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First described half a century ago, anisakiasis (using the term in the broad sense) is caused mainly by the accidental ingestion of larvae of the nematodes (roundworms) Anisakis simplex and Pseudoterranova decipiens. Anasakiasis occurs worldwide, with a higher incidence in regions where raw fish is commonly eaten (e.g., Japan, the Pacific coast of South America, and the Netherlands).
Anisakis simplex is the most common helminth infection in humans resulting from the consumption of raw or undercooked fish (Pseudoterranova decipiens is less frequent but still common). Human anisakid infections frequently cause gastrointestinal symptoms, which may be associated with mild to severe immunological, usually allergic-type, reactions. In addition, some patients show more-generalized hypersensitivity reactions, without any associated digestive disorders. Episodes of allergy have been described in association with exposure to even very small doses of A. simplex antigens and without the involvement of living parasites. Allergic reactions range from rapid onset and potentially lethal anaphylactic reactions to chronic, debilitating conditions. Dead, and occasionally live, nematodes were found to be not rare in a survey of fish served in Seattle sushi restaurants in the 1990s. (Audican et al. 2002; Audicana and Kennedy 2008 and references therein)
Adult stages of Anisakis simplex and Pseudoterranova decipiens reside in the stomachs of marine mammals, where they are embedded in the mucosa in clusters. Unembryonated eggs produced by adult females are passed in the feces of marine mammals. The eggs become embryonated in water, and first-stage larvae are formed in the eggs. The larvae molt, becoming second-stage larvae, and after the larvae hatch from the eggs, they become free-swimming. Larvae released from the eggs are ingested by crustaceans. The ingested larvae develop into third-stage larvae that are infective to fish and squid. The larvae migrate from the intestine to the tissues in the peritoneal cavity and grow up to 3 cm in length. Upon the host's death, larvae migrate to the muscle tissues, and through predation, the larvae are transferred from fish to fish. Fish and squid maintain third-stage larvae that are infective to humans and marine mammals. When fish or squid containing third-stage larvae are ingested by marine mammals, the larvae molt twice and develop into adult worms. The adult females produce eggs that are shed by marine mammals. Humans become infected by eating raw or undercooked infected marine fish. After ingestion, the anisakid larvae penetrate the gastric and intestinal mucosa, causing the symptoms of anisakiasis.