Fasciola Hepatica
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''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
trematode (fluke or flatworm, a type of helminth) of the
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...
Trematoda 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 ...
, 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, unsegme ...
. It infects the livers of various mammals, including humans, and is transmitted by sheep and cattle to humans the world over. The disease caused by the
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
is called
fasciolosis Fasciolosis is a parasitic worm infection caused by the common liver fluke '' Fasciola hepatica'' as well as by '' Fasciola gigantica''. The disease is a plant-borne trematode zoonosis, and is classified as a neglected tropical disease (NTD). ...
or fascioliasis, which is a type of
helminthiasis Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths. There are numerous species of these parasites, which are br ...
and has been classified as a
neglected tropical disease Neglected tropical diseases (NTDs) are a diverse group of tropical disease, tropical infections that are common in low-income populations in Developing country, developing regions of Africa, Asia, and the Americas. They are caused by a variety ...
.
Fasciolosis Fasciolosis is a parasitic worm infection caused by the common liver fluke '' Fasciola hepatica'' as well as by '' Fasciola gigantica''. The disease is a plant-borne trematode zoonosis, and is classified as a neglected tropical disease (NTD). ...
is currently classified as a plant/food-borne
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 ...
infection, often acquired through eating the parasite's
metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
encysted on plants. ''F. hepatica'', which is distributed worldwide, has been known as an important parasite of sheep and cattle for decades and causes significant economic losses in these livestock species, up to £23 million in the UK alone. Because of its relatively large size and economic importance, it has been the subject of many scientific investigations and may be the best-known of any trematode species. ''F. hepatica's'' closest relative is ''
Fasciola gigantica ''Fasciola gigantica'' is a parasitic flatworm of the class Trematoda, which causes tropical fascioliasis. It is regarded as one of the most important single platyhelminth infections of ruminants in Asia and Africa. Estimates of infection rates ...
''. These two flukes are sister species; they share many morphological features and can mate with each other.


