Fluke is also called trematode or blood fluke. Liver fluke definition is given as, it is any member of the invertebrate class Trematoda (or phylum Platyhelminthes), which is a parasitic flatworm family that developed millions of years ago from free-living species. There are above 10,000 species of flukes, and they are present worldwide and range in size from up to 5 millimetres (0.2 inches) to many centimetres; most do not exceed more than 100 millimetres (4 inches) in length.
Flukes parasitize the members of all vertebrate classes but most commonly, frogs, parasitize fish, and turtles; they also parasitize domestic animals, humans, and invertebrates such as crustaceans and mollusks. A few are external parasites (or ectoparasites); a few attach themselves to the internal organs (or endoparasites); others are semi-external, attaching themselves to the lining of the mouth, to the cloaca (the end of the digestive tract), or to the gills. A few attack a single host, while others need two or more hosts.
A Liver fluke (Fasciola hepatica) is represented below:
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The fluke's symmetrical body is protected by a noncellular cuticle. Many are flattened and ribbonlike or leaflike, although a few are circular and stout in cross-section. Muscular suckers on the ventral (or bottom) surface, spines, and hooks are used for attachment. The body is solid and is filled with a spongy connective tissue (called mesenchyme), which surrounds all the body organs.
A circulatory system is not present. The digestive system has a simple sac with a mouth either in the middle of the anterior end of the ventral surface. Usually, an anus is absent, but a few species have either one or two anal pores. The nervous system has a pair of nerve centres or anterior ganglia and usually three pairs of lengthwise nerve cords.
Several species are hermaphroditic; it means functional reproductive organs of both sexes take place in the same individual. However, in some, the sexes are separate. Most of the species pass through larval, egg, and mature stages.
Liver Fluke Classification
Blood flukes take place in most types of vertebrates; three species attack humans: the intestinal blood fluke (called S. mansoni), the urinary blood fluke (called Schistosoma haematobium) and the Oriental blood fluke (called S. japonicum). Human diseases, which are caused by them, are called schistosomiasis (bilharziasis); they affect millions of persons, specifically in East Asia and Africa.
The urinary blood fluke (called S. haematobium) that lives in the veins of the urinary bladder takes place mainly in southern Europe, Africa, and the Middle East. Eggs laid in the veins break through the vein wall into the bladder and they are voided during urination. The larval fluke develops in the snail body (chiefly of the genera Physopsis and Bulinus), which is the intermediate host. The mature larva will make its way into the final host's body, man, either through the mouth or skin.
Intestinal Blood Fluke
The intestinal blood fluke (called S. mansoni) - lives in the veins around large and small intestines, takes place primarily in northern South America and Africa. With the faeces, the eggs pass from the host. The larva enters the snail's body (any of many genera), the intermediate host, and returns to the human host from the skin.
Oriental Blood Fluke
The Oriental blood fluke that takes place primarily in Japan, China, Taiwan, the Philippine Islands, and East Indies differs from S. haematobium and S. mansoni in that it can attack vertebrates other than man, including different domestic animals, mice and rats. Snails of the genus Oncomelania are described as the intermediate host. The adult takes place in the veins of the small intestine. A few eggs are carried in the bloodstream to multiple organs and can cause a variety of symptoms, including liver enlargement. Human hosts can die from severe infestations.
The widely distributed big liver fluke of cattle (named Fasciola hepatica) and the Oriental or Chinese liver fluke are both economically significant flukes (Clonorchis sinensis or Opisthorchis sinensis). In sheep, as well as other domestic animals, F. hepatica causes highly damaging "liver rot." By eating raw vegetables, a person can become infected with this fluke.
Chinese Liver Fluke
The Chinese liver fluke infests a wide range of mammals, including man. In addition to snails as the intermediate hosts, the Chinese liver fluke infests fish as the second intermediate host prior to passing to the final host. The cat liver fluke, called Opisthorchis felineus, which may also infest the man as the final host, also needs a freshwater snail (called Bithynia leachii) and a carp as its secondary intermediate hosts.
Trematodes are worm-like and flattened oval animals, usually no more than some centimetres in length, although species smaller as 1 millimetre (0.039 in) are known. Their most distinctive external feature is given as the presence of two suckers, one close to the mouth and the other one on the animal's underside.
The body surface of the trematodes comprises a tough syncytial tegument that helps protect against the digestive enzymes in those species, which inhabit the gut of larger animals. Also, it is the surface of gas exchange; there are no respiratory organs.
