- Fasciola hepatica is a parasitic flatworm that belongs to the phylum Platyhelminthes.
- It is commonly known as the common liver fluke.
- The adult Fasciola hepatica parasite lives in the bile ducts of the liver of various mammals including sheep, cows, and humans.
- The intermediate host of Fasciola hepatica is a snail, in which the parasite undergoes several developmental stages.
- Fasciola hepatica can be found in Asia, Europe, Africa, and the Americas.
- The body of Fasciola hepatica is flat and leaf-like with a size that varies from 1.0 to 2.5 cm in length and a width of about 1 cm.
- The anterior end of Fasciola hepatica is conically projected to form the head lobe, which has two suckers.
- Fasciola hepatica has a single median genital pore that is situated ventrally and between the two suckers.
- The digestive system of Fasciola hepatica consists of a mouth in the middle of the anterior sucker, followed by a suctorial pharynx, short esophagus, and bifurcating intestine that gives rise to many caeca.
- Fasciola hepatica does not have an anus and excretes waste through a single excretory aperture located at the extreme posterior end of the body.
- Fasciola hepatica has a nervous system consisting of prominent masses called cerebral ganglia that are joined together by a nerve ring around the esophagus.
- Fasciola hepatica is a hermaphrodite, with both male and female sex organs.
- The male organs of Fasciola hepatica consist of a pair of much-branched testes, two vasa deferentia, a pear-shaped seminal vesicle, a convoluted ejaculatory duct, and a muscular penis (cirrus).
- The female organs of Fasciola hepatica consist of a single and branched ovary, a convoluted oviduct, shell gland, a uterus, and accessory glands, yolk or vitelline glands.
- Cross-fertilization is the rule in Fasciola hepatica.
- The eggs of Fasciola hepatica are passed out with feces and hatch in water, releasing ciliated larvae called miracidia.
- The miracidia of Fasciola hepatica infect the snail intermediate host and go through several developmental stages to form cercariae.
- The cercariae of Fasciola hepatica leave the snail and encyst on vegetation as metacercariae.
- When a mammal ingests metacercariae of Fasciola hepatica, the parasite excysts in the small intestine and migrates to the liver, where it develops into an adult.
- Fasciola hepatica infection in humans can cause liver damage, anemia, and other health problems.
Life Cycle of Fasciola Hepatica
Fasciola hepatica completes its life cycle in two hosts. The definitive or primary host is a sheep or cow, while the intermediate or secondary host is a small of the genus Limnaea. Its life cycle consists of a number of larval stages that reproduce asexually (polyembryony).
Copulation, Fertilization and Capsule Formation
- Sheep copulate within their biliary ducts. Coupling is bilateral, and cross-fertilization is the norm. Self-fertilization is also a possibility.
- During copulation, the cirrus or penis of each worm is inserted into the vaginal orifice of the opposing worm, and sperm is expelled.
- Reportedly, the cirrus of a worm is inserted into the Laurer’s canal of another individual, after which sperm is expelled. Sperms can persist in the uterus because of the prostatic secretion. Spermatozoa migrate down the uterus.
- Approximately thirty yolk cells and shell globules are derived from vitelline glands via numerous vitelline ducts. On one side of the shell, a covering or operculum is formed.
- According to Rowen (1956-57), there is a viscous granular cushion immediately beneath the operculum at the opercular end of the egg. The capsule consists of the zygote and a bulk of yolk-cells enclosed by the shell. When finished, the capsule enters the uterus.
- In the uterus or in the ootype, ova are fertilized. The yolk cells generated by the vitelline glands in the ootype surround the eggs.
- The yolk cells contain yolk as well as numerous shell globules (proteins and phenol). The egg yolk cells release the shell globules. They create a shell or capsule around the embryo.
- The secretion of the Mehlis gland makes the embryos slippery and lubricates them. A single fluke can produce approximately 200,000 eggs in approximately 11 years, or between 30,000 and 35,000 eggs annually.
- The fully formed, fertilized, capsulated eggs are light brown ovals measuring approximately 130 x 150µ with a distinct line separating the capsule or shell from the operculum.
- These embryos are known as operculate. A granular cushion is positioned directly beneath the operculum. In a later stage of development, the eggs are deposited in the bile ducts of the host, transported to the intestine with the bile, and then expelled with feces.
- While the embryos are still in the uterus, they begin to divide or cleave. As a consequence of the first unequal division, a small granular propagatory cell and a large somatic or ectodermal cell are produced. The somatic cell divides multiple times to produce the larva’s ectoderm.
- The divisions of the propagatory cell produce germ cells and somatic cells. The somatic cells comprise the organs of the body. There is no further development of the egg in the uterus.
