Anisakiasis is a parasitic (nematode) infestation caused by 2 helminths of the family Anisakidae, Anisakis spp and Pseudoterranova spp. Common species are Anisakis simplex, Anisakis physeteris, and Pseudoterranova decipiens. Anisakiasis is also known as sushi worm, herring worm (Anisakis spp), cod worm (P decipiens), whale worm, and seal worm disease.

Anisakiasis occurs worldwide, but the highest incidence rates are where raw marine fish is a staple diet. Most cases have been reported in Japan. In Europe, cases have been described in the Netherlands, but the infection was controlled after the implementation of adequate public health measures. Regions with lower incidence rates include the United States and countries in South America.

Infections in humans occur by ingestion of inadequately refrigerated and insufficiently cooked marine fish or squid that harbor the infective larva. Of note, humans are incidental hosts and are not needed to complete the cycle of the worm in nature.

The adult worm resides in the stomach of definite hosts: marine mammals (whales, seals, dolphins, walruses). Eggs are passed in seawater, where they mature and hatch as free-swimming larvae. These larvae are ingested by crustaceans, where they evolve into the infective larvae. Crustaceans are then eaten by fish and squid that continue to harbor these infective larvae. When the fish and squid are eaten by marine mammals, the larvae develop into the adult worm. When humans eat the infected raw, undercooked, or pickled marine fish, such as cod, halibut, tuna, sardines, mackerel, and greenling, the larva invades into the mucosal layers of the gastrointestinal tract and causes symptoms. Since humans are incidental hosts, the larva dies in the human body without developing into an adult worm. Salmon, herring, cod, mackerel, and squid transmit Anisakis spp., whereas halibut, cod, and red snapper transmit P decipiens.

Clinical symptoms of anisakiasis are associated with allergic reaction or direct tissue injury from parasitic larvae. Depending on the time after ingestion of infected raw fish, the clinical syndrome could be divided into allergic reaction to larvae, gastric anisakiasis, intestinal anisakiasis, and extra-gastrointestinal anisakiasis.

Some affected persons may present with allergic symptoms such as urticaria, angioedema, pruritus, or tingling of the back of the throat immediately after ingestion of the infected raw fish. Infrequently, systemic anaphylactic reaction may occur. Patients may experience a foreign body sensation in the throat, resulting in retching and regurgitation of the worm during paroxysms of coughing.

Individuals with gastric anisakiasis usually present with acute abdominal pain, nausea, or vomiting, typically 1-8 hours after ingestion of infected raw fish. Frequently, the larva may be expelled in the vomitus, followed by immediate resolution of the symptoms.

If larva is not expelled in vomitus, intestinal anisakiasis may present a few days after ingestion of raw fish with abdominal pain, distension, or a palpable inflammatory mass from the intense host reaction to the larva. Intestinal obstruction may occur secondary to mechanical obstruction from the mass or intense inflammatory reaction. Other intestinal symptoms include diarrhea with blood or mucus. The inflammatory abdominal mass may be palpable on clinical examination. It may mimic a tumor on radiological imaging. This inflammatory mass may resolve spontaneously in some cases, and the suspected "tumor" may not be noted on subsequent clinical examination or radiological test (evanescent or vanishing tumor).

Anisakis larvae have a tendency to perforate the gut wall and invade the peritoneal cavity or other internal visceral organs (extra-gastrointestinal anisakiasis), causing peritonitis or eosinophilic granuloma formation. These granulomas are often mistaken for malignant neoplasms.