Fasciola hepatica

Fasciola hepatica, also known as the common liver fluke or sheep liver fluke, is a parasitic trematode (fluke or flatworm, a type of helminth) of the class Trematoda, phylum Platyhelminthes. It infects the livers of various mammals, including humans. The disease caused by the fluke is called fasciolosis or fascioliasis, which is a type of helminthiasis and has been classified as a neglected tropical disease. Fasciolosis is currently classified as a plant/food-borne trematode infection, often acquired through eating the parasite's metacercariae 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. These two flukes are sister species; they share many morphological features and can mate with each other. Fasciola hepatica occurs in the liver of a definitive host and its lifecycle is indirect. Definitive hosts of the fluke are cattle, sheep, and buffaloes. Wild ruminants 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. 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: The metacercariae are released from the freshwater snail as cercariae, and form cysts on various surfaces including aquatic vegetation. 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 such as watercress. Inside the duodenum of the mammalian host, the metacercariae are released from within their cysts. From the duodenum, they burrow through the lining of the intestine and into the peritoneal cavity. 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 3 to 4 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, allowing them to hatch as miracidia, which then find a suitable intermediate snail host of the Lymnaeidae 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. 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 is a larger sucker than the oral sucker and is located at the anterior end. The outer surface of the fluke is called the tegument. This is composed of scleroprotein, and its primary function is to protect the fluke 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, 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, Golgi bodies, 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 named Teg) can actually suppress the immune response of the mammalian 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. 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 on the ventral side of the fluke. This mouth leads to the pharynx, which is then followed by a narrow oesophagus. The oesophagus, which is lined with a thin layer of epithelial cells, then opens up into the large intestine. 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 and bile ducts when feeding within the host. They use their mouth suckers to pull off and suck up food, bile, lymph, and tissue pieces from the walls of the bile ducts. F. hepatica relies on extracellular digestion which occurs within the intestine of the host. The waste materials are egested through the mouth. The nonwaste matter is adsorbed 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. F. hepatica has no respiratory organs: the adult flukes respire anaerobically (without oxygen). Glycogen taken from within the host is broken down by glycolysis to produce carbon dioxide and fatty acids. 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. F. hepatica's excretory system 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 role of F. hepatica's excretory system is excretion and osmoregulation. Each tubule within the excretory system is connected to a flame cell, 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.