LIVER FLUKE

Fasciola hepatica (Sheep liver fluke)

· Digenetic endoparasite, cosmopolitan in distribution.
· Adult Fasciola lives in the liver and bile passages of the sheep (the primary host).

It may also occur in other vertebrates like goats, horses, monkeys, man, etc. Larval stages are found in an invertebrate host, a freshwater gastropod, commonly Snail. (either Limnea truncatula or Planorbis spp. or Bulinus spp.)
· A single sheep may contain about 200 adult flukes in its body as a result of which its liver may cease to work.

This condition is known as liver rot (complete breakdown of the liver) or cirrhosis [IOM 2006].

In case of heavy infection, sheep become dull and sluggish followed by swelling and pain in the abdomen, weight is lost, eye-sockets become pale and lives enlarges and sheep die.

External morphology

· Body is leaf-like, dorsoventrally flattened, soft, oval, and long.
· Colour is usually pinkish but appears brownish due to ingested bile of the host.
· Anterior end is somewhat broad and rounded, while the posterior is bluntly pointed.
· The anterior end is modified into a prominent conical projection called the oral cone or head lobe.
· Two suckers i.e. anterior and ventral are present without hooks and spines.
· The mouth is present in the middle of the oral or anterior sucker. It helps in ingestion and adhesion. About 3-4 mm behind the oral sucker, the ventral or posterior sucker (also called the acetabulum) is present mid-ventrally.
· Gonopore lies just in front of the acetabulum and an excretory pore at the posterior end slightly towards the ventral surface.

Body wall

· Body wall lacks ciliated epidermis and consists of tegument, basement membrane, musculature, and mesenchyme.
· Tegument is thick and non-ciliated external covering of the body. It contains mitochondria, ER, vacuoles, and pinocytic vesicles.

It is capable of withstanding the action of the host’s digestive juices. Instead, it contains all over numerous microscopic and backwardly directed spinules which help in anchoring the fluke in the host’s bile passage, provide protection as well as facilitate locomotion.
· Thin and delicate basement membrane lies below the tegument. The electron microscope shows only its outer edge, while the inner edge appears to merge imperceptively with underlying tissues. The basement membrane has integumentary muscles.
· Musculature consists of outer circular and inner longitudinal layers. In Fasciola, the third layer of oblique or diagonal muscle fibres is also present inside the longitudinal layer.
· Mesenchyme (Parenchyma) is mesodermal in origin and found surrounding various internal organs. It helps in the transport of nutrients and waste materials.

Mesenchyme is present in place of the coelom in Platyhelminthes.

Digestive system

· Alimentary canal is incomplete due to the absence of anus and consists of the mouth, pharynx, oesophagus, and intestine. No stomach.
· Digestion is extracellular and takes place mainly in the intestine.
· Absorption of food takes place through the intestinal caeca with the help of mesenchyme from where the distribution of simple food takes place throughout.
· Food is stored in the form of glycogen and fats in the mesenchyme and muscles.

Excretory system

· It is composed of a large number of excretory cells or flame cells or protonephridia.

Flame cells arise from mesoderm; and are unicellular, uninucleate, and ovoid [IOM 2001]. The main excretory products are CO2, fatty acids, and ammonia.

Respiratory system

· Respiration in Fasciola is usually anaerobic (occurring in absence of oxygen).

Reproductive system

· Fasciola is a hermaphrodite animal. Male and female genital ducts lead into the common genital atrium which opens outside through the common genital aperture or gonopore.
· Male reproductive system consists of a pair of highly branched tubular testes, tandem in position (lying one behind the other), a pair of vasa deferentia, a seminal vesicle (vesicula seminalis), a narrow and twisted tube ejaculatory duct, and a stout muscular, copulatory organ, the penis or cirrus, genital atrium, male genital pore.

Laurer’s canal arises from the oviduct. During copulation, it serves as a vagina.
· Prostate glands surround and open into the ejaculatory duct. Their alkaline secretion helps in the free movement of sperms during copulation.
· Female reproductive system consists of a single ovary, oviduct, vitelline ducts, vitelline glands, Mehlis’s or shell glands, vitelline or yolk reservoir, ootype, uterus, meta term, and female genital pore situated in the right half of the body.
· Vitelline glands contain abundant yolk for embryos and numerous shell globules that form eggshells.
· Junction of the oviduct, median vitelline duct, and the uterus is surrounded by Mehlis’s glands (shell glands), secretion of which helps in the lubricating uterus for smooth passage of eggs and in activating the sperms.
· Cross-fertilization occurs in flukes and the fertilization takes place in the lower part (distal end) of the oviduct.