Life cycle

''Fasciola hepatica'' occurs in the liver of a
definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
and its lifecycle is indirect. Definitive hosts of the fluke are
cattle Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult ma ...
,
sheep Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to domesticated ...
, and buffaloes. Wild
ruminant Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
s and other mammals, including humans, can act as definitive hosts as well. The life cycle of ''F. hepatica'' goes through the intermediate host and several environmental larval stages. Intermediate hosts of ''F. hepatica'' are air-breathing freshwater snails from the family
Lymnaeidae Lymnaeidae, common name the pond snails, is a taxonomic family of small to large air-breathing freshwater snails, aquatic pulmonate gastropod mollusks, that belong to the clade Hygrophila. Lymnaeidae is the only family within the superf ...
. Although several lymnaeid species susceptible to ''F. hepatica'' have been described, the parasite develops only in one or two major species on each continent. ''
Galba truncatula ''Galba truncatula'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails.Neubauer, Thomas A. (2014). Galba (Galba) truncatula (Müller, 1774). Accessed through: World R ...
'' is the main snail host in Europe, partly in Asia, Africa, and South America. '' Lymnaea viator'', '' L. neotropica'', ''
Pseudosuccinea columella ''Pseudosuccinea columella'' , the American ribbed fluke snail, is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails. This snail is an intermediate host for '' Fasciola ...
'', and '' L. cubensis'' are most common intermediate hosts in Central and South America. Several other lymnaeid snails may be naturally or experimentally infected with ''F. hepatica'', but their role in transmission of the fluke is low. The list of lymnaeid snails that may serve as natural or experimental intermediate hosts of ''F. hepatica'' include: * ''
Austropeplea ollula ''Austropeplea ollula'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails. Distribution This species occurs in: * Korea * Philippines Parasites In Korea, ' ...
'' * ''
Austropeplea tomentosa ''Austropeplea tomentosa'' is a species of gastropod belonging to the family Lymnaeidae Lymnaeidae, common name the pond snails, is a taxonomic family of small to large air-breathing freshwater snails, aquatic pulmonate gastropod mollu ...
'' * ''
Austropeplea viridis ''Austropeplea viridis'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails. Distribution This species occurs in: * Russian Federation: Kuril Islands * Japan ...
'' * '' Fossaria bulimoides'' * ''
Galba truncatula ''Galba truncatula'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails.Neubauer, Thomas A. (2014). Galba (Galba) truncatula (Müller, 1774). Accessed through: World R ...
'' * '' Lymnaea cousini'' * ''
Lymnaea cubensis ''Galba cubensis'' is a species of air-breathing freshwater snail, an aquatic gastropod mollusk in the family Lymnaeidae, the pond snails. Distribution ''Galba cubensis'' is native to parts of South America, Mexico, the southern Coastal Pl ...
'' * '' Lymnaea diaphana'' * '' Lymnaea humilis'' * '' Lymnaea neotropica'' * '' Lymnaea occulta'' * ''
Lymnaea stagnalis ''Lymnaea stagnalis'', better known as the great pond snail, is a species of large air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae. The great pond snail is a model organism to study parasitology, ...
'' * ''
Lymnaea viatrix ''Galba viatrix'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or ...
'' * '' Omphiscola glabra'' * ''
Pseudosuccinea columella ''Pseudosuccinea columella'' , the American ribbed fluke snail, is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails. This snail is an intermediate host for '' Fasciola ...
'' * ''
Radix auricularia ''Radix auricularia'', the big-ear radix, is a species of medium-sized freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae.Neubauer, Thomas A.; Rosenberg, G.; Gofas, S. (2014). Radix auricularia (Linnaeus, 1758) ...
'' * ''
Radix lagotis ''Ampullaceana lagotis'' is a species of freshwater gastropods belonging to the family Lymnaeidae Lymnaeidae, common name the pond snails, is a taxonomic family of small to large air-breathing freshwater snails, aquatic pulmonate gastro ...
'' * ''
Radix natalensis ''Radix natalensis'' is a species of freshwater snail, an aquatic gastropod mollusc in the family Lymnaeidae. This species occurs widely in Africa. It is a major intermediate host for '' Fasciola gigantica'' in Africa. Placement of this s ...
'' * ''
Radix peregra ''Peregriana peregra'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod in the family Lymnaeidae, the pond snails. Distribution and habitat This small pond snail is found in Europe, Newfoundland and northern A ...
'' * ''
Radix rubiginosa ''Radix rubiginosa'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod in the family Lymnaeidae, the pond snails. This species is sometimes treated as a subspecies of ''Radix auricularia''. Distribution This spe ...
'' * '' Stagnicola caperata'' * '' Stagnicola fuscus'' * '' Stagnicola palustris'' * '' Stagnicola turricula'' The
metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
are released from the freshwater snail as
cercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
, and form cysts on various surfaces including
aquatic vegetation Aquatic plants are plants that have adapted to living in aquatic environments (saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that g ...
. The mammalian host then eats this vegetation and can become infected. Humans can often acquire these infections through drinking contaminated water and eating freshwater
plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
such as
watercress Watercress or yellowcress (''Nasturtium officinale'') is a species of aquatic flowering plant in the cabbage family Brassicaceae. Watercress is a rapidly growing perennial plant native to Europe and Asia. It is one of the oldest known leaf v ...
. Inside the duodenum of the mammalian host, the
metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
are released from within their cysts. From the duodenum, they burrow through the lining of the intestine and into the
peritoneal cavity The peritoneal cavity is a potential space between the parietal peritoneum (the peritoneum that surrounds the abdominal wall) and visceral peritoneum (the peritoneum that surrounds the internal organs). The parietal and visceral peritonea are la ...
. They then migrate through the intestines and
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 ...
, and into the
bile ducts 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. It ...
. Inside the
bile ducts 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. It ...
, they develop into an adult
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
. In humans, the time taken for ''F. hepatica'' to mature from
metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
into an adult
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
is roughly 3 to 4 months. The adult flukes can then produce up to 25,000 eggs per
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
per day. These eggs are passed out via stools and into freshwater. Once in freshwater, the eggs become
embryonated Embryonated, unembryonated and de-embryonated are terms generally used in reference to eggs or, in botany, to seeds. The words are often used as professional jargon rather than as universally applicable terms or concepts. Examples of relevant fields ...
, allowing them to hatch as
miracidia Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
, which then find a suitable intermediate snail host of the
Lymnaeidae Lymnaeidae, common name the pond snails, is a taxonomic family of small to large air-breathing freshwater snails, aquatic pulmonate gastropod mollusks, that belong to the clade Hygrophila. Lymnaeidae is the only family within the superf ...
family. Inside this snail, the
miracidia Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
develop into sporocysts, then to rediae, then to
cercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
. The
cercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
are released from the snail to form
metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
and the life cycle begins again.


