Question

The infective stage of Fasciola hepatica to sheep is
A. Sporocyst
B. Cercaria
C. Metacercaria
D. Hexacanth

Answer 1

Hint:- The common name of Fasciola hepatica is Liver fluke. The stage of life cycle at which the parasite starts to infect its host is known as the infective stage. The hosts of Fasciola hepatica are herbivores such as cows, sheeps and buffaloes. The infective stage of a parasite is the one in which the parasite is competent in entering the host and keeps growing within the host.

Complete step-by-step solution:- Fasciola hepatica is found in the liver of animals. Immature Fasciola eggs are released in the biliary ducts and stool.
- Eggs become embryonated in water and discharge miracidia which attack the appropriate snail host.
- In the snail, the parasites go through several developmental stages such as sporocysts, rediae and cercariae. Sporocysts and Cercariae are the developmental stages.
- The cercariae are discharged from the snail and are encysted as metacercariae on aquatic vegetation. Mammals develop the disorder by eating vegetation that contains the metacercariae.
- Humans can be affected by swallowing metacercariae present in freshwater plants, especially watercress.
- After ingestion, the metacercariae encysts inside the duodenum and moves through the intestinal wall to the greater peritoneal sac and on the way, it attaches to the liver parenchyma present in the biliary ducts where they develop into adult flukes.
- The adult flukes reside within the large biliary ducts of the mammalian host.
According to their life cycle, we can conclude that the infective stage of Fasciola hepatica is metacercaria. So, the correct answer is option (C).

Note:- Metacercaria is the larval stage. In the growth cycle, cercariae are discarded in the water around the snail and the tails are lost when they encyst to form metacercariae. This can infect effortlessly as it has a tough outer cyst wall and can survive for long durations as metacercariae in the liver.