Question

Which of the following protozoa has a cystic stage in its life-cycle?

• A. Tritrichomonas foetus
• B. Histomonas meleagridis
• C. Tetratrichomonas gallinarum
• D. Giardia intestinalis

Hint:

Remember that all members of the family Trichomonadidae (such as Tritrichomonas and Tetratrichomonas) and Histomonas meleagridis lack a cystic stage.
Giardia is the classic example of a flagellate with a highly resistant, infective cystic stage.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Many protozoan parasites exhibit a life cycle consisting of two main developmental stages: the active, motile, feeding stage known as the trophozoite, and the dormant, environmentally resistant, infective stage known as the cyst.
Certain flagellated protozoa do not produce a cystic stage and are transmitted solely via their trophozoite forms.

Step 2: Detailed Explanation:
Let us evaluate each protozoan listed:
- Tritrichomonas foetus: This venereal parasite of cattle exists only in the trophozoite form.
It does not form a protective cyst in its life cycle.
- Histomonas meleagridis: The causative agent of blackhead in poultry exists in flagellated and amoebic trophozoite forms, but does not produce true cysts.
Instead, it utilizes the eggs of the cecal worm, Heterakis gallinarum, to survive in the environment.
- Tetratrichomonas gallinarum: Similar to other trichomonads of birds, this flagellate does not produce a true cystic stage.
- Giardia intestinalis (also known as G. duodenalis or G. lamblia): This enteric parasite possesses a classic, well-defined life cycle consisting of flagellated trophozoites that colonize the small intestine, and thick-walled, environmentally stable infective cysts excreted in feces.

Step 3: Final Answer:
Therefore, only Giardia intestinalis has a true cystic stage in its life cycle, corresponding to option (D).