Morphology and anatomy

''Fasciola hepatica'' is one of the largest
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
s of the world, reaching a length of 30 mm and a width of 13 mm (''
Fasciola gigantica ''Fasciola gigantica'' is a parasitic flatworm of the class Trematoda, which causes tropical fascioliasis. It is regarded as one of the most important single platyhelminth infections of ruminants in Asia and Africa. Estimates of infection rates ...
'', though, is even bigger and can reach up to 75 mm). It is leaf-shaped, pointed at the back (posteriorly), and wide in the front (anteriorly). The oral sucker is small but powerful and is located at the end of a cone-shape projection at the anterior end. The acetabulum is a larger sucker than the oral sucker and is located at the anterior end.


Tegument

The outer surface of the
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
is called the tegument. This is composed of
scleroprotein In molecular biology, fibrous proteins or scleroproteins are one of the three main classifications of protein structure (alongside globular and membrane proteins). Fibrous proteins are made up of elongated or fibrous polypeptide chains which f ...
, and its primary function is to protect the
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
from the destructive digestive system of the host. Its also used for renewal of the surface plasma membrane and the active uptake of nutrients, and the uptake of some compounds (e.g. taurine) make flukes even more resistant to be killed by the digestive system of host. On the surface of the tegument are also small spines. Initially, these spines are single-pointed, then, just prior to the fluke entering the
bile ducts 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. It ...
, they become multipointed. At the anterior end of the
fluke Fluke may refer to: Biology * Fluke (fish), a species of marine flatfish * Fluke (tail), the lobes of the tail of a cetacean, such as dolphins or whales, ichthyosaurs, mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ...
, the spines have between 10 and 15 points, whereas at the posterior end, they have up to 30 points. The tegument is a syncytial epithelium. This means it is made from the fusion of many cells, each containing one
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
, to produce a multinucleated cell membrane. In the case of ''F. hepatica'', no nuclei are in the outer cytoplasm between the basal and apical membranes. Thus, this region is referred to as anucleate. Instead, the nuclei are found in the cell bodies, also known as tegumental cells, these connect to the outer cytoplasm via thin cytoplasmic strands. The tegumental cells contain the usual cytoplasmic organelles ( mitochondria,
Golgi bodies The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles in ...
, and endoplasmic reticulum). The tegument plays a key role in the fluke's infection of the host. Studies have shown that certain parts of the tegument (in this case, the
antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respons ...
named Teg) can actually suppress the
immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...
of the mammalian host. This means that the fluke is able to weaken the
immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...
, and increase its chances of a successful infection. A successful infection is needed for the fluke to have enough time to develop into an adult and continue its lifecycle.


