Which parasite is responsible for causing gigantism in its intermediate host?
(a) Plasmodium vivax
(b) Ascaris lumbricoides
(c) Sacculina
(d) Fasciola hepatica

Hint: The parasite responsible for causing gigantism in its intermediate host completes its life cycle in two hosts where the primary hosts are vertebrates while the intermediate host is an invertebrate.

Complete step by step answer:
Fasciola hepatica or liver fluke is a platyhelminth that is found in the liver and bile duct of its primary hosts i.e sheep and goat. The secondary or intermediate hosts is a pond snail that may belong to the genus Limnaea and Planorbis. Liver fluke causes gigantism in snails i.e increased size of an infected snail with respect to its non- infected counterparts. The liver fluke undergoes five developmental stages as larvae before attaining maturity.
- The adult liver fluke in its primary host sexually reproduces and the zygote gives rise to the first larva which is referred to as Miracidium.
- Miracidium enters the snail and changes into the second larva ‘sporocyst.’ It undergoes various developmental changes with the production of the third larva Rediae, followed by the fourth larva Cercariae, and finally leaves its intermediate host.
- In the outer environment, the fourth larva changes into Metacercariae and gets attached to the water weeds.
- These water weeds are eaten by goats and sheep and thus the fifth larva enters their primary host. These larvae grow into a young fluke.
- This young fluke attains sexual maturity and starts laying eggs in about 11- 13 weeks after its entry into the vertebrate host.
- Plasmodium vivax is a protozoan carried by mosquitoes and causing malaria in humans. Ascaris lumbricoid or roundworm is an endoparasite in the small intestines of children. Sacculina is a crustacean which is a class under the phylum Arthropoda. It lives as an ectoparasite on crabs.
So, the correct answer is ‘Fasciola hepatica.’

Note:
- The symptoms of liver rot or acute fascioliasis appear about one month after the infection. It includes jaundice, anemia and in severe cases, it may lead to death.
- Drugs like carbon tetrachloride and tetrachloroethane are given and are fairly effective in treating the cases of liver rot.