Anisakis

Sexual reproduction occurs once the larvae have reached a definitive host and completed their final molt, at which point sexual organs are developed.[3] Females are capable of producing 2.6 million eggs, depending on species and size at sexual maturity.[4]

The known diversity of the genus has increased greatly since mid-1980s with the advent of modern genetic techniques in species identification.[5] Each final host species was discovered to have its own biochemically and genetically identifiable "sibling species" of Anisakis, which is reproductively isolated. This finding has allowed the proportion of different sibling species in a fish to be used as an indicator of population identity in fish stocks.

Anisakiasis
Differential symptoms of parasite infection by raw fish: Clonorchis sinensis (trematode/fluke), Anisakis (nematode/roundworm) and Diphyllobothrium (cestode/tapeworm),[7] all have gastrointestinal, but otherwise distinct, symptoms.[8][9][10][11]
Specialty	Infectious disease

Anisakiasis is a human parasitic infection of the gastrointestinal tract caused by the consumption of raw or undercooked seafood containing larvae of the nematode Anisakis simplex. The first case of human infection by a member of the family Anisakidae was reported in the Netherlands by Van Thiel, who described the presence of a marine nematode in a patient suffering from acute abdominal pain.[12] It is frequently reported in areas of the world where fish is consumed raw, lightly pickled, or salted. The areas of highest prevalence are Scandinavia (from cod livers), Japan (after eating sashimi), the Netherlands (by eating infected fermented herrings (maatjes)), Spain (from eating anchovies and other fish marinated in escabeche), and along the Pacific coast of South America (from eating ceviche). The frequency in the United States is unknown, because the disease is not reportable and can go undetected or be mistaken for other illnesses. Anisakiasis was first recognized in the 1960s. During the 1970s, about 10 cases per year were reported in the literature. The frequency is probably much higher, due to home preparation of raw or undercooked fish dishes. In Japan, more than 1,000 cases are reported annually.[13] Development of better diagnostic tools and greater awareness has led to more frequent reporting of anisakiasis.

Within a few hours of ingestion, the parasitic worm tries to burrow though the intestinal wall, but since it cannot penetrate it, it gets stuck and dies. The presence of the parasite triggers an immune response; immune cells surround the worms, forming a ball-like structure that can block the digestive system, causing severe abdominal pain, malnutrition, and vomiting. Occasionally, the larvae are regurgitated. If the larvae pass into the bowel or large intestine, a severe eosinophilic granulomatous response may also occur one to two weeks following infection, causing symptoms mimicking Crohn's disease.[14]

Diagnosis can be made by gastroscopic examination, during which the 2-cm larvae are visually observed and removed, or by histopathologic examination of tissue removed at biopsy or during surgery.

Raising consumer and producer awareness about the existence of anisakid worms in fish is a critical and effective prevention strategy. Anisakiasis can be easily prevented by adequate cooking at temperatures greater than 60 °C or freezing. The FDA recommends all shellfish and fish intended for raw consumption be blast frozen to −35 °C or below for 15 hours or be regularly frozen to −20 °C or below for seven days.[13] Salting and marinating will not necessarily kill the parasites, as in Italy where two-thirds of cases were attributed to anchovies marinated in lemon or vinegar.[15] Humans are thought to be more at risk of anisakiasis from eating wild fish rather than farmed fish. Many countries require all types of fish with potential risk intended for raw consumption to be previously frozen to kill parasites. The mandate to freeze herring in the Netherlands has virtually eliminated human anisakiasis in the Netherlands.[16]

Even when the fish is thoroughly cooked, Anisakis larvae pose a health risk to humans. Anisakids (and related species such as the sealworm, Pseudoterranova species, and the codworm Hysterothylacium aduncum) release a number of biochemicals into the surrounding tissues when they infect a fish. They are also often consumed whole, accidentally, inside a fillet of fish.

Anisakid larvae in the body cavity of a herring (Clupea harengus)

Acute allergic manifestations, such as urticaria and anaphylaxis, may occur with or without accompanying gastrointestinal symptoms. The frequency of allergic symptoms in connection with fish ingestion has led to the concept of gastroallergic anisakiasis, an acute IgE-mediated generalized reaction.[12] Occupational allergy, including asthma, conjunctivitis, and contact dermatitis, has been observed in fish processing workers.[17] Sensitivization and allergy are determined by skin-prick test and detection of specific antibodies against Anisakis. Hypersensitivity is indicated by a rapid rise in levels of IgE in the first several days following consumption of infected fish.[12] A 2018 review of cases in France has shown that allergic cases were more commonly found, although the number of human Anisakis infections was decreasing.[18]