Platyhelminthes: Fasciola | Zoology Optional Notes for UPSC PDF Download

Habitat and Behavior of Fasciola Hepatica

Habitat of Fasciola Hepatica:

• Scientific Name: Fasciola hepatica
• Common Name: Sheep liver fluke
• Habitat: Fasciola hepatica is an endoparasitic flatworm that resides within the bile passages of its primary host, which is the sheep. It completes its life cycle in two different hosts, involving a vertebrate host (sheep) and an invertebrate host (a gastropod mollusc). The adult parasite inhabits the primary vertebrate host, while various larval stages are found in the invertebrate host.

Host Range:

• Fasciola hepatica not only infects sheep but also a range of other vertebrates, including goats, deer, horses, dogs, asses, oxen, and occasionally humans.
• Its secondary hosts, which are invertebrate gastropod molluscs, include Planorbis species, Bulinus species, and Limnaea truncatula. These are all freshwater snails.
• Fasciola hepatica has a global distribution, with a particular presence in areas where sheep and cattle are raised, making it a concern for human infections in those regions.

Other Indian Species:

• Another species in India, known as F. gigantica (or F. indica), inhabits the bile passages of buffaloes, cows, goats, and pigs.

Structure of Fasciola Hepatica

1. Shape, Size, and Color:

• Body Shape: Fasciola hepatica has a dorsoventrally flattened, elongated, oval, and leaf-shaped body.
• Size: It measures approximately 25 to 30 mm in length and 4 to 12 mm in width.
• Color: Usually pinkish, but it may appear brownish due to the ingestion of bile from the host.

2. External Morphology:

• Anterior End: The anterior end of the body is characterized by a triangular oral cone or head lobe, giving it a shouldered appearance. At the tip of the head lobe, there is a somewhat triangular aperture called the mouth.
• Suckers: Fasciola hepatica has two muscular suckers:
  • An oral sucker located at the anterior end, encircling the mouth.
  • A large ventral sucker or acetabulum positioned mid-ventrally, situated about 3 to 4 mm behind the oral sucker. These suckers are muscular cup-like organs used for attachment to the host through a vacuum mechanism.
• Apertures: The body features two permanent apertures:
  • A common genital aperture or gonopore located mid-ventrally in front of the ventral sucker.
  • An excretory pore situated at the posterior end of the body.
• Laurer’s Canal: During the breeding season, a temporary opening of Laurer’s canal appears on the dorsal surface just anterior to the middle of the body.
• Anus: Fasciola hepatica lacks an anus as its alimentary canal is incomplete.

Body Wall of Fasciola Hepatica

The body wall of Fasciola hepatica has several distinctive layers and structures that play vital roles in its physiology and survival.

(i) Cuticle:

• The cuticle is a tough, resistant outer layer made of a homogeneous scleroprotein.
• It serves to protect the fluke from the digestive juices of the host.
• Small spines, spinules, or scales on the cuticle help anchor the fluke to the bile duct lining of the host, offer protection, and assist in locomotion.
• In F. indica, the cuticle features broad, stout, and blunt scales.
• Notably, the epidermis is absent during the development of the cercaria stage, and the cuticle is secreted by specialized mesenchymal cells located beneath muscle layers.

(ii) Basement Membrane:

• Below the cuticle, there is a thin and delicate basement membrane that separates the cuticle from the muscle layers.

(iii) Muscle Layer:

• The muscle layer is located beneath the basement membrane.
• It consists of different muscle fiber layers, including:
  • Outer layer of circular muscle fibers.
  • Middle layer of longitudinal muscle fibers.
  • Inner layer of diagonal muscle fibers, which are more developed in the anterior half of the body.
• All muscles are smooth in nature, and they form sturdy bundles of radial fibers in the suckers.

(iv) Mesenchyme:

• Below the muscles lies the parenchyma, known as mesenchyme.
• Mesenchyme comprises numerous loosely arranged uninucleate and bi-nucleate cells, forming a syncytial network of fibers with fluid-filled spaces.
• Some of these cells are large and have large processes that extend up to the base of the cuticle, where they are involved in secretion.
• The mesenchyme serves as a packing material between the muscle layer and internal organs, aiding in the transport of nutrients and waste substances.
• This tissue is essential for gaseous exchange, excretion of nitrogenous wastes, and the absorption of amino acids to some extent.

(v) Structure of Body Wall Under Electron Microscope:

• Electron microscopic studies of the body wall of Fasciola hepatica have shown that the cuticle is not just a protective layer; it is metabolically active and referred to as the integument.
• The integument consists of a syncytial layer of protoplasm with mitochondria, endoplasmic canals, vacuoles, and pinocytic vesicles.
• The tegument is continuous with tegument-secreting cells in the mesenchyme.
• The outer integumental surface has fine projections that increase its surface area for the absorption of the host's fluids.
• The tegument is equipped with numerous fine pore canals through which dissolved substances in the form of a solution are absorbed into the mesenchyme.

