through the fillets aids in the detection and reduces the chances of fillets containing seal worms reaching the consumer. While the operation is routinely checked by federal government inspectors, it is not 100% effective and in spite of taking every reasonable precaution, fillets containing the worms do reach the market.
Are such fish safe to eat? They most certainly are. The seal worm is destroyed by commercial freezing and storage (frozen to -5 ºF or -20 ºC at the center and stored at -15 ºF or -26 ºC) as well as by normal cooking temperatures.
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. To date, only A. simplex and P. decipiens are reported from human cases in North America.
Anisakiasis is generally used when referring to the acute disease in humans. Some purists utilize generic names (e.g., contracaeciasis) in referring to the disease, but the majority consider that the name derived from the family is specific enough. The range of clinical features is not dependent on species of anisakid parasite
in cases reported to date.
In North America, anisakiasis is most frequently diagnosed when the affected individual feels a tingling or tickling sensation in the throat and coughs up or manually extracts a nematode. In more severe cases there is acute abdominal pain, much like acute appendicitis accompanied by a nauseous feeling. Symptoms occur from as little as an hour to about 2 weeks after consumption of raw or undercooked seafood. One nematode is the usual number recovered from a patient. With their anterior ends, these larval nematodes from fish or shellfish usually burrow into the wall of the digestive tract to the level of the muscularis mucosae (occasionally they penetrate the intestinal wall completely and are found in the body cavity). They produce a substance that attracts eosinophils and other host white blood cells to the area. The infiltrating host cells form a granuloma in the tissues surrounding the penetrated worm. In the digestive tract lumen, the worm can detach and reattach to other sites on the wall. Anisakids rarely reach full maturity in humans and usually are eliminated spontaneously from the digestive tract lumen within 3 weeks of infection. Penetrated worms that die in the tissues are eventually removed by the host's phagocytic cells.
In cases where the patient vomits or coughs up the worm, the disease may be diagnosed by morphological examination of the nematode. (Ascaris lumbricoides, the large roundworm of humans, is a terrestrial relative of anisakines and sometimes these larvae also crawl up into the throat and nasal passages.) Other cases may require a fiber optic device that allows the attending physician to examine the inside of the stomach and the first part of the small intestine. These devices are equipped with a mechanical forceps that can be used to remove the worm. Other cases are diagnosed upon finding a granulomatous lesion with a worm on laparotomy. A specific radioallergosorbent test has been developed for anasakiasis, but is not yet commercially marketed.
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 sea water. These larvae are infective to copepods (minute crustaceans related to shrimp) and other small invertebrates. The larvae grow in the invertebrate and 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. Some evidence exists that the nematode larvae move from the viscera to the flesh if the fish hosts are not gutted promptly after catching. The life cycles of all the other anisakid genera implicated in human infections are similar. 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. However, it is suspected that many other cases go undetected. The disease is transmitted by raw, undercooked or insufficiently frozen fish and shellfish, and its incidence is expected to increase with the increasing popularity of sushi and sashimi bars.
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 apparently persist after the worm dies since some lesions are found upon surgical removal that contain only nematode remnants. Stenosis (a narrowing and stiffening) of the pyloric sphincter was reported in a case in which exploratory laparotomy had revealed a worm that was not removed.
The target population consists of consumers of raw or underprocessed seafood.
Candling or examining fish on a light table is used by commercial processors to reduce the number of nematodes in certain white-flesh fish that are known to be infected frequently. This method is not totally effective, nor is it very adequate to remove even the majority of nematodes from fish with pigmented flesh.