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Liver fluke is a collective name of a
polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
group of parasitic
trematode Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive h ...
s under the phylum
Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegm ...
. They are principally parasites of the
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
of various
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s, including humans. Capable of moving along the blood circulation, they can occur also in
bile duct A bile duct is any of a number of long tube-like structures that carry bile, and is present in most vertebrates. Bile is required for the digestion of food and is secreted by the liver into passages that carry bile toward the hepatic duct. ...
s,
gallbladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath the liver, although ...
, and liver
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word � ...
. In these organs, they produce pathological lesions leading to parasitic diseases. They have complex life cycles requiring two or three different hosts, with free-living larval stages in water.


Biology

The body of liver flukes is leaf-like and flattened. The body is covered with a tegument. They are
hermaphrodites In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have sep ...
having complete sets of both male and female reproductive systems. They have simple digestive systems and primarily feed on
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
. The anterior end is the oral sucker opening into the mouth. Inside, the mouth leads to a small pharynx which is followed by an extended intestine that runs through the entire length of the body. The intestine is heavily branched and the anus is absent. Instead, the intestine runs along an excretory canal that opens at the posterior end. Adult flukes produce eggs that are passed out through the excretory pore. The eggs infect different species of snails (as intermediate hosts) in which they grow into larvae. The larvae are released into the environment from where the definitive hosts (humans and other mammals) get the infection. In some species, another intermediate host is required, generally a
cyprinid fish Cyprinidae is a family of freshwater fish commonly called the carp or minnow family. It includes the carps, the true minnows, and relatives like the barbs and barbels. Cyprinidae is the largest and most diverse fish family and the largest vert ...
. In this case, the definitive hosts are infected from eating infected fish. Hence, they are food-borne parasites.


Pathogenicity

Liver fluke infections cause serious medical and veterinary diseases. Fasciolosis of sheep, goats and cattle, is the major cause of economic losses in dairy and
meat industry The meat industry are the people and companies engaged in modern industrialized livestock agriculture for the production, packing, preservation and marketing of meat (in contrast to dairy products, wool, etc.). In economics, the meat industry is ...
. Fasciolosis of humans produces clinical symptoms such as fever, nausea, swollen liver, extreme abdominal pain, jaundice and anemia. Clonorchiasis and opisthorchiasis (due to '' Opisthorchis viverrini'') are particularly dangerous. They can survive for several decades in humans causing chronic inflammation of the bile ducts, epithelial hyperplasia, periductal fibrosis and bile duct dilatation. In many infections these symptoms cause further complications such as stone formation, recurrent pyogenic cholangitis and cancer (
cholangiocarcinoma Cholangiocarcinoma, also known as bile duct cancer, is a type of cancer that forms in the bile ducts. Symptoms of cholangiocarcinoma may include abdominal pain, yellowish skin, weight loss, generalized itching, and fever. Light colored stoo ...
). Opisthorchiasis is the leading cause of cholangiocarcinoma in
Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...
and
Laos Laos (, ''Lāo'' )), officially the Lao People's Democratic Republic ( Lao: ສາທາລະນະລັດ ປະຊາທິປະໄຕ ປະຊາຊົນລາວ, French: République démocratique populaire lao), is a socialist s ...
. ''
Clonorchis sinensis ''Clonorchis sinensis'', the Chinese liver fluke, is a liver fluke belonging to the class Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, feeding on ...
'' and '' Opisthorchis viverrini'' are both categorized by the
International Agency for Research on Cancer The International Agency for Research on Cancer (IARC; french: Centre International de Recherche sur le Cancer, CIRC) is an intergovernmental agency forming part of the World Health Organization of the United Nations. Its role is to conduct and ...
(IARC) as Group 1 carcinogens.


Species

Species of liver fluke include: * ''
Clonorchis sinensis ''Clonorchis sinensis'', the Chinese liver fluke, is a liver fluke belonging to the class Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, feeding on ...
'' (the Chinese liver fluke, or the Oriental liver fluke) * '' Dicrocoelium dendriticum'' (the lancet liver fluke) * '' Dicrocoelium hospes'' * '' Fasciola hepatica'' (the sheep liver fluke) * '' Fascioloides magna'' (the giant liver fluke) * '' Fasciola gigantica'' * '' Fasciola jacksoni'' * '' Metorchis conjunctus'' * '' Metorchis albidus'' * ''
Protofasciola robusta Fasciolidae is a family of trematodes and includes several parasites involved in the veterinary and medical sciences, which cause the disease Fasciolosis. Fasciolidae is divided into five genera by Olson et al. 2003. The family's various species ...
'' * '' Parafasciolopsis fasciomorphae'' * '' Opisthorchis viverrini'' (Southeast Asian liver fluke) * '' Opisthorchis felineus'' (Cat liver fluke) * '' Opisthorchis guayaquilensis''


See also

* The Integrated Opisthorchiasis Control Program


References

{{Helminthiases Digenea Hepatology Infectious causes of cancer Foodborne illnesses Veterinary helminthology Invertebrate common names