The body wall serves multiple functions, including protection, gaseous exchange, elimination of waste substances, and aiding in nutrient absorption.

Digestive System of Fasciola Hepatica

(i) Alimentary Canal:

• The oral sucker contains a ventral mouth leading into a funnel-shaped mouth cavity.
• This is followed by a round, muscular pharynx with thick walls and a small lumen. The pharynx contains pharyngeal glands.
• F. indica has a shorter muscular pharynx, from which arises an oral pouch, about half the size of the pharynx.
• An oesophagus, short and narrow, leads to the intestine.
• The intestine further divides into two branches, known as intestinal caeca or crura. These caeca run along each side to the posterior end and end blindly.
• To facilitate food distribution to all parts of the body, many branching diverticula are released from the intestinal caeca, as Fasciola hepatica lacks a circulatory system.
• There is no anus present.
• The foregut, up to the oesophagus, is lined with cuticle and functions as a suctorial structure, while the intestine is lined with endodermal columnar epithelial cells. The caecal epithelium contains secretory gland cells.

(ii) Food, Feeding, and Digestion:

• Fasciola hepatica feeds on bile, blood, lymph, and cell debris from its host.
• The combination of the oral sucker and pharynx constitutes an effective suctorial apparatus for feeding.
• Digestion is extracellular and takes place within the intestine.
• The digested nutrients are distributed to different parts of the body through the branching diverticula, as there is no circulatory system.
• The digestive system functions as a gastrovascular system, allowing the absorption of digested nutrients into the parenchyma, which further diffuses them into various organs.
• Reserve food, primarily glycogen and fats, is stored in the parenchyma, while monosaccharide sugars like glucose and fructose are directly diffused into the fluke's body.
• Indigestible food remains, if any, are thought to be ejected through the mouth.

The digestive system of Fasciola hepatica plays a crucial role in nutrient acquisition and distribution throughout the body.

Respiration in Fasciola Hepatica

The respiration in Fasciola hepatica is unique and can be described as anaerobic or anoxybiotic respiration. This means that the fluke doesn't rely on oxygen for respiration. Instead, it metabolizes glycogen to produce carbon dioxide and fatty acids, generating energy in the form of heat. The respiration process occurs in several steps:

1. Glycogen undergoes anaerobic glycolysis to form pyruvic acid.
2. Pyruvic acid is decarboxylated, resulting in the production of carbon dioxide and an acetyl group.
3. The acetyl group combines with coenzyme A to form acetyl coenzyme A.
4. Finally, acetyl coenzyme A is condensed and reduced to form fatty acids.

The carbon dioxide produced during this process is then diffused out through the general body surface, while the fatty acids are excreted through the excretory system. This unique anaerobic respiration allows Fasciola hepatica to obtain energy without relying on oxygen.

Excretion in Fasciola Hepatica

The excretory system of Fasciola hepatica serves both excretory and osmoregulatory functions. It consists of flame cells, also known as flame bulbs or protonephridia, connected to a network of excretory ducts.

(i) Flame Cells:

• Flame cells are unique structures in the excretory system of Fasciola hepatica. They are considered to be modified mesenchymal cells.
• These flame cells are numerous and have an irregular, bulb-like shape. They are distributed throughout the body of the fluke.
• Each flame cell possesses a thin elastic wall with pseudopodia-like processes, a nucleus, and an intracellular cavity. The cavity contains many long cilia that arise from basal granules.
• In a living state, the cilia of flame cells vibrate in a manner resembling a flickering flame, hence the name "flame cell."

(ii) Excretory Ducts:

• The excretory system includes an excretory pore located at the posterior end of the fluke.
• From this pore, a longitudinal excretory canal originates, which then gives rise to four main branches: two dorsal and two ventral. These branches further subdivide into numerous small capillaries, and these capillaries anastomose (interconnect) with each other.
• While the longitudinal excretory canal is non-ciliated, the capillaries are lined with cilia.

(iii) Process of Excretion:

• The excretory wastes, which typically consist of fatty acids and ammonia, are diffused from the surrounding mesenchyme into the flame cells.
• These excretory wastes are then collected into the intracellular cavities of the flame cells.
• The vibrating movement of the cilia in the flame cells propels the flow of excretory wastes from the intracellular cavities into the excretory ducts.
• The wastes are transported from the excretory ducts to the main excretory canal and eventually expelled from the fluke's body through the excretory pore due to hydrostatic pressure.

The excretory system of Fasciola hepatica is crucial for eliminating waste products from the parasite and regulating the amount of fluid in its body. This system plays an important role in maintaining the fluke's internal environment.