Digestive system

The alimentary canal of ''F. hepatica'' has a single mouth which leads into the blind gut; it has no anus. The mouth is located within the anterior
sucker Sucker may refer to: General use * Lollipop or sucker, a type of confection * Sucker (slang), a slang term for a very gullible person * Hard candy ** Cough drop ** Mint (candy) Biology * Sucker (botany), a term for a shoot that arises undergro ...
on the
ventral side Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
of the fluke. This mouth leads to the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
, which is then followed by a narrow
oesophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to th ...
. The oesophagus, which is lined with a thin layer of
epithelial cells Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...
, then opens up into the
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before bein ...
. As no anus is present, the intestine branches, with each branch ending blindly near the posterior end of the body. Flukes migrate into smaller
capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
and
bile ducts 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. It ...
when feeding within the host. They use their mouth suckers to pull off and suck up food, bile,
lymph Lymph (from Latin, , meaning "water") is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues ...
, and tissue pieces from the walls of the bile ducts. ''F. hepatica'' relies on
extracellular digestion Extracellular phototropic digestion is a process in which saprobionts feed by secreting enzymes through the cell membrane onto the food. The enzymes catalyze the digestion of the food ie diffusion, transport, osmotrophy or phagocytosis. Since dige ...
which occurs within the intestine of the host. The waste materials are egested through the mouth. The nonwaste matter is
adsorbed Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
back in through the tegument and the general surface of the fluke. The tegument facilitates this adsorption by containing many small folds to increase the surface area.


Respiratory system

''F. hepatica'' has no
respiratory organs The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gre ...
: the adult flukes respire anaerobically (without oxygen). Glycogen taken from within the host is broken down by glycolysis to produce
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
and
fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
. This process provides the fluke with energy. In contrast, the free-living
miracidia Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
stages of the parasite generally develop within oxygen-rich environments. The free-living stages of the parasite are thought to respire aerobically, to gain the most energy from their environment.


Excretory system

''F. hepatica's''
excretory system The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The dual function of excreto ...
contains a network of
tubule In biology, a tubule is a general term referring to small tube or similar type of structure. Specifically, tubule can refer to: * a small tube or fistular structure * a minute tube lined with glandular epithelium * any hollow cylindrical body stru ...
s surrounding one main excretory canal. This canal leads to the excretory pore at the posterior end of the fluke. This main canal branches into four sections within the dorsal and ventral regions of the body. The role of ''F. hepatica's'' excretory system is
excretion Excretion is a process in which metabolic waste is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after ...
and
osmoregulation Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration o ...
. Each tubule within the
excretory system The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The dual function of excreto ...
is connected to a
flame cell A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms, rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, ...
, otherwise known as
protonephridia The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...
. These cells are modified parenchyme cells. In ''F. hepatica'', their role is to perform excretion, but more importantly, osmoregulatory functions.
Flame cell A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms, rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, ...
s are therefore primarily used to remove excess water.


Nervous system and sensory organs

The nerve system of ''F. hepatica'' consists of a pair of nerve ganglia, each one is located on either side of the
oesophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to th ...
. Around the oesophagus is a
nerve ring A circumesophageal or circumpharyngeal nerve ring is an arrangement of nerve ganglia around the esophagus/ pharynx of an animal. It is a common feature of nematodes, molluscs, and many other invertebrate animals, though it is absent in all vertebra ...
, which connects the two nerve ganglia together. The nerves stem from this ring, reaching the posterior end of the body. At the posterior end, one pair of nerves becomes thicker than the others; these are known as the lateral nerve cords. From these lateral nerve cords, the other nerves branch. Sensory organs are absent from ''F. hepatica''.


