Question

During its life cycle, Fasciola hepatica (Liver fluke) infects its intermediate host and primary host at the following larval stages respectively
A. Metacercaria and cercaria
B. Miracidium and metacercaria
C. Redia and miracidium
D. Cercaria and redia

Answer 1

Hint:- Liver fluke or Fasciola hepatica belongs to phylum Platyhelminthes. These are flat parasitic worms that complete its life cycle in two or three hosts. Fasciola hepatica is known to cause Fasciolosis. The life cycle of Fasciola hepatica involves more than one larval stage. The larvae at its different stages of development infect the primary and intermediate host of the parasite.

Complete step-by-step solution:-
The life cycle of the platyhelminth parasite Fasciola hepatica is completed in a primary host and an intermediate host. The primary or definitive hosts include cattle, sheep, ruminants as well as human beings. Intermediate hosts are generally snails.
In order to understand the infective larval stages of Fasciola hepatica, we must first go through the larval stages found in trematodes in general.
The life cycle of trematode parasites involves a number of larval stages. They can be described as the following:
Miracidium – The eggs of the trematodes hatch to give birth to the miracidium larvae. These cannot feed because they lack mouth. Miracidium needs to find the intermediate host of the worm quickly and infect the same.
Redia – The miracidium is enclosed by a sporocyst once inside the intermediate host. In the sporocyst, the first developmental form of the larva is formed. It is known as Redia.
Cercaria – This stage is formed within the intermediate host. The Redia develop into cercaria by forming well-developed penetration glands and a tail. Cercaria larvae that are encysted are known as metacercaria. They are found in a resting stage on aquatic vegetation. When the infected aquatic vegetation is consumed by the primary or definitive hosts like humans or cattle or sheep, they are in turn infected.
The metacercaria develop into adult worms in the primary hosts. They infect the liver and intestine. The eggs of the worms are passed out into the environment through faeces. The eggs then become embryonated and hatch in the form of miracidium larvae to continue their life cycle.
Therefore, it can be concluded that the larval stage of Fasciola hepatica that infects intermediate hosts is the miracidium and the definitive or primary host is metacercaria.
Hence, option (b) is correct.

Note:- Liver fluke or Fasciola hepatica is a trematode parasite that belongs to phylum Platyhelminthes. These worms are known causative agents of the disease Fasciolosis. The life cycle of Liver fluke comprises two hosts and a number of larval forms. Fasciolosis is characterized by fever, anemia, abdominal pain, jaundice. The cure to Fasciolosis is mediated through drugs that are easily available at present.