Question

Redia stage is absent in the life cycle of which of the following parasites:
A. Dicrocoelium dendriticum
B. Prosthogonimus ovatus
C. Chlonorchis sinensis
D. Schistosoma incognitum
Choose the correct answer from the options given below:

• A. A, B and C only
• B. B, C and D only
• C. A, B and D only
• D. A and C only

Hint:

Remember that the families Schistosomatidae, Dicrocoeliidae, and Plagiorchiidae (which includes Prosthogonimus) completely lack the redia stage, going straight from sporocyst to cercaria.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Trematodes (flukes) typically exhibit indirect life cycles involving specific snail intermediate hosts.
In classical digenetic flukes, the larval stages develop as follows: Miracidium $\rightarrow$ Sporocyst $\rightarrow$ Redia $\rightarrow$ Cercaria $\rightarrow$ Metacercaria.
However, evolutionary adaptations have led to the deletion of certain larval stages in some families.
Identifying whether a redia stage is present or absent is a key taxonomical marker in veterinary parasitology.

Step 2: Detailed Explanation:
Let us review the developmental life cycles of the four listed trematodes:
1. Dicrocoelium dendriticum (Lancet fluke):
This fluke utilizes terrestrial snails as its first intermediate host.
Inside the snail, the miracidium hatches and develops directly into a first-generation mother sporocyst.
The mother sporocyst buds to produce second-generation daughter sporocysts, which directly yield cercariae.
Thus, the redia stage is completely absent in Dicrocoelium dendriticum.
2. Prosthogonimus ovatus (Poultry oviduct fluke):
This parasite develops in aquatic snails.
The miracidium undergoes development into sporocysts, which subsequently produce cercariae without forming rediae.
Thus, the redia stage is absent in Prosthogonimus ovatus.
3. Chlonorchis sinensis (Chinese liver fluke):
In this fluke, the miracidium develops inside a suitable freshwater snail into a sporocyst.
The sporocyst then gives rise to a redia stage, which further produces cercariae.
Thus, the redia stage is present in the life cycle of Chlonorchis sinensis.
4. Schistosoma incognitum (Blood fluke of domestic animals):
Schistosomes have a distinct life cycle pattern.
The miracidium develops into a mother sporocyst, which produces daughter sporocysts in the snail intermediate host.
The daughter sporocysts directly release fork-tailed cercariae (schistosomules).
There is no redia stage in the genus Schistosoma.
Thus, the redia stage is absent in Schistosoma incognitum.
Combining these observations, the redia stage is absent in parasites A, B, and D.

Step 3: Final Answer:
The redia stage is absent in Dicrocoelium dendriticum, Prosthogonimus ovatus, and Schistosoma incognitum.
Therefore, the correct answer is option (C).