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Anisakis life cycle.
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1. Adults reside in the stomach of marine mammals. The adults live in clusters, embedded in the mucosa. Adult females produce unembryonated eggs that are passed in feces of the marine mammals.
2a. The eggs become embryonated in water, and first-stage, then 2nd-stage larvae form in the eggs.
2b. After the larvae hatch from the eggs, they become free-swimming.
3. Free-swimming larvae are ingested by crustaceans. The larvae develop into 3rd-stage larvae and migrate from the intestine to tissues in the peritoneal cavity.
4. When the host is eaten by fish or squid, larvae migrate to the muscle tissues, and through predation, larvae are transferred from fish to fish.
5. Third-stage larvae, which are infective to humans and marine mammals, reside in fish and squid.
6. When such a fish or squid is ingested by marine mammals, the larvae develop into adult worms. Adult females produce eggs that are shed by marine mammals.
7. Humans become infected by eating raw or undercooked infected marine fish. After ingestion, the larvae penetrate the gastric and intestinal mucosa, causing symptoms.
Image from the Centers of Disease Control and Prevention Image Library.