Liver Flukes: History, Habits and Habitat (With Diagram)

In this article we will discuss about:- 1. History of Liver Flukes 2. Systematic Position Liver Flukes 3. Habits and Habitat 4. Digenetic Life Cycle.

History of Liver Flukes:

Amongst the trematodes, this was the first to be discovered by Jehan de Brie in 1379. Fasciola hepatica, the sheep liver fluke was the first digenetic trematode, whose life history has been exten­sively studied and reviewed by Pantelouris (1965), Boray (1969), Kendall (1970), Smith (1981), Malone, Wescott and Foreyt (1986).

Liver fluke is cosmopolitan in distribution. Fasciola hepatica is the common liver-fluke of sheep. Fasciola gigantica (F. indica), the liver fluke of cattle is however a more familiar species in India. The Chinese liver fluke is the causa­tive organism of Clonorchiasis in human beings in Oriental countries like Japan, Korea, South­eastern China and Vietnam.

Liver fluke is of great parasitological and pathological impor­tance as it is the causative organism of fascioliasis, a disease that causes immense damage to the liver tissues and bile ducts of sheep and cattle.

Several attempts have been made in the past to create new species in this genus. The two principal species are F. hepatica and F. gigantica which represent the two opposite ends of a range of forms. The new species which have been created at various times include F. indica in the Indian subcontinent and F. halli and F californica in the U.S.A.

Systematic Position Liver Flukes:

Phylum – Platyhelminthes

Class – Trematoda

Subclass – Digenea

Genus – Fasciola

Species – hepatica

Common name:

The sheep liver fluke.

Habits and Habitat of Liver Flukes:

F. hepatica Linnaeus (1758), is an endoparasite which completes its life cycle in two hosts. Adult F hepatica is known as the sheep liver fluke because it is found attached to the inner wall of bile ducts and in the liver of sheep, cow etc., the definitive hosts. It may also occur in some other vertebrates like goat, pig, hare, rabbit, beaver, coypu, elephant, horse, cat, dog, monkey, man etc.

Man is not usu­ally considered to be a host of F. hepatica but, in-fact, this infection is not unusual in humans and infec­tions have been reported in many countries inclu­ding Europe and USA. The eating of water-cress is found to be a common source of human infection. The usual site of infec­tion is the liver but in aberrant hosts (i.e. man) other sites, such as the lung, may be involved. As Fasciola is observed in liver it is called liver fluke and causes a disease called liver-rot.

F. hepatica spends a part of its life history in an intermediate host, a fresh water snail, which is either Lymnaea truncatula or some specific species of Planorbis or Bulinus. But in countries, where Fasciola has been introduced in infected stock, such as North-America, Canada, Australia and some Pacific Islands, a new parasitic relationship appears to have been established with indigenous snail populations. In China, species of Radix serves as intermediate hosts.

Digenetic Life Cycle of Liver Flukes:

Fasciola is digenetic, i.e., the life cycle is com­pleted in two alternating hosts. One is the primary or definitive host, the sheep or cow and the other is the secondary or intermediate host, the fresh water snail of genus Lymnaea, This kind of life cycle, involving two different types of hosts, is para­sitic termed as digenetic.

One is the primary host, the sheep or cow and the other is the intermediate host, the snail of genus Lymnaea. Many species of Lymnaea are susceptible to some degree but only when young. In China, species of Radix serve as intermediate hosts.

A. In Primary host (sheep):

1. Copulation and fertilization:

In liver-fluke self-fertilization is of rare occurrence though they are hermaphrodite. Cross fertilization preceded by copulation is of normal occurrence. In F. hepatica copula­tion takes place in bile ducts of the defi­nitive host. During copulation two liver flukes bring their genital pores in oppo­sition.

Within the body of the host the cirrus of one is introduced in the opening of Laurer’s canal and the sperms migrate from here to uterus. Fertilization is internal. In cross fertilization sperms received in Laurer’s canal during copula­tion, enter the distal end of oviduct where the eggs from ovary reach and finally fertilization occurs there. During self-fertilization sperms from the same fluke enter the uterus through the female genital pore to fertilize the eggs in the distal part of the oviduct.

2. Capsule formation:

The fertilized eggs get covered by yolk of vitelline glands and chitinous secretion of shell glands form the shell around the eggs. It is operculate i.e., provided with a lid or operculum. The covering of the egg or capsule has an opening covered by operculum. About 3,000 or more such capsules may occur at a time in the uterus of a single fluke. In the liver of one sheep there may be as many as 200 flukes.

3. Cleavage and early development:

Segmen­tation of the fertilized egg (i.e., cleavage) starts while it is still inside the uterus of the parent. The fertilized egg by com­plete and unequal division forms a large somatic cell and a small propagatory cell.

By repeated divisions of the somatic cell forms the larval ectoderm and tissues. Propagatory cell divides further into 2 daughter cells. One daughter cell by its divisions forms the larval body and the other daughter cell divides several times to produce a mass of germ cells which cluster in the posterior part of larval body.

So long as the eggs remain in the body of the sheep, they do not undergo any further development. Eggs then pass through bile ducts, reach the intestine and are deposited on soil with faeces of the host (Fig. 8.3).

B. Development in water: Miracidium larva:

This is the first larva formed in the life cycle of Fasciala. Under favourable circumstances, the ciliated larva, Miracidium is produced within 9 to 20 days and comes out through the opening of egg-capsule in water. It is a free-swimming, ciliated larva with broader anterior end which is provided with a conical projection called apical lobe or apical papilla. The posterior end is narrow and slightly coni­cal. The entire body, except the apical papilla, is covered with cilia by means of which it swims actively (Fig. 8.4).

The ectoderm is formed of 5 rows of closely fitting hexagonal cells (6 + 6 + 3 + 4 + 2) below which is the muscular layer. It has a brain, a sense organ in the form of a double eye-spot arranged like ‘X’, a pair of excretory organs or protonephridia and a group of germ cells.

Miracidium larva swims freely in search of the intermediate host, Lymnaea. The life of a ha led Miracidium is only about 24 hrs. If it fails to out the host, it dies. If it comes in contact with he specific host (Lymnaea), it penetrates the host’s soft-body and reaches the pulmonary chamber.

According to Ulmer, 1977, the Miracidium larva is attached by the mucous secreted by the host and attaches itself to the body of the snail by apical papilla. By the secretion of the penetration gland it enters into the body of host and by gradual cytolysis it reaches the pulmonary chamber of the snail. From here it comes to the digestive gland and throws off the cilia and changes to the second lar­val stage called Sporocyst larva.