Reproductive system

''F. hepatica'' adult flukes are hermaphrodite; each contains both male and female
reproductive organs A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
. The male and female reproductive organs open up into the same chamber within the body, which is called the
genital A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
atrium. The genital atrium is an ectodermal sac which opens up to the outside of the fluke via a genital pore. The
testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...
are formed of two branched tubules, these are located in the middle and posterior regions of the body. From the epithelium lining of the tubules, sperm is produced. The sperm then passes into the
vas deferens The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube ...
and then into the
seminal vesicle The seminal vesicles (also called vesicular glands, or seminal glands) are a pair of two convoluted tubular glands that lie behind the urinary bladder of some male mammals. They secrete fluid that partly composes the semen. The vesicles are 5 ...
. From the
seminal vesicle The seminal vesicles (also called vesicular glands, or seminal glands) are a pair of two convoluted tubular glands that lie behind the urinary bladder of some male mammals. They secrete fluid that partly composes the semen. The vesicles are 5 ...
projects the
ejaculatory duct The ejaculatory ducts (''ductus ejaculatorii'') are paired structures in male anatomy. Each ejaculatory duct is formed by the union of the vas deferens with the duct of the seminal vesicle. They pass through the prostate, and open into the ureth ...
, and this opens into the genital atrium, and many
prostate The prostate is both an accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found only in some mammals. It differs between species anatomically, chemically, and phys ...
glands surround this opening. The right side of the anterior
testis A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...
has a branched, tubular ovary. From here, a short
oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
passes to the
vitelline duct In the human embryo, the vitelline duct, also known as the vitellointestinal duct, the yolk stalk, the omphaloenteric duct, or the omphalomesenteric duct, is a long narrow tube that joins the yolk sac to the midgut lumen of the developing fetus. ...
. This duct connects, via a junction, the ovaries, the
uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
, and the yolk reservoir. From this junction, the uterus opens into the
genital A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
atrium; this opening is surrounded by Mehlis glands. In some flukes, the terminal end of the uterus is strengthened with
muscles Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of musc ...
and spines. ''F. hepatica'' reproduces both sexually, via the hermaphrodite adult flukes, and asexually. The
miracidia Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
can reproduce asexually within the intermediate
snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class G ...
host.


Genome

With its draft genome sequence published in 2015, ''F. hepatica'' is known to have the largest nuclear genome size among trematodes so far sequenced. It is about 1.3 Gb, which is two times that of ''Opisthorchis viverrini'' with 634.5 Mb, the second largest genome among trematodes. The genome is contained in 10 pairs of chromosomes. The protein-coding sequence covers about 21.8 Mb and repetitive DNA sequence about 32% of the total genome. The number of genes predicted is 14,642. The mitochondrial genome consists of 14462 bp, containing 12 protein-encoding, 2 ribosomal and 22 transfer RNA genes.


Prevalence

Currently, ''F. hepatica'' has one of the widest geographical spread of any parasitic and vector-borne disease. Originating in Europe, it has expanded to colonize over 50 countries, covering all continents except
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...
. In contrast, '' F. gigantica'' is generally considered more geographically restricted to the tropical regions of
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
,
Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...
, and the
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabia (including the Arabian Peninsula and Bahrain), Asia Minor (Asian part of Turkey except Hatay Province), East Thrace (Europ ...
, with some overlap between the two species.
Climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
affects both ''F. hepatica'' and its intermediate host, the snail. For example, the development of ''F. hepatica'' miracidia and larvae, and the reproduction of ''
Galba truncatula ''Galba truncatula'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails.Neubauer, Thomas A. (2014). Galba (Galba) truncatula (Müller, 1774). Accessed through: World R ...
'', require a temperature range of 10 to 25 °C. In addition, they both require high levels of moisture in the air, as both are at risk of desiccation. Due to this, the
prevalence In epidemiology, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number o ...
, along with the intensity of infection, of ''F. hepatica'' is primarily dependent on rainfall levels and temperature.


Parasitic adaptations

''F. hepatica's'' tegument protects it from the
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s of the host's digestive system, whilst still allowing water to pass through. Free-swimming larvae have cilia and the
cercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
have a muscular tail to help them swim through the aquatic environment and also allow them to reach the plants on which they form a cyst. To attach within the host, ''F. hepatica'' has oral suckers and body spines. Their pharynges also help them to suck onto the tissues within the body, particularly within the
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. It ...
s. The adult fluke's respiration is anaerobic; this is ideal, as no
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
is available in 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 ...
. ''F. hepatica'' is adapted to produce a large number of eggs, which increases its chances of survival, as many eggs are destroyed on release into the environment. Also, ''F. hepatica'' is hermaphrodite, thus all flukes can produce eggs, increasing the number of offspring produced by the population. The genome for ''F. hepatica'' was published in 2015. At 1.3 Gb, its genome is one of the largest known
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
s. The genome contains many polymorphisms, and this represents the potential for the fluke to evolve and rapidly
adapt ADAPT (formerly American Disabled for Attendant Programs Today) is a United States grassroots disability rights organization with chapters in 30 states and Washington, D.C. They use nonviolent direct action in order to bring about disability just ...
to changes in the environment, such as host availability and drug or
vaccine A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified.
interventions.