- The eggs are then discharged in the host’s bile ducts, from which they travel to the intestine and exit the body with their faces.
- Eggs will develop more rapidly if they are placed in moist environments. As long as the embryos remain in the feces, their development is halted. They can persist for several months in moist feces.
- If rinsed away, the embryo’s development continues. The optimal temperature range for development is 10 to 30 degrees Celsius.
- In eight days at 30°C, the encapsulated embryo transforms into a miracidium larva. Oxygen availability is another factor.
- The mature miracidium is a small animal that is roughly conical and covered with ciliated epithelium. It does not resemble an adult fluke at all, but does resemble a free-living flat worm.
1. Miracidium Larva
- It is the first larval stage of Fasciola hepatica’s life cycle. It actively swims in water in pursuit of its secondary host, the Limnaea snail. In response to exposure to light, the hatching enzyme is secreted during the formation of the miracidium.
- The enzyme dissolves the cementing material that holds the operculum in place, thereby releasing it. The operculum is dislodged by the expansion of the granular cushion and the exosmosis of ions and other substances from within the egg. The intermediate host is infected by free-swimming miracidium, which hatches in water.
Miracidium of Fasciola is approximately 0.07mm long and ovoid or conical in shape. It produces an apical lobe or apical papilla or terebratorium at its broad anterior extremity.
It merits the inauguration of:
- Multinucleated pouch-like apical organ and
- A number of unicellular organs of penetration.
The remainder of the body, excluding the apical lobe, is ciliated. It is covered with 21 hexagonal plates of epidermis arranged in five layers. The first row consists of six plates (two dorsal, two ventral, and two lateral), the second row of six cells (three dorsal and three ventral), the third row of three cells (one dorsal and two ventrolateral), the fourth row of four cells (two right and two left), and the fifth row consists of two cells (one right and one left).
Below the epidermal plates is a thin layer of sub-epidermal musculature composed of the following:
- An outermost stratum that is circular and
- The innermost stratum of longitudinal muscle fibers.
There is a sub-epithelial layer underneath. There are two protonephridia and clusters of germ cells within the larva. In addition, the anterior portion of the body contains a pair of large pigmented eyespots, a large larval brain, and a simple nervous system. In the body’s anterior region are a massive apical gland and a pair of penetration glands or cephalic glands. These open on the lobe or cone of the apex.
Infection to Secondary Host
- Miracidium does not consume. It expires within 24 hours if it is unable to reach its secondary host, which is typically Limnaea truncatula (common in freshwater bodies and wet environments). If it locates the snail, it will penetrate the mollusk’s soft skin and respiratory tissues.
- The apical lobe and tissue dissolving (or histolytic) action of penetration gland secretion aid in penetration.
- The snail might even consume the miracidium or capsule containing the miracidium. Inside the host’s tissue, the larva sheds its ciliated epidermis and penetrates deeper, eventually reaching the lymph vessels or pulmonary chamber. Here, it forms a sac-like organism called the sporocyst.
2. Sporocyst Larva
- Miracidium loses its apical gland, penetration granule, brain, and eyespots and transforms into a sporocyst larva resembling a sac. It resembles an elongated bladder approximately 0.7 mm in length.
- Its body wall retains all but the ciliated epithelium of the miracidium’s body wall. It consists of a thin cuticle, a layer of circular and longitudinal muscles, and a layer of circular and longitudinal muscles.
- Protonephridia and reproductive cells populate the vast interior. Currently, each protonephridium is composed of two flame cells. There is a common fissure on the surface of these.
- A primitive stomach is also present. Rarely, the germ cells may also generate daughter sporocysts in addition to producing radia larvae. A sporocyst may contain between 5 and 18 radia. Radia larvae migrate to the snail’s digestive tract.
3. Redia Larva
- Redia is an elongated larva with a small mouth, a suctorial pharynx, and a simple closed intestine located in the body’s anterior region.
- Numerous unicellular pharyngeal organs have openings in the pharynx. The collar is a muscular, ring-like ridge that encircles the anterior portion of the body. It facilitates locomotion. Just behind the collar is a birth fissure from which the larvae of the next generation emerge.
- Lappets or procrusta are a pair of lobe-like processes located in the posterior region. This serves as an anchor. The larval excretory system is composed of anterior and posterior flame cells that each have a single nephridiopore opening to the outside. The body cavities continue to be filled with parenchyma. Redia also contains germ cell clusters.
- Protonephridia divide further and form a system with numerous branches. All of the flame cells on one side exit via a single excretory duct. A birth pore is located laterally and close to the pharynx. In the egg chamber, there are germ pellets.
