Anisakis simplex

The Anisakis simplex is a parasitic roundworm classified under the phylum Nematoda.[2][5] It possesses the typical characteristics of its phylum, including an unsegmented, cylindrical body that occasionally fills up with fluids and allows it to swim freely.[5] Like other nematodes, Anisakis simplex communicates through its sense organs that detect chemicals and motion.[5] These nematodes have the tendency to infect and inhabit the organs of other animals, who act as their host.[2]

A. simplex is also part of the Anisakidae family and the Anisakis genus, which are specifically known for their potential to spread diseases from one species to another, starting with the infection of animals, such as fish, and eventually leading to transmission in humans.[9]

Anisakis simplex, like typical worms from the Anisakidae has an oral pore, oesophagus, ventriculus, intestine and anal pore to aid with ingestion and excretion.[3][6]

The morphological features of A. simplex change throughout its four stages of development.[3] However, these features have only been examined in detail in its third and fourth larval stages.[3]

The anterior region of Anisakis simplex as seen on a scanning electron micrograph

In its early stages, the larva can be found in the faeces of its host, and is around 5mm in length, presenting a boring tooth and a bundle of sensory nerves at the front tip of its body.[5][4] Once it develops into a third stage larva, it grows to a length of 20 to 30 mm, and is found in the body cavity of the host, either freely floating or enclosed in a protective sac attached to the main organs.[3] At this stage, the larva appears to be cylindrical with narrowed ends, and are visibly pink and white in colour, with a small white mark in the frontal region.[3] Its outer surface is textured with irregular indents and striations.[6] There are three protruding segments around the mouth that form the lips, one that is dorsal and two that are ventrolateral, each with lobules on them.[6] The two ventrolateral segments encase the triangular boring tooth and an excretory opening in front of it.[6] The mouth then connects to the tubular esophagus that is visible from the frontal region and leads to the wideset ventriculus.[6] A defining characteristic of this species is that the ventriculus is diagonally adjoined to the center of the intestine.[6] At its rear end, the species has an anal opening, as well as spine-like structure known as the mucron.[3][6]

Third stage larvae develop into their fourth and last stage days after being ingested by their final host.[6] In fourth stage larvae, the three lip segments are still present, but the dorsal segment has grown to be larger than the ventrolateral ones, and the boring tooth is no longer present within them.[6] At the rear end, the mucron is replaced by a cone-shaped protrusion with small globular structures on it, and the previously irregular striations and indents on the outer surface of the organism become regularly patterned.[6] At this stage, female larvae begin to develop reproductive organs at the center of their bodies, including internal genitalia, uteri and surrounding papillae.[6]

There are four stages in the life cycle of A. simplex. The first stage includes emerging as an egg and hatching as larvae, and the following stages include being consumed by an intermediate host, a paratenic host, and lastly, a final host.[4]

The egg of the A. simplex is first found in the fecal matter of its final host in an unembryonated form, and is visibly transparent and circular, with an evenly-surfaced shell.[3][4] Once they are passed, embryos begin to develop within these eggs until they grow into second-stage larvae.[4] The egg then hatches, and second stage larvae emerge into the sea enclosed in a protective sheath.[4]

Anisakis simplex lodged in the liver of a cod fish

The temperature of the water affects the speed of hatching of the eggs. In warmer temperatures, eggs take between 4 and 8 days to hatch, but in temperatures below 5 °C, it may take up to 82 days.[3] Once they are hatched, they can survive up to a month in warmer temperatures, and can last even longer in cooler temperatures below 10 °C.[4] However, the release of eggs from the final host is unaffected by seasonal factors or temperatures, so the rate of infection by the species is stable throughout the year.[5]

Once the eggs are hatched, second stage larvae are ingested by an intermediate host. This host is most commonly a krill or some other type of marine crustaceans.[4]  Upon ingestion, the larvae shed their protective sheath, then travel freely to the body cavity of the host to inhabit it.[4] The change in the environment of the second stage larvae from the open water to the body of its host initiates their development into third stage larvae.[5]

The krill infected by third stage larvae are then ingested by common predators, such as squid and fish, specifically teleosts, who then become paratenic hosts.[3] These hosts are referred to as paratenic because third stage larvae do not develop further when inhabiting them.[5] At this stage, the larvae no longer roam freely in the gut of the host, but are rather embedded in their viscera, specifically in the mucus layers of the stomach, and eventually make their way into the inner muscles.[7] Certain species of paratenic hosts are sold and consumed by humans, including salmon, mackerel, cod, anchovy, sardine and squids.[3]

Anisakis simplex reaches the end of its life cycle when the paratenic host is consumed by the final host, which are usually large marine mammals, such as whales, dolphins, and seals.[5][3] Here, the third stage larvae make their way into the abdomen of the host, and moult two times to become fully mature organisms.[4] They then embed themselves into the mucus lining of the stomach, and produce digestive enzymes to ingest it.[5] At this stage, the worms are able to reproduce. Females attract a mate by releasing chemical signals, and a male then wraps around a female to reproduce.[5]

The immediate habitat of A. simplex is within the body cavity of another marine animal, as it spends a majority of its life cycle being carried by these hosts.[5] Its secondary habitats are the waterbodies in which these hosts reside.[5] This includes the Pacific Ocean, the Atlantic Ocean, the Arctic Ocean and other regions characterised by cold marine waters.[4][5] It is found along the coast and within the deeper regions of these waters.[7]

The geographical distribution of this species is inferred based on the rates of human infection in different locations. The first case of human infection was discovered in the Netherlands in the 1960s.[3] Since then, cases have been detected in other parts of Europe, including Germany, Spain, Italy and the United Kingdom.[10] It has also been found in Egypt, Korea and parts of the America, such as Hawaii, Alaska and South America.[10] However, it is most commonly detected in Japan.[10]

The main factors affecting the distribution and survival of this species are salinity and average sea surface temperature. While higher temperatures reduce the time taken for their eggs to hatch, it also reduces their chance of long-term survival.[11] However, higher salinity predicts longer survival times.[11] Its distribution is also determined by locality, as it is specific to the aforementioned areas.[12]

A. simplex can cause zoonotic diseases.[7] The consumption or management of raw seafood puts humans at risk for developing an infection or allergic reaction caused by Anisakis simplex.[3][5] The presence of a single worm in a human's body is enough to cause infection and elicit symptoms.[3] In fact, this species is responsible for the majority of human infection cases within its family.[3]  Although such infections can be transmitted from animals to humans, it is not transmissible from one human to another.[7]

• List of parasites of humans