Anisakis is a
genusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of parasitic
nematodeThe nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s, which have a life cycle involving
fishFish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and
marine mammalMarine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...
s. They are infective to humans and cause anisakiasis. People who produce
immunoglobulin EImmunoglobulin E is a class of antibody that has been found only in mammals. IgE is a monomeric antibody with 4 Ig-like domains...
(IgE) in response to this parasite may subsequently have an allergic reaction, including anaphylaxis, after eating fish that have been infected with
Anisakis spp.
Life cycle
Anisakis spp. have a complex
life cycleA life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
which passes through a number of hosts through the course of their lives. Eggs hatch in sea water, and
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e are eaten by crustaceans, usually
euphausidsKrill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
. The infected crustacean is subsequently eaten by a fish or squid, and the nematode burrows into the wall of the
gutIn zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
and
encystsA cyst is a closed sac, having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst could go away on its own or may have to be removed through surgery.- Locations :* Acne...
in a protective coat, usually on the outside of the visceral organs, but occasionally in the
muscleMuscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
or beneath the
skin-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
. The life cycle is completed when an infected fish is eaten by a marine mammal, such as a
whaleWhale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
,
sealPinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...
, or
dolphinDolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...
. The nematode excysts in the intestine, feeds, grows, mates and releases eggs into the sea water in the host's
fecesFeces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
. As the gut of a marine mammal is functionally very similar to that of a human,
Anisakis spp. are able to infect humans who eat raw or undercooked fish.
The known
diversityBiodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
of the genus has increased greatly over the past 20 years, with the advent of modern
geneA gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
tic techniques in species identification. Each final host species was discovered to have its own biochemically and genetically identifiable "sibling species" of
Anisakis spp., which is reproductively isolated. This finding has allowed the proportion of different sibling species in a fish to be used as an indicator of
populationA population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
identity in fish stocks.
Morphology
Anisakids share the common features of all nematodes; the vermiform body plan, round in
cross sectionIn geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
and a lack of
segmentationSegmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
. The body cavity is reduced to a narrow
pseudocoelBy the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...
. The mouth is located anteriorly, and surrounded by projections used in feeding and sensation, with the
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
slightly offset from the posterior. The
squamous epitheliumIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
secretes a layered
cuticleA cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
to protect the body from digestive juices.
As with all parasites with a complex life cycle involving a number of hosts, details of the morphology vary depending on the host and which life cycle stage is in. In the stage which infects fish,
Anisakis spp. are found in a distinctive "watch-spring coil" shape. They are roughly 2 cm long when uncoiled. When in the final host, anisakids are longer, thicker and more sturdy, to deal with the hazardous environment of a mammalian gut.
Health implications
Anisakids pose a risk to human health in two ways: through intestinal infection with worms from the eating of underprocessed fish, and through allergic reactions to chemicals left by the worms in fish flesh.
Anisakiasis
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. It is frequently reported in areas of the world where fish is consumed raw, lightly pickled or salted. The areas of highest prevalence are
ScandinaviaScandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
(from
codCod is the common name for genus Gadus, belonging to the family Gadidae, and is also used in the common name for various other fishes. Cod is a popular food with a mild flavor, low fat content and a dense, flaky white flesh. Cod livers are processed to make cod liver oil, an important source of...
livers),
JapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
(after eating
sushiis a Japanese food consisting of cooked vinegared rice combined with other ingredients . Neta and forms of sushi presentation vary, but the ingredient which all sushi have in common is shari...
and
sashimiSashimi is a Japanese delicacy. It consists of very fresh raw meat, most commonly fish, sliced into thin pieces.-Origin:The word sashimi means "pierced body", i.e...
), the
NetherlandsThe Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
(by eating infected
fermented herringsThe term soused herring usually means 'soaked in a mild preserving liquid', and can be used to refer to raw herring in a mild vinegar pickle or the famous Dutch brined herring...
(
maatjes)), and along the Pacific coast of
South AmericaSouth America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
(from eating
cevicheCeviche is a seafood dish popular in the coastal regions of the Americas, especially Central and South America. The dish is typically made from fresh raw fish marinated in citrus juices such as lemon or lime and spiced with chilli peppers. Additional seasonings such as onion, salt,...
). Fewer than ten cases occur annually in the United States. Development of better diagnostic tools and greater awareness has led to more frequent reporting of anisakiasis.
Within hours after ingestion of infective larvae, violent abdominal pain, nausea, and vomiting may occur. Occasionally, the larvae are regurgitated. If the larvae pass into the bowel, a severe
eosinophilicEosinophilic refers to the staining of certain tissues, cells, or organelles after they have been washed with eosin, a dye.Eosin is an acidic dye; thus, the structure being stained is basic....
granulomatous response may also occur one to two weeks following infection, causing symptoms mimicking
Crohn's diseaseCrohn's disease, also known as regional enteritis, is a type of inflammatory bowel disease that may affect any part of the gastrointestinal tract from mouth to anus, causing a wide variety of symptoms...
.
Diagnosis can be made by gastroscopic examination, during which the 2 cm larvae are visualized 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 fifteen hours or be regularly frozen to -20°C or below for seven days. Salting and marinating will not necessarily kill the parasites. Humans are thought to be more at risk of anisakiasis from eating wild fish 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.
Allergic reactions
Even when thoroughly cooked,
Anisakis larvae pose a health risk to humans. Anisakids (and related species such as the sealworm,
Pseudoterranova spp., 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.
Acute allergic manifestations, such as
urticariaUrticaria is a kind of skin rash notable for pale red, raised, itchy bumps. Hives is frequently caused by allergic reactions; however, there are many non-allergic causes...
and
anaphylaxisAnaphylaxis is defined as "a serious allergic reaction that is rapid in onset and may cause death". It typically results in a number of symptoms including throat swelling, an itchy rash, and low blood pressure...
, 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. Occupational allergy, including asthma, conjunctivitis, and contact dermatitis, has been observed in fish processing workers.
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.
Treatment
For the worm, humans are a dead-end host.
Anisakis and
Pseudoterranova larvae cannot survive in humans, and will eventually die. In some cases, the infection will resolve with only symptomatic treatment. In other cases, however, infection can lead to small bowel obstruction, which may require surgery, although there are case reports of treatment with
albendazoleAlbendazole, marketed as Albenza, Eskazole, Zentel and Andazol, is a member of the benzimidazole compounds used as a drug indicated for the treatment of a variety of worm infestations. Although this use is widespread in the United States, the U.S. Food and Drug Administration has not approved...
alone (avoiding surgery) being successful. Intestinal perforation (an emergency) is also possible.
Occurrence
Larval anisakids are common parasites of marine and anadromous fish (e.g.
salmonSalmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...
,
sardineSardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines are named after the Mediterranean island of Sardinia, around which they were once abundant....
), and can also be found in squid and cuttlefish. In contrast, they are absent from fish in waters of low salinity, due to the physiological requirements of Euphausiids, which are needed to complete their life cycle.
Anisakids are also uncommon in areas where cetaceans are rare, such as the southern
North SeaIn the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
.
Similar parasites
- Cod or seal worm Pseudoterranova (Phocanema, Terranova) decipiens
- Contracaecum spp.
- Hysterothylacium (Thynnascaris) spp.
External links