- Redia is an extremely active creature. It moves by means of body contractions and lappets. It nourishes itself by drawing fluid and host cells through its muscular pharynx.
- It quickly migrates to the snail’s digestive organs. In the summer months, when sustenance is abundant, mother radiae mature in 12 to 18 days. The germ pellets produce a second generation of rediae daughters. In the winter, offspring rediae germ balls transform into the next larval stage, the cercariae.
4. Cercaria Larva
- A completely developed cercaria larva has an oval, tadpole-like body with a long, elastic tail extending from its posterior end.
- The body is covered by a thin layer of cuticle that contains numerous spines pointing backwards. In the anterior portion of the body, there are a mouth, a muscular pharynx, a small oesophagus, and a bifid intestine.
- There is an oral sucker around the mouth and a ventral sucker between the segments of the intestine. There are numerous embryo cells, flame cells, and excretory ducts in the internal cavity. All excretory ducts connect to the excretory bladders, which are connected to the exterior via a minor excretory duct.
- The cercaria travel from the snail’s digestive organs to the pulmonary sac before escaping into the surrounding water. In the summer, Cercaris swims for between five minutes and one hour. After that, it rests on some green water plant leaves.
- The tail’s excretory duct is located at its base. There are numerous unicellular cystogenous structures beneath the body wall. The substance secreted by these organs forms a cyst around the larva, resulting in the transformation of the cercaria into a metacercaria larva.
- The cercariae, emerging with the water current, swim for some time before attacking the foliage of an aquatic plant. Their appendages disintegrate and cysts form around them as a result of the cystogenous glands’ secretions. The larva is now called a metacercaria.
The metacercaria has a dense, cyst-like cuticle covering that is somewhat rounded. The cercaria’s cystogenous cells dissolve, while the flame cells multiply.
Transmission to the Last Host
When a sheep grazes on aquatic vegetation and reaches its intestine, the metacercaria enters its final or definitive host, the sheep. In the intestine, digestive enzymes dissolve the cyst, allowing the juvenile fluke to emerge. It enters the liver via the hepatic portal system and begins its life in the biliary passage.
Nature of Existence
- Due to the prevalence of multiple larval stages, Fasciola hepatica’s life cycle is extremely complex. Previously, it was believed that radiae and cercariae develop via parthenogenesis from propagatory or germ cells, which were believed to be ova. This type of asexual reproduction by larval forms is known as heterogamy.
- This viewpoint was widely held for a time, and as a result, three terms were coined. This category consists of parthenitae for sparocyst and radiae larvae, adolescariae for cercaria larvae, and maritae for adult flukes.
- This viewpoint, however, is no longer held and has been abandoned. It was later proposed that the germ cells present in larval forms are not ova. Instead, they are diploid cells derived from the mitotic divisions of a proliferating cell that has been separated from the zygote at the first cleavage.
- Thus, it can be said that they are primordial components. Consequently, one zygote produces multiple larvae. Ishii (1934), Chen (1937), Rees (1940), and Cart (1944) have referred to this reproduction process in sporocysts and radiae as polyembryony.
- The phenomenon should be referred to as delayed polyembryony due to the fact that all the progeny resulting from a zygote are not formed simultaneously but at intervals.
Effect or Pathogenesis on the Host
- Fasciola hepatica primarily affects the liver, but it also causes other diseases while residing in the host’s intestine.
- They induce hepatitis and inflammation in the bile ducts, resulting in the thickening and destruction of the bile ducts’ epithelium. The eventual calcification of the biliary ducts leads to the formation of gallstones.
- When invading the substance of the liver and other tissues during severe infections, they cause hemorrhaging. The by-products produced by the worms are extremely toxic and can induce anemia, diarrhea, eosinophilia, and other conditions.
- Additionally, worms contain proteolytic, glycolytic, and fat-splitting enzymes, and decomposing worms are extremely hazardous.
- F. hepatic causes liver decay or fascioliasis by irritating the liver with its cuticular spines and disrupting the liver’s normal functions.
- Acute hepatic rot is a prevalent disease among sheep. The symptoms, which are more severe in lambs than in sheep, manifest one month after infection. Sheep become listless, lethargic, accompanied by abdominal swelling and discomfort, weight loss, pale eye sockets, an enlarged liver, and eventually death.
- Chronic liver rot is the other form of liver rot. The disease is induced by the gradual and sequential entry of cercariae into sheep or cattle. The patient loses strength. Anemia is caused by blood loss and substances secreted by the worms.
- Both milk production and reproduction are drastically reduced. Occasionally, fever and an increase in respiratory activity are present. The dry, brittle fiber sheds. The jaws develop watery swellings (bottle jaw), the appetite decreases, rumination becomes irregular, and the animal expires within a few months.