HOME

TheInfoList



OR:

''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 (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...
trematode ( fluke or
flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...
, a type of helminth) of the
class Class, Classes, 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 d ...
Trematoda Trematoda is a class of flatworms known as trematodes, and commonly 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 a mol ...
,
phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
Platyhelminthes Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called f ...
. It infects the livers of various
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s, including humans, and is transmitted by sheep and cattle to humans all over the world. The disease caused by the fluke 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 macroparasite, macroparasitic disease of humans and other animals in which a part of the body is infected with parasitism, parasitic worms, known as helminths. There are numerous species of the ...
and has been classified as a
neglected tropical disease Neglected tropical diseases (NTDs) are a diverse group of tropical infections that are common in low-income populations in developing regions of Africa, Asia, and the Americas. They are caused by a variety of pathogens, such as viruses, bacteri ...
.
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 (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, life cycle requiring at least two Host ( ...
infection, often acquired through eating the parasite's
metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two Sucker (zoology), suckers: one ventral and the other Mouth, oral. Trematodes are covered by a Tegument (helminth), tegument, that protects the o ...
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. The closest relative of ''Fasciola hepatica'' is '' F. gigantica''. 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 The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
and bile ducts 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, bovid ungulates widely kept as livestock. They are prominent modern members of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Mature female cattle are calle ...
,
sheep Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal 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 d ...
and
goat The goat or domestic goat (''Capra hircus'') is a species of Caprinae, goat-antelope that is mostly kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the ...
s as well as
water buffalo The water buffalo (''Bubalus bubalis''), also called domestic water buffalo, Asian water buffalo and Asiatic water buffalo, is a large bovid originating in the Indian subcontinent and Southeast Asia. Today, it is also kept in Italy, the Balkans ...
. Wild
ruminant Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microb ...
s, such as
African buffalo The African buffalo (''Syncerus caffer)'' is a large sub-Saharan African bovine. The adult African buffalo's horns are its characteristic feature: they have fused bases, forming a continuous bone shield across the top of the head, referred to ...
, and other mammals, including humans, can act as definitive hosts as well. Rabbits and hares can serve as wild life
reservoirs A reservoir (; ) is an enlarged lake behind a dam, usually built to store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of water, interrup ...
. The life cycle of ''F. hepatica'' goes through the
intermediate 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 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 Taxonomy (biology), taxonomic family (biology), family of small to large air-breathing freshwater snails, Aquatic animal, aquatic pulmonate gastropod mollusks, that belong to the clade Hygrophila (cl ...
. 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'' is the main snail host in Europe, partly in Asia, Africa, and South America. '' Lymnaea viator'', '' L. neotropica'', '' Pseudosuccinea columella'', 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 tomentosa'' * '' Austropeplea viridis'' * '' Fossaria bulimoides'' * '' Galba truncatula'' * '' Lymnaea cousini'' * '' Lymnaea cubensis'' * '' Lymnaea diaphana'' * '' Lymnaea humilis'' * '' Lymnaea neotropica'' * '' Lymnaea occulta'' * ''
Lymnaea stagnalis ''Lymnaea stagnalis'', common name, better known as the great pond snail, is a species of large air-breathing freshwater snail, an Aquatic animal, aquatic pulmonate gastropod mollusk in the family (biology), family Lymnaeidae. The great pond sn ...
'' * '' Lymnaea viatrix'' * '' Omphiscola glabra'' * '' Pseudosuccinea columella'' * ''
Radix auricularia In a positional numeral system, the radix (radices) or base is the number of unique digits, including the digit zero, used to represent numbers. For example, for the decimal system (the most common system in use today) the radix is ten, becaus ...
'' * '' Radix lagotis'' * '' Radix natalensis'' * '' Radix peregra'' * '' Radix rubiginosa'' * '' Stagnicola caperata'' * '' Stagnicola fuscus'' * ''
Stagnicola palustris ''Stagnicola palustris'' is a species of air-breathing freshwater snail, an aquatic animal, aquatic pulmonate gastropod mollusc in the family Lymnaeidae, the pond snails. MolluscaBase eds. (2023). MolluscaBase. Stagnicola palustris (O. F. Mülle ...
'' * '' Stagnicola turricula'' The
metacercariae Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two Sucker (zoology), suckers: one ventral and the other Mouth, oral. Trematodes are covered by a Tegument (helminth), tegument, that protects the o ...
are released from the
freshwater snail Freshwater snails are gastropod mollusks that live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs t ...
as cercariae, and form cysts on various surfaces including
aquatic vegetation Aquatic plants, also referred to as hydrophytes, are vascular plants and Non-vascular plant, non-vascular plants that have adapted to live in aquatic ecosystem, aquatic environments (marine ecosystem, saltwater or freshwater ecosystem, freshwater ...
. The
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian 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 the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars f ...
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 Eurasia. It is one of the oldest known leaf vegetabl ...
. Inside the
duodenum The duodenum is the first section of the small intestine in most vertebrates, including mammals, reptiles, and birds. In mammals, it may be the principal site for iron absorption. The duodenum precedes the jejunum and ileum and is the shortest p ...
of the mammalian host, the metacercariae are released from within their cysts. From the duodenum, they burrow through the lining of the
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...
and into the
peritoneal cavity The peritoneal cavity is a potential space located between the two layers of the peritoneum—the parietal peritoneum, the serous membrane that lines the abdominal wall, and visceral peritoneum, which surrounds the internal organs. While situated ...
. They then migrate through the intestines and liver, and into the bile ducts. Inside the bile ducts, they develop into an adult fluke. In humans, the time taken for ''F. hepatica'' to mature from metacercariae into an adult fluke is roughly three to four months. The adult flukes can then produce up to 25,000 eggs per fluke 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, which then find a suitable intermediate snail host of the
Lymnaeidae Lymnaeidae, common name the pond snails, is a Taxonomy (biology), taxonomic family (biology), family of small to large air-breathing freshwater snails, Aquatic animal, aquatic pulmonate gastropod mollusks, that belong to the clade Hygrophila (cl ...
family. Inside this snail, the miracidia develop into sporocysts, then to rediae, then to cercariae. The cercariae are released from the snail to form metacercariae and the life cycle begins again.


