Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Inverse age susceptibility is seen in babesiosis in animals, except those infected with Babesia canis
Reason (R) : A well developed complement system is lacking in young ones, due to which parasites can't penetrate the RBCs
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

The complement system is crucial for Babesia entry into erythrocytes. Young calves are clinically resistant to babesiosis because their immature complement system prevents massive RBC parasitemia.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Typically, young animals are more susceptible to infectious diseases due to their naive immune systems.
However, babesiosis exhibits a phenomenon known as "inverse age susceptibility", where young animals (calves, foals) are naturally resistant to clinical disease, while adult animals develop severe, often fatal infections.
Detailed Explanation:
Let us analyze the statements:
- Assertion (A) analysis: Calves (under 6-9 months) are naturally resistant to clinical babesiosis, a classic example of inverse age susceptibility. However, this resistance is not seen in puppies infected with Babesia canis, who can develop severe and acute disease. Thus, Assertion (A) is true.
- Reason (R) analysis: Babesia merozoites rely on host serum factors, specifically components of the complement system, to successfully attach to and penetrate red blood cells (RBCs). Young animals have a physically immature and less developed complement system. Because they lack these necessary complement proteins, the parasite cannot infect their RBCs as efficiently, protecting them from clinical disease. Thus, Reason (R) is true.
- Evaluating the connection: Since the lack of a mature complement system in young animals explains the biological mechanism behind their clinical resistance (inverse age susceptibility), (R) is the correct explanation of (A).

Step 2: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A). This corresponds to Option (A).