Epidemiology

''For more information on the epidemiology – see the disease page,
fasciolosis Fasciolosis is a parasitic worm infection caused by the common liver fluke '' Fasciola hepatica'' as well as by '' Fasciola gigantica''. The disease is a plant-borne trematode zoonosis, and is classified as a neglected tropical disease (NTD). ...
'' Infection begins when cyst-covered aquatic vegetation is eaten or when water containing metacercariae is drunk. In the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...
, ''F. hepatica'' frequently causes disease in
ruminants Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...
, most commonly between March and December. Humans become infected by eating
watercress Watercress or yellowcress (''Nasturtium officinale'') is a species of aquatic flowering plant in the cabbage family Brassicaceae. Watercress is a rapidly growing perennial plant native to Europe and Asia. It is one of the oldest known leaf v ...
or by drinking 'Emoliente', a Peruvian drink that uses drops of watercress juice. Cattle and sheep are infected when they consume the infectious stage of the parasite from low-lying, marshy
pasture Pasture (from the Latin ''pastus'', past participle of ''pascere'', "to feed") is land used for grazing. Pasture lands in the narrow sense are enclosed tracts of farmland, grazed by domesticated livestock, such as horses, cattle, sheep, or sw ...
. Human infections have been reported from more than 75 countries around the world. In Asia and Africa, people are infected both by ''F. hepatica'' and ''F. gigantica'' whereas human fasciolosis is caused only by ''F. hepatica'' in South and Central America and Europe. The presence of ''F. hepatica'' can interfere with the detection of
bovine tuberculosis Bovines (subfamily Bovinae) comprise a diverse group of 10 genera of medium to large-sized ungulates, including cattle, bison, African buffalo, water buffalos, and the four-horned and spiral-horned antelopes. The evolutionary relationship betwee ...
in cattle. Cattle co-infected with ''F. hepatica'', compared to those infected with ''M. bovis'' alone, react weakly to the single intradermal comparative cervical tuberculin (SICCT) test. Therefore, an infection from ''F. hepatica'' can make it difficult to detect
bovine tuberculosis Bovines (subfamily Bovinae) comprise a diverse group of 10 genera of medium to large-sized ungulates, including cattle, bison, African buffalo, water buffalos, and the four-horned and spiral-horned antelopes. The evolutionary relationship betwee ...
; this is, of course, a major problem in the farming industry.


Fasciolosis

Both ''F. hepatica'' and ''F. gigantica'' can cause fasciolosis. Human symptoms vary depending on whether the disease is chronic or acute. During the acute phase, the immature worms begin penetrating the gut, causing symptoms of fever, nausea, swollen liver (caused by Fh8), skin rashes, and extreme abdominal pain. The chronic phase occurs when the worms mature in the bile duct, and can cause symptoms of intermittent pain, jaundice, and anemia. In cattle and sheep, classic signs of fasciolosis include persistent diarrhea, chronic weight loss, anemia, and reduced milk production. Some remain asymptomatic. ''F. hepatica'' can cause sudden death in both sheep and cattle, due to internal hemorrhaging and liver damage.
Fasciolosis Fasciolosis is a parasitic worm infection caused by the common liver fluke '' Fasciola hepatica'' as well as by '' Fasciola gigantica''. The disease is a plant-borne trematode zoonosis, and is classified as a neglected tropical disease (NTD). ...
is an important cause of both production and economic losses in the dairy and meat industries. Over the years, the prevalence has increased and it is likely to continue increasing in the future. Livestock are often treated with flukicides, chemicals toxic to flukes, including
bromofenofos Bromofenofos is an anthelminthic agent used in veterinary medicine. It is used to treat Fasciola hepatica, common liver fluke (''Fasciola hepatica'') infections in cattle and sheep. It is teratogenic. References