Life cycle

The life cycle of Liver Fluke
A. Egg B. Miracidium C. Sporocyst D. Redia E. Cercaria F. Metacercaria G. Adult fluke

· Fasciola is harmless to snails but is pathogenic in vertebrate hosts including man (rarely).
· Shelled eggs of Fasciola hepatica are called capsules. Cleavage begins while the eggs are still in the uterus and are of holoblastic and equal type.
· Capsules are ejected out along with the faecal matter of the host. During the suitable conditions, the encapsulated embryo differentiates into the miracidium larva within 4-15 days.
· Miracidium is microscopic, oval, elongated, dorso-ventrally flattened, free swimming, ciliated and conical in shape. It is a multicellular organism with 21 flattened epidermal cells or plates arranged in five rows.
Miracidium swims by cilia until it is attached to the water snail of the genus Limnea, Planorbis, or Bulinus. Its further development occurs when it penetrates snail within 8 hours.
· Miracidium casts off its ciliated epidermis and changes into sporocyst larva within 14 days. Sporocyst inside the snail gives rise to 5-8 Redia larva.
· Redia gives rise to cercaria (heart-shaped with bifid intestine) larva which penetrates the snail and comes out of water. Each redia forms about 14-20 cercariae.
· Cercaria larva (L. Kerkos-tail) is then transformed into Metacercaria lying on the nearby grasses to be eaten by sheep or goats.
· It takes 35 to 65 days, after entry of miracidium into the snail’s body to the exit of cercariae from the host’s body.

In short,
· Egg capsules - Gonopore - Intestine - Outside through faecal matter - Miracidium (in 4-15 days) - Swims (snail) - Sporocyst (5-8 rediae) - Rediae - 14-20 Cercariae - Leave snail’s body - Metacercaria (Sheep)

· Miracidium is the infective stage for the intermediate host (snail) and metacercaria for the primary host (sheep)

Metacercaria → intestine → coelomic cavity → liver → bile duct laying eggs

· Fasciola exhibits both alternation of generation (sexual generations with asexual but parthenogenetic generations called Heterogamy) and alternation of the host.
· Paedogenesis- larval forms with power of multiplication
· Polyembryony- more than one larval form within the life cycle of one individual
· Germinal lineage Hypothesis (based on the development of different larval forms) has been accepted for Fasciola hepatica.

Schistosoma or Bilharzia (Blood or lymphatic fluke)

Blood Fluke Male and Female

· Discovered by Theodor Bilharz ( in1851 AD)
· Endoparasite in the blood of small branches of the urinary bladder, urinary tract, mesenteric and portal veins, and lymph nodes.
· Digenetic, man- primary host, snail- secondary host
· The three species that infect man are:
· Schistosoma haematobium- present in the blood vessels and urinary tract, discovered by Bilharz as the causative agent of urinary schistosomiasis
· Schistosoma mansoni- present in small branches of mesenteric and portal veins
· Schistosoma japonicum- present in small branches of the portal and mesenteric vessels
· Unlike other flukes, it is dioecious, sexual dimorphism, Male is smaller and thicker.

The Female is longer, slender, and cylindrical and lives permanently in the gynaecophoric canal of the male.
·Life cycle begins in humans when the female lays eggs in thin-walled vessels of the large or small intestine (Schistosoma mansoni and Schistosoma japonicum) and in the urinary bladder (Schistosoma haematobium)
· Eggs pass out along with urine or faeces and hatch when they come in contact with water and form ciliated free-swimming larva called miracidium.
· Miracidium burrows into the tissues of the snail and develops into a sporocyst.
· Capsules (eggs) pass out through urine and faeces.
· Infection occurs from contaminated water by penetration of cercaria through the skin.
· Different larval stages of blood flukes are miracidium, sporocyst, and cercaria. Redia and metacercaria stages are absent.
· Blood fluke causes schistosomiasis (Bilharziasis).
· Symptoms are skin rash, bronchial cough, anaemia, haematuria, bloody stool, etc.
· Blood flukes sometimes produce a severe cercarial dermatitis called swimmer’s itch.
· Schistosomiasis can be treated with the drug Praziquantel, Tartar emetic, etc.
· All trematodes are hermaphrodite except blood flukes.