Question

Starting from the largest to smallest, arrange the following parasites on the basis of the size of their oocyst
(A) Eimeria maxima
(B) Eimeria leuckarti
(C) Eimeria subspherica
(D) Cryptosporidium parvum
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (B), (C), (A), (D).
• C. (B), (A), (C), (D).
• D. (C), (B), (D), (A).

Hint:

Remember that Eimeria leuckarti of horses is a giant among coccidia ($>$80 $\mu$m), while Cryptosporidium is tiny (4--5 $\mu$m) and is often mistaken for yeast cells due to its small size.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Coccidian parasites produce microscopic egg-like stages called oocysts that are shed in feces.
The physical size of these oocysts is a critical morphological feature used for species identification in diagnostic veterinary parasitology.

Step 2: Detailed Explanation:
Let us compare the average dimensions of the oocysts of each of the specified parasites:
- Eimeria leuckarti (B): This is a unique equine coccidian species characterized by extremely large, dark brown, thick-walled oocysts. They are oval or pyriform and typically measure around 75--88 $\mu$m $\times$ 50--59 $\mu$m. This is by far the largest oocyst among the options.
- Eimeria maxima (A): This is a common pathogen of chickens. It produces relatively large, egg-shaped, yellowish-brown oocysts measuring approximately 30--42 $\mu$m $\times$ 20--30 $\mu$m (with an average size of around 35 $\mu$m).
- Eimeria subspherica (C): This is the smallest species of Eimeria found in cattle. Its small, subspherical oocysts measure only about 9--14 $\mu$m $\times$ 8--13 $\mu$m.
- Cryptosporidium parvum (D): This is a tiny zoonotic protozoan parasite. Its spherical oocysts are extremely small, measuring only 4--5 $\mu$m in diameter.
Arranging these parasites in decreasing order of their oocyst size (from largest to smallest): \[ \text{Eimeria leuckarti (B)} \rightarrow \text{Eimeria maxima (A)} \rightarrow \text{Eimeria subspherica (C)} \rightarrow \text{Cryptosporidium parvum (D)} \] This corresponds to the sequence (B), (A), (C), (D).

Step 3: Final Answer:
The correct descending order is (B), (A), (C), (D), which corresponds to option (C).