Question

Which one of the following life cycle stages of liver fluke is infective to the intermediate host?
(a) Metacercaria
(b) Miracidia
(c) Sporocyst
(d) Cercaria

Answer 1

Hint: These are non-feeding, free-swimming ciliated larval stages in which the parasitic fluke passes from the egg to its intermediate host, typically a snail.

Correct step by step answer:
The lifecycle of liver fluke is complex and involves an intermediate host, the snail. Miracidium is the larval stage of liver fluke, it is released into freshwater when eggs are hatched. This miracidium infects the snail present in that freshwater. During favorable temperature, a miracidium develops within the egg, hatches, and then migrates in thin films of moisture, actively seeking the snail host. Outside the snail, a miracidium can only survive for a few hours.
- Liver fluke disease known as fasciolosis is caused by the trematode parasite Fasciola hepatica.
- This disease can result only when large numbers of immature flukes migrate through the liver, or due to the presence of adult flukes in the bile ducts, or by both.
- Within the snail, the miracidia further undergoes two developmental stages, eventually becoming infective cercariae. When the temperature and moisture levels are suitable, these cercariae emerge from the snail.
- The cercariae migrate onto wet herbage, encysting as metacercariae, which is the highly resilient infective stage of the liver fluke. Following ingestion, the young flukes migrate to the liver, causing considerable tissue damage.
- The infection will remain about 10- 12 weeks after the metacercariae are ingested. The whole cycle takes 18- 20 weeks to complete. Adult fluke lay eggs that are passed out onto pasture in the feces.
So, the correct answer is, '(b) Miracidia'.

Note:
- The liver fluke can infect all grazing animals and men but mainly affects sheep and cattle. It is most pathogenic in sheep.
- The cases of fasciolosis are highest in years when rainfall is above average during the month of May, June, and July.
- The hatching of fluke eggs and the multiplication of snails depends on adequate moisture and temperatures greater than 10ºC.