Morphology and anatomy

''Fasciola hepatica'' is one of the largest flukes of the world, reaching a length of 30 mm and a width of 13 mm ('' Fasciola gigantica'', 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 The acetabulum (; : acetabula), also called the cotyloid cavity, is a wikt:concave, concave surface of the pelvis. The femur head, head of the femur meets with the pelvis at the acetabulum, forming the Hip#Articulation, hip joint. Structure The ...
is a larger sucker than the oral sucker and is located at the anterior end.


Tegument

The outer surface of the fluke is formed by the tegument. This is composed of scleroprotein, and its primary function is to protect the fluke from the destructive
digestive system The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
of the host. It is enveloped in a
glycocalyx The glycocalyx (: glycocalyces or glycocalyxes), also known as the pericellular matrix and cell coat, is a layer of glycoproteins and glycolipids which surround the cell membranes of bacteria, epithelial cells, and other cells. Animal epithe ...
. The tegument renews its plasma membrane as a protection against compromise and is modified to actively or passively uptake nutrients. The uptake of some compounds (e.g.
taurine Taurine (), or 2-aminoethanesulfonic acid, is a naturally occurring amino sulfonic acid that is widely distributed in animal tissues. It is a major constituent of bile and can be found in the large intestine. It is named after Latin (cogna ...
) makes flukes even more resistant to being killed by the digestive system of the 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, they become multipointed. At the anterior end of the fluke, 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, 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 A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
,
Golgi bodies The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...
, and
endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...
). 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, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
named Teg) can actually suppress the
immune response An immune response is a physiological reaction which occurs within an organism in the context of inflammation for the purpose of defending against exogenous factors. These include a wide variety of different toxins, viruses, intra- and extracellula ...
of the
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian host. This means that the fluke is able to weaken the immune response, 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 In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...
. The mouth is located within the anterior sucker on the ventral side of the fluke. This mouth leads to the
pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...
, which is then followed by a narrow
oesophagus The esophagus (American English), oesophagus (British English), or œsophagus ( archaic spelling) ( see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), colloquially known also as the food pipe, food tube, or gullet, ...
. The oesophagus, which is lined with a thin layer of
epithelial cells Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
, 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 Digestion, digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces befor ...
. 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 in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the in ...
and bile ducts when feeding within the host. They use their mouth suckers to pull off and suck up food,
bile Bile (from Latin ''bilis''), also known as gall, is a yellow-green/misty green fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is primarily composed of water, is pro ...
,
lymph Lymph () 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 to be recirculated. At the ori ...
, and tissue pieces from the walls of the bile ducts. ''F. hepatica'' relies on extracellular digestion which occurs within its
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...
. The waste materials are egested through the mouth. The nonwaste matter is absorbed back in through the tegument and the general surface of the fluke. The tegument facilitates this absorption by containing many small folds to increase the surface area.


Respiratory system

''F. hepatica'' has no respiratory organs: the adult flukes respire anaerobically (without oxygen).
Glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. It is the main storage form of glucose in the human body. Glycogen functions as one of three regularly used forms ...
taken from within the host is broken down by
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...
to produce
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
and
fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...
. This process provides the fluke with energy. In contrast, the free-living miracidia 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