Nervous System of Fasciola Hepatica

The nervous system of Fasciola hepatica is relatively simple due to its parasitic lifestyle. It consists of a few distinct components:

• Nerve Ring: There is a nerve ring that surrounds the esophagus, indicating a concentration of nervous tissue in this area.

• Cerebral Ganglia: There are a pair of cerebral ganglia located dorsolaterally. These ganglia are clusters of nerve cells and play a role in processing sensory information and coordinating responses.

• Ventral Ganglion: Below the esophagus, there is a ventral ganglion. This ganglion is positioned ventrally and is involved in motor control and coordination.

• Nerve Cords: Three pairs of longitudinal nerve cords arise from the ganglia. These include a dorsal pair, a lateral pair, and a ventral pair of nerve cords. The lateral nerve cords are the most well-developed and extend to the posterior end of the body.

• Transverse Commissures: Nerve cords are connected by transverse commissures that facilitate communication and coordination between different parts of the nervous system.

• Nerve Branches: Numerous small branches extend from the nerve cords, and some of them form plexuses. These smaller nerve structures help in relaying signals and coordinating various functions within the organism.

  

Due to its parasitic life and adaptation, adult Fasciola hepatica lacks well-developed sense organs, as they are not needed in its host-specific environment. The simplicity of its nervous system reflects its specialized lifestyle as a parasitic fluke.

Reproductive System of Fasciola Hepatica

Fasciola hepatica is a hermaphroditic organism, meaning it has both male and female reproductive organs, but it typically undergoes cross-fertilization. Its reproductive system is well-developed and complex, consisting of male and female components.

(i) Male Reproductive System: The male reproductive system includes the following structures:

• Testes: Fasciola hepatica possesses two ramified tubular testes located in the posterior-middle part of the body. They are situated one behind the other and are responsible for sperm production.

• Vasa Deferentia: Narrow vas deferens or sperm ducts arise from each testis and extend forward.

• Seminal Vesicle: The vasa deferentia unite to form a dilated seminal vesicle near the ventral sucker (acetabulum). The seminal vesicle serves as a storage organ for sperm.

• Ejaculatory Duct: The seminal vesicle continues as a narrow and coiled ejaculatory duct.

• Cirrus (Penis): Fasciola hepatica possesses a muscular and elongated structure called the cirrus, which serves as the penis. The ejaculatory duct opens into the cirrus.

• Prostate Glands: Numerous unicellular prostate glands surround the ejaculatory duct. Their secretions are alkaline and help in the free movement of sperm during copulation.

• Genital Atrium: The genital atrium is a common chamber for both male and female genital apertures and opens externally through a gonopore situated ventrally in front of the acetabulum. The cirrus, seminal vesicle, and prostatic glands are enclosed in a common cirrus sheath or cirrus sac.

(ii) Female Reproductive System: The female reproductive system includes the following structures:

• Ovary: Fasciola hepatica has a single, highly branched ovary situated anterior to the testes, in the right anterior one-third of the body.

• Oviduct: The branches of the ovary open into a narrow oviduct, which descends obliquely and joins the median vitelline duct.

• Uterus: The junction of the oviduct and median vitelline duct leads to a wide, convoluted uterus where fertilized eggs or capsules are stored. The uterus opens into the common genital atrium, located to the left of the male genital aperture. The terminal part of the uterus, called the metraterm, is muscular and functions in ejecting eggs and sometimes receiving the cirrus during copulation.

• Vitelline Glands: Situated on both sides of the body, as well as behind the testes, are numerous follicles that form the vitellaria, yolk glands, or vitelline glands. These glands produce albuminous yolk and shell material for the eggs. Ducts connect the vitelline glands to form the vitelline reservoir. A median vitelline duct runs forward to join the oviduct.

• Mehlis's Glands: Mehlis's glands are found around the junction of the median vitelline duct, oviduct, and uterus. Their secretions are thought to lubricate the passage of eggs in the uterus, possibly harden egg shells, and may activate spermatozoa. In some flukes, an ootype (egg assembly site) is formed at this junction.

• Laurer's Canal: A narrow Laurer's canal runs vertically upwards from the oviduct and opens dorsally during the breeding season. It acts as a vestigial vagina, serving as a copulation canal.

Fasciola hepatica's reproductive system is specialized for cross-fertilization and is adapted to its parasitic lifestyle, where it can reproduce efficiently within its host.

Life History of Fasciola Hepatica

Fasciola hepatica has a complex life cycle, involving several stages and adaptations for its parasitic lifestyle. Here's an overview of its life history:

(i) Copulation and Fertilization:

• Fasciola hepatica is hermaphroditic, but cross-fertilization is common. Copulation occurs in the bile duct of the sheep.
• During copulation, the cirrus (penis) of one Fasciola is inserted into the Laurer's canal of another Fasciola, leading to cross-fertilization.
• In cases of self-fertilization, where cross-fertilization doesn't occur, sperms from the same Fasciola enter the female genital aperture and travel down the uterus to fertilize the eggs in the oviduct.

