Fasciola hepatica is hermaphrodite. The gonads are well developed and the male and female genital ducts open into a common chamber, the genital atrium. It is situated anteriorly in the body and opens to the exterior through the common genital aperture or gonopore, located ventrally in front of the acetabulum.
1. The Male Reproductive Organs
The male reproductive organs are:
(i) Testis. These are greatly ramified and tubular structures situated in the middle part of the body, one behind the other.
(ii) Vasa deferentia. A narrow delicate duct, the vas deferens, emerges out from each testis and rims forward to meet the fellow of opposite side forming the common sperm duct just below the ventral sucker.
(iii) Seminal vesicle. The common sperm duct immediately leads into a large muscular pear-shaped sac, the seminal vesicle, which lies in front of ventral sucker.
(iv) Ejaculatory duct. The ejaculatory duct arises as fine convoluted tube from the seminal vesicle and runs forward in a zigzag fashion through the cirrus to open into the genital chamber through the male genital pore.
(v) Cirrus and cirrus sac. The cirrus or penis is a muscular cylindrical structure traversed internally by the passage of ejaculatory duct. It can be pushed out and drawn in through the genital pore and thus helps in copulation. The cirrus and the seminal vesicle both are enclosed in a bag-like cirrus sac.
(vi) Prostate glands. Numerous unicellular prostate glands are found around the ejaculatory duct.
1. The Female Reproductive Organs
The female reproductive organs are:
(i) Ovary. The ovary is a large, highly branched tubular structure which is situated on the right side in front of testes in the mesenchyme.
(ii) Oviduct. From the inner side of ovary arises a short, narrow and convoluted duct, the oviduct. It runs down ward to join the uterus.
(iii) Uterus. It is a long wide and highly convoluted tube that extends up to the genital atrium, opening into it through the female genital aperture, close to male genital pore on the left-side. It contains a large number of capsules containing fertilized eggs.
(iv) Mehlis gland. Mehli’s glands, though also known as shell glands, do not play any role in shell-formation, remain surrounding the base of the oviduct.
(iv) Laurer’s canal. The Laurer’s canal arises from the oviduct and acts as a sperm duct. It develops a temporary opening on the dorsal body surface during breeding season. It may serves as vagina.
Copulation and fertilization
Digenetic life cycle
Life cycle of Fasciola hepatica is completed in two hosts. Primary host, in which the matured fluke lives, is sheep and the intermediate host, in which numerous larval stages are passed, is snail (Lymnaea). As this type of life history involves two different kinds of hosts, is termed digenetic.
In Fasciola hepatica, cross fertilization between two flukes takes place in the bile ductsof the host. Cross-fertilization takes place by insertion of cirrus of one fluke into the Laurer’s canal of another fluke. Fertilization takes place in oviduct. Self-fertilization is rare.
Formation of capsule
Fertilized egg gets about 30 or so yolk- cells and shell globules from vitelline glands by the way of various vitelline ducts. The shell is formed with a lid or operculum on one side.
The zygote and a mass of yolk – cells enveloped by the shell constitute the capsule. When complete the capsule passes into the uterus. Capsule formation occurs throughout the year and one fluke may produce about 50,000 capsules.
Embryonic development (Cleavage)
Cleavage starts immediately after fertilization within the capsule in the uterus leading to a mass of embryonic cells. Encapsulated embryos or capsules do not develop further in fluke’s uterus. Capsules then escape through the gonopores and are liberated into the bile capillaries from where they reach the intestine of the host and are passed out with the faeces. The embryos may survive for few months in wet faecal matter only without any development. Further development occurs when the capsules come in contact with water. In optimum temperature (30 degree C) the encapsulated embryo differentiates into miracidium larva within eight days. The fully formed miracidium is a more or less conical little animal covered with ciliated epithelium
It is the first larval stage. The encapsulated embryo, in 4-15 days, differentiates into a miracidium larva. It hatches out and swim in water. It is a minute, about 0.07 mm long, oval, elongate and richly ciliated active creature. It is a multicellular organisms. Within the miracidium body, are present glands, nervous tissue, protonephridia and germ cells. A pair of large, unicellular penetration glands open near the apical papilla. Miracidium does not feed. During swimming, miracidia tries to penetrate any object or organism, but only those survive that come in contact with a specific intermediate snail host (Lymnaea, Succinea, etc.). Other who can not penetrate the desired hosts, die within 24 hours. For penetration, miracidium attaches its apical papilla and performs boring mechanisms and the larva enters the host’s body through the small opening. It soon reaches the digestive gland of snail, after various changes in about 14 days, develops, in the second larval stage, the sporocyst larva.