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 excret ...
of ''F. hepatica'' contains a network of tubules 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 excretory system functions in
excretion Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, Kidney (vertebrates), kidneys, and skin. This is in contrast with secretion, where the substa ...
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 ...
. Each tubule within the excretory system is connected to a
flame cell A flame cell is a specialized excretory cell found in simple invertebrates, including flatworms ( Platyhelminthes), rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney ...
, otherwise known as protonephridia. These cells are modified parenchyme cells. In ''F. hepatica'', their role is to perform excretion, but more importantly, osmoregulatory functions. Flame cells 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), oesophagus (British English), or œsophagus ( archaic spelling) ( see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), colloquially known also as the food pipe, food tube, or gullet, ...
. Around the oesophagus is a nerve ring, 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 A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...
; each contains both male and female reproductive organs. The male and female reproductive organs open up into the same chamber within the body, which is called the genital atrium. The genital atrium is an
ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
al sac which opens up to the outside of the fluke via a genital pore. The
testes A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone. The ...
are formed of two branched tubules, these are located in the middle and posterior regions of the body. From the
epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
lining of the tubules,
sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
is produced. The sperm then passes into the
vas deferens The vas deferens (: vasa deferentia), ductus deferens (: ductūs deferentes), or sperm duct is part of the male reproductive system of many vertebrates. In mammals, spermatozoa are produced in the seminiferous tubules and flow into the epididyma ...
and then into the
seminal vesicle The seminal vesicles (also called vesicular glands or seminal glands) are a pair of convoluted tubular accessory glands that lie behind the urinary bladder of male mammals. They secrete fluid that largely composes the semen. The vesicles are 5 ...
. From the seminal vesicle projects the
ejaculatory duct The ejaculatory ducts (''ductus ejaculatorii'') are paired structures in the male reproductive system. Each ejaculatory duct is formed by the union of the vas deferens with the Excretory duct of seminal gland, duct of the seminal vesicle. They pa ...
, and this opens into the genital atrium, and many
prostate The prostate is an male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found in all male mammals. It differs between species anatomically, chemica ...
glands surround this opening. The right side of the anterior
testis A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is Homology (biology), homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of Androgen, androgens, p ...
has a branched, tubular
ovary The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/ oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are end ...
. From here, a short
oviduct The oviduct in vertebrates is the passageway from an ovary. In human females, this is more usually known as the fallopian tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, or will dege ...
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'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...
, and the yolk reservoir. From this junction, the uterus opens into the genital atrium; this opening is surrounded by Mehlis glands. In some flukes, the terminal end of the uterus is strengthened with
muscles Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
and spines. ''F. hepatica'' reproduces both sexually, via the hermaphrodite adult flukes, and asexually. The miracidia can reproduce asexually within the intermediate
snail A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial molluscs, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gas ...
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 (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
. 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. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...
,
Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
, and the
Middle East The Middle East (term originally coined in English language) is a geopolitical region encompassing the Arabian Peninsula, the Levant, Turkey, Egypt, Iran, and Iraq. The term came into widespread usage by the United Kingdom and western Eur ...
, with some overlap between the two species.
Climate Climate is the long-term weather pattern in a region, 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 meteoro ...
affects both ''F. hepatica'' and its
intermediate 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 ...
, the snail. For example, the development of ''F. hepatica'' miracidia and larvae, and the reproduction of '' Galba truncatula'', require a temperature range of . In addition, they both require high levels of moisture in the air, as both are at risk of
desiccation Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...
. 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'' has a tegument that protects it from the
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s of the host's
digestive system The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
, whilst still allowing water to pass through. Free-swimming larvae have
cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
and the cercariae 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. The bile duct is separated into three main parts: the fundus (superior), the body (middle), and the neck (inferior). Bile is requ ...
s. The adult fluke's respiration is
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...
; this is ideal, as no
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
is available in the
liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
. ''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 A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...
, 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 (, "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 theory of d ...
genome 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 genes, other functional regions of the genome such as ...
s. The genome contains many polymorphisms, and this represents the potential for the fluke to evolve and rapidly adapt to changes in the environment, such as host availability and drug or
vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifi ...
interventions.


Epidemiology

Infection begins when cyst-covered aquatic vegetation is eaten or when water containing metacercariae is drunk. In the United Kingdom, ''F. hepatica'' frequently causes disease in ruminants, 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 Eurasia. It is one of the oldest known leaf vegetabl ...
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. 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 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; 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, triclabendazole, and bithionol. Ivermectin, which is widely used for many helminthic parasites, has low effectivity against ''F. hepatica'', as does praziquantel. 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 Eurasia. It is one of the oldest known leaf vegetabl ...
. This is particularly important in highly Endemic (epidemiology), endemic areas. Some farms are Irrigation, irrigated with Pollution, polluted water, hence, vegetables farmed from such land should be thoroughly washed and cooked before being eaten. The best way to prevent fasciolosis 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 ELISA, enzyme-linked immunosorbent assay (ELISA) test is the diagnostic test of choice. ELISA is available commercially and can detect antihepatica antibody, 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 or Arc2 immunodiffusion. Proteases secreted by ''F. hepatica'' have been used experimentally in immunizing antigens.


See also

*List of parasites (human) *Veterinary parasitology *
Trematoda Trematoda is a class of flatworms known as trematodes, and commonly 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 a mol ...


References


External links


University of Michigan Animal Diversity Web

Encyclopedia of LifeTaxonomy and nomenclature at ITIS.govMolecular database at UniProt
* {{Authority control Fasciola, Hepatica Animal diseases Veterinary helminthology Animals described in 1758 Taxa named by Carl Linnaeus