Anthelmintics Bromoaren ...
,
triclabendazole Triclabendazole, sold under the brand name Egaten among others, is a medication used to treat fascioliasis and paragonimiasis. It is very effective for both conditions. Treatment in hospital may be required. It is taken by mouth with typically ...
, and
bithionol Bithionol is an antibacterial, anthelmintic, and algaecide. It is used to treat '' Anoplocephala perfoliata'' (tapeworms) in horses and ''Fasciola hepatica'' (liver flukes). __TOC__ Mechanism of action Bithionol has been shown to be a potent in ...
.
Ivermectin Ivermectin (, '' EYE-vər-MEK-tin'') is an antiparasitic drug. After its discovery in 1975, its first uses were in veterinary medicine to prevent and treat heartworm and acariasis. Approved for human use in 1987, today it is used to treat i ...
, which is widely used for many helminthic parasites, has low effectivity against ''F. hepatica'', as does
praziquantel Praziquantel (PZQ), sold under the brandname Biltricide among others, is a medication used to treat a number of types of parasitic worm infections in mammals, birds, amphibians, reptiles, and fish. In humans specifically, it is used to treat sc ...
. For humans, the type of control depends on the setting. One important method is through the strict control over the growth and sales of edible water plants such as
watercress Watercress or yellowcress (''Nasturtium officinale'') is a species of aquatic flowering plant in the cabbage family Brassicaceae. Watercress is a rapidly growing perennial plant native to Europe and Asia. It is one of the oldest known leaf v ...
. This is particularly important in highly
endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...
areas. Some farms are
irrigated Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been devel ...
with
polluted Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...
water, hence,
vegetables Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems ...
farmed from such land should be thoroughly washed and cooked before being eaten. The best way to prevent
fasciolosis Fasciolosis is a parasitic worm infection caused by the common liver fluke '' Fasciola hepatica'' as well as by '' Fasciola gigantica''. The disease is a plant-borne trematode zoonosis, and is classified as a neglected tropical disease (NTD). ...
is by reducing the lymnaeid snail population or separating livestock from areas with these snails. These two methods are not always the most practical, so control by treating the herd before they are potentially infected is commonly practiced.


Diagnosis

A diagnosis may be made by finding yellow-brown eggs in the stool. They are indistinguishable from the eggs of '' Fascioloides magna'', although the eggs of ''F. magna'' are very rarely passed in sheep, goats, or cattle. If a patient has eaten infected liver, and the eggs pass through the body and out via the faeces, a false positive result to the test can occur. Daily examination during a liver-free diet will unmask this false diagnosis. An enzyme-linked immunosorbent assay (ELISA) test is the diagnostic test of choice. ELISA is available commercially and can detect antihepatica antibodies in serum and milk; new tests intended for use on faecal samples are being developed. Using ELISA is more specific than using a
Western blot The western blot (sometimes called the protein immunoblot), or western blotting, is a widely used analytical technique in molecular biology and immunogenetics to detect specific proteins in a sample of tissue homogenate or extract. Besides detect ...
or Arc2
immunodiffusion Immunodiffusion is a diagnostic test which involves diffusion through a substance such as agar which is generally soft gel agar (2%) or agarose (2%), used for the detection of antibodies or antigen. The commonly known types are: :# Single diffusi ...
.
Protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
s secreted by ''F. hepatica'' have been used experimentally in immunizing antigens.


See also

*
List of parasites (human) Endoparasites Protozoan organisms Helminths (worms) Helminth organisms (also called helminths or intestinal worms) include: Tapeworms Flukes Roundworms Other organisms Ectoparasites References {{Portal bar, Bio ...
* Veterinary parasitology *
Trematoda 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 ...


References


External links


University of Michigan Animal Diversity Web

Encyclopedia of LifeTaxonomy and nomenclature at ITIS.govMolecular database at UniProt
* {{Taxonbar, from=Q334149
Hepatica ''Hepatica'' (hepatica, liverleaf, or liverwort) is a genus of herbaceous perennials in the buttercup family, native to central and northern Europe, Asia and eastern North America. Some botanists include ''Hepatica'' within a wider interpre ...
Animal diseases Veterinary helminthology Animals described in 1758 Taxa named by Carl Linnaeus