(ii) Formation of Egg Capsules:

• Fertilized eggs in the oviduct receive yolk cells from vitelline glands.
• They get enclosed in a chitinous shell formed by yolk cell granules, which hardens and becomes brownish yellow.
• These completed eggs are called capsules.
• Capsules pass into the uterus, where development begins.

(iii) Development of Fasciola Hepatica:

• Development starts in the uterus and continues outside the host's body.
• Fertilized eggs divide into a propagatory cell and a somatic cell.
• The somatic cell forms the ectoderm of the larva.
• The propagatory cell divides into two cells: one forms the endoderm and mesoderm of the larva, and the other forms a mass of germ cells at the posterior end of the larva.
• Within two weeks, a ciliated miracidium larva is formed and escapes from the eggshell.

(iv) Miracidium Larva:

• The miracidium is a free-swimming stage, covered with ciliated epidermal cells.
• It has rings of epidermal plates with various features, including an apical papilla with a glandular sac, penetration glands, eye spots, a nervous system, protonephridia with flame cells, and a pair of nephridiopores.
• The miracidium doesn't feed and has a short lifespan unless it finds a suitable intermediate host, typically amphibious snails of the genera Limnaea, Bulinus, or Planorbis.
• Once it locates a suitable host, it adheres and enters the snail's body, eventually transforming into a sporocyst.

(v) Sporocyst:

• The sporocyst is an elongated germinal sac within the snail's body.
• It lacks features like those found in the miracidium.
• Germ cells in the sporocyst develop into a third type of larva called the redia.

(vi) Redia:

• The redia is elongated with lappets or procruscula, a birth pore, and a collar used for locomotion.
• It possesses a mouth, pharynx with pharyngeal glands, a sac-like intestine, protonephridia with flame cells, and germ cells.
• Germ cells in the redia can produce either a second generation of daughter rediae (in summer) or cercaria larvae (in winter).
• Multiple rediae are typically formed from a sporocyst.

(vii) Cercaria:

• The cercaria has an oval body with a simple long tail.
• It possesses rudimentary features of adult organs, including suckers, an alimentary canal, an excretory bladder with protonephridial canals, penetration glands, and the beginnings of reproductive organs.
• Cercariae exit the snail, enter the water, and swim briefly, then lose their tails and encyst themselves to become metacercariae.

(viii) Metacercaria:

• The metacercaria is an encysted juvenile fluke and can have a long lifespan if formed in water.
• Metacercariae are infectious when ingested by the final host, such as sheep.

Life History of Fasciola HepaticaInfection of the Primary Host (Sheep)

• Further development of the metacercaria occurs when it is ingested by the final host, such as a sheep.
• After ingestion, the metacercaria sheds its cyst wall and enters the host's body.
• It moves to the liver and then to the bile ducts, where it matures into an adult fluke.

Characteristic Features of Life History:

• Fasciola hepatica displays complex life history features to ensure the survival and reproduction of the species.
• The life cycle involves multiple stages, including parasitic adaptation for each stage.
• The parasite produces a large number of eggs to overcome wastage during transference and ensure the continuity of its race.
• Various larval stages have specific adaptations to facilitate their development and transmission to the next host.
• The life history of Fasciola hepatica features a combination of sexual and asexual reproduction throughout its various stages.

Parasitic Adaptations of Fasciola Hepatica

Adaptations of the Adult Fluke:

1. Dorsoventrally flattened body shape increases the surface area for nutrient absorption.
2. Thick cuticle protects the fluke from the host's immune system.
3. Adhesive suckers ensure firm attachment to host tissues.
4. Anterior mouth and pharynx for feeding on pre-digested host tissues.
5. Absence of a developed alimentary canal and digestive glands due to nutrient availability in host tissue.
6. Anaerobic respiration in an oxygen-poor environment.
7. Lack of respiratory organs due to the parasitic lifestyle.
8. Simple nervous system and the absence of sense organs.
9. Lack of locomotory organs due to a protected lifestyle.
10. Complex excretory system to manage metabolic waste products.
11. Well-developed reproductive system for reproduction within the host.

Adaptations in Life History:

1. Production of a high number of eggs to ensure survival.
2. Shelled capsules protect eggs from host enzymes.
3. Miracidia and cercariae have adaptations for free swimming and finding suitable hosts.
4. Parthenogenetic reproduction ensures the continuity of the parasite's life cycle.
5. Metacercariae can survive for extended periods to await ingestion by the final host.
6. The life history combines sexual and asexual reproduction for maximum efficiency.
7. The presence of secondary hosts, such as snails, aids in the continuation of the parasite's life cycle.