The sporocyst is an elongated, hollow,sac-like larva. Its body wall consists of non-cellular cuticle. The brood chamber is hollow space in the centre of the sporocyst within which are found the germ balls that eventually give rise to the next generation. Sporocyst contains no alimentary canal, nervous system or reproductive systems.
The sporocyst moves about by muscular contractions in the host tissues, absorbing nourishment from them and causing them a considerable damage. Its germ balls develop into the next larval form, the rediae. Each sporocyst produces 5-8 rediae.
Redia is a very active little creature. It nourishes itself by sucking fluid and cells of the host tissues with its muscular pharynx. It soon migrates into the digestive glands of the snail. During summer months when the food is sufficiently available, mother rediae mature in 12-18 days, the germ balls give rise to a second generation of daughter rediae. Germ balls of daughter rediae in the winter develop into next larval stage, the cercariae.
Cercariae leave the mother or rediae through their birth pores. From each redia 14-20 cercariae are produced. Cercaria is about 0.25-0.35 mm long and is an oval larva with a tail. On the anterior side, there is a mouth with a sucker, a suctorial pharynx, long oesophagus and a blind intestine. A ventral sucker or acetabulum is present on the ventral side. From the digestive glands of the snail, the cercariae pass into the pulmonary sac and then escape into the surrounding water. Cercaria swims about 5 minutes to an hour in summer. Afterwards, it settles down on some green leaves of water plants. Tail is cast off and the body becomes rounded and forms around itself a thick brownish cyst wall from the cystogenous glands. The encysted cercaria is now called metacercaria.
It is a juvenile fluke having a rounded form with a diameter of about 0.2 mm. It resembles cercaria larva. The metacercaria, encysted on vegetation, may live for as long as a year at low temperature, or for as short time as two or three weeks at 25 degree C.
Infection of primary host
Metacarearia develops into adult fluke only inside its definitive host or sheep. Sheep gets infected during grazing on leaves and grass to which metacarearia cysts are attached. Metacarearia survives from action of host’s gastric juice and the cyst is insoluble in it. Cyst wall finally dissolves in intestine and liberates the larva. It penetrates the wall of the intestine and enters into the coelomic cavity and infects the liver, feeds on the liver tissue and grows in size in 5-6 weeks. In then takes shelter in the bile duct, where it finally attains sexual maturity. In 11-13 weeks after its penetration to the primary host (sheep) the adult flukes start laying eggs (capsules).
Pathogenesis as a direct result of the flukes activities may be either chronic or acute. Acute fascioliasis or liver rot occurs during the preadult migration of the flukes in the parenchyma of the liver, and sometimes other organs, for about 8 weeks. The liver becomes swollen. The by-products produced by worms are very toxic and cause anaemia, diarrhoea, eosinophilia etc.
Chronic fascioliasis occurs beyond 12 weeks, when the flukes have reached the bile ducts and are maturing sexually. Although milk and wool production looses are the usual results of chronic infections. Deaths also occur.
Carbon tetrachloride has been used in sheep at 500 mg/kg. Hexachlorethane was developed specially for use in cattles. Other drugs used are emetine hydrochloride, filcin. tetrachloroethane, etc.
Control of liver flukes is effected in three major ways:
1. Snail control.
2. Treatment of infected sheeps.
3. Herd management through predicting out breaks of disease.