The mouth is located at the animal's forward end and opens into a muscle, which is a pumping pharynx. The pharynx connects, through the short oesophagus, to either one or two blind-ending caeca that occupy most of the body length. In a few species, the caeca are themselves branched. There is no anus, and waste should be ejected by the mouth, as with other flatworms.
Although the excretion of nitrogenous waste takes place mostly through the tegument, trematodes do possess an excretory system that is instead primarily concerned with osmoregulation. This has either two or more protonephridia, with those on everybody's side opening into a collecting duct. Typically, the two collecting ducts meet up at a single bladder, opening to the exterior through either one or two pores near the animal's posterior end.
In the head region, the brain consists of a pair of ganglia, from which either two or three pairs of nerve cords run down the body's length. The nerve cords which are running along the ventral surface are always the largest, while the dorsal cords are present in the Aspidogastrea only. Generally, trematodes lack any specialised sense organs, although a few ectoparasitic species do possess either one or two pairs of simple ocelli.
Most trematodes are given as simultaneous hermaphrodites, having male and female organs. Usually, there are two testes with sperm ducts that join together on the underside of the animal's front half. This final part of the male system differs considerably in structure between species but can include accessory glands and sperm storage sacs, in addition to the copulatory organ that is either reversible and termed as non-reversible or a cirrus and termed a penis.
Usually, there is only a single ovary and the eggs pass from it into an oviduct. The distal part of the oviduct, which is called the ootype, is dilated. It is connected through a pair of ducts to numerous vitelline glands on either side of the body that produce yolk cells. After the egg is surrounded by the yolk cells, its shell can be formed from the secretion of another gland known as the shell gland or Mehlis' gland, the duct of which opens in the ootype as well.
The ootype is connected to the elongated uterus, which opens to the exterior in the genital pore, close to the male opening. In many trematodes, sperm cells travel via the uterus to reach the ootype, where fertilization takes place. Sometimes, the ovary is also associated with a storage sack for sperm and a copulatory duct termed the canal of Laurer.
Almost all the trematodes infect molluscs as a first host in the life cycle, and most have a complex life cycle involving the other hosts. Most trematodes are monoecious and they alternately reproduce sexually and asexually. The two primary exceptions to this are given as the Aspidogastrea that have no asexual reproduction and the schistosomes, which are dioecious.
In the definitive host, where sexual reproduction takes place, eggs are commonly shed along with the host faeces. Eggs, which are shed in water, release free-swimming larval forms (called Miracidia) that are infective to the intermediate host, where the asexual reproduction takes place.
A species, which exemplifies the remarkable life history of the trematodes is given as the bird fluke and Leucochloridium paradoxum. The definitive hosts, where the parasite reproduces, are different woodland birds, while the hosts in which the parasite multiplies (called the intermediate host) are different species of snail. The adult parasite in the gut of the bird produces eggs and these end up on the ground in the faeces of the bird eventually.
A few eggs can be swallowed by a snail and hatch into larvae (called miracidia). These larvae grow and take on the appearance of sac-like. This stage is called sporocyst, and it forms a central body in the digestive gland of the snail that extends into a brood sac in the head of the snail, eye-stalks and muscular foot. It is present in the central body of the sporocyst, where the parasite replicates itself by producing several tiny embryos (called redia). These embryos move to the brood sac and mature into the cercaria.
Life Cycle Adaptations
Trematodes contain a large variety of forms on their whole life cycles. Individual trematode parasite life cycles may differ from this list.
The trematodes get released from the definitive host as eggs that have evolved to withstand the harsh environment.
Miracidium is released from the egg and this infects the first intermediate host in one of two ways, active or passive transmission. a) Active transmission has adapted for the dispersal in space as a free-swimming ciliated miracidium with adaptations for penetrating and recognising the first intermediate host. b) Passive transmission has adapted for the dispersal in time and it infects the first intermediate host contained within the egg.
The sporocyst forms inside the snail's first intermediate host and feeds through the diffusion across the tegument.
Also, the rediae form inside the snail's first intermediate host and feed via the developed pharynx. Either the sporocyst or the rediae develop into the cercariae via polyembryony in the snail.
The cercariae can be adapted for dispersal in space and exhibit a large variety in morphology. They can be adapted to penetrate and recognise the second intermediate host, and they contain physiological and behavioral adaptations not present in the earlier life stages.
The metacercariae are given as an adapted cystic form dormant in the secondary intermediate host.
The adult is fully developed, which infects the definitive host.