Question

Explain the lifecycle of a typical Trematode fluke, Fasciola heptatica. What happens during the Fasciola lifecycle if the larvae get into the wrong host?

Answer 1

Fasciola hepatica has two hosts and life cycle is completed within these hosts. The primary host is sheep, and the intermediate host is a snail. The liver fluke undergoes copulation in the bile duct of sheep. In liver fluke, self as well as cross fertilization takes place by copulation. Typical life cycle stages of Fasciola hepatica:

Egg: Discharged either in open water or in intestine of definitive host.

Miracidium: A free-living motile form, covered with cilia, and settles in the mollusc to become a sporocyst.

Sporocyst: It is an elongated sac and produces either rediae or more sporocysts.

Redia: A larval form with an oral sucker, it produces either more rediae, or cercariae.

Cercaria: It is a larval form of the parasite that develops within the germinal cells of the sporocyst or redia. A cercaria has a tapering head with large penetration glands. The motile cercaria finds and settles in a host where it will become either an adult, or a mesocercaria, or a metacercaria, according to species.

Mesocercaria: A cercaria little modified but resting.

Metacercaria: A cercaria encysted and resting.

When the sheep eats the plants with metacercaria stages, they enter into its digestive system. When the cyst wall is digested in intestine, it penetrates through the intestine wall and reaches the liver. It takes six weeks to grow into adult and 12 weeks to attain sexual maturity. Thus Fasciola hepatica completes its life cycle.

When it enters the wrong host, then it won't be able to carry out the larval stage and eventually won't be able to grow and increase their population.