Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Vaccination of bulls with Brucella abortus S19 is of no value in protecting them against brucellosis.
Reason (R): Residual vaccine titre is a major disadvantage of adulthood vaccination against brucellosis.
In light of the above statements, choose the most appropriate answer from the options given below:

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

Hint:

Remember that the Brucella S19 vaccine is strictly for female calves. It is contraindicated in male animals because it can cause permanent infertility due to orchitis.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Brucellosis is a major zoonotic bacterial disease affecting livestock.
The Brucella abortus Strain 19 (S19) vaccine is widely used to control the disease, but its application is restricted based on the sex and age of the animals.

Step 2: Detailed Explanation:
Let us analyze both statements:
- Assertion (A): Vaccination of bull calves or adult bulls with the S19 vaccine is not recommended and is of no value. In male animals, the vaccine can cause localization of the bacteria in the testes and accessory sex glands, leading to vaccine-induced orchitis, epididymitis, and permanent infertility. Furthermore, vaccinated bulls can shed the vaccine strain in their semen, posing a transmission risk. Thus, Assertion (A) is correct.
- Reason (R): When female animals are vaccinated as adults (rather than during calfhood, which is ideally between 4 and 8 months of age), they develop persistent, long-lasting antibody titers. These "residual vaccine titers" interfere with diagnostic serological tests, making it difficult to distinguish between vaccinated and naturally infected animals. This is a major disadvantage of adult vaccination. Thus, Reason (R) is correct.
- Causal Link: While both statements are true, the reason why S19 is not used in bulls is due to the risk of orchitis and infertility, not the issue of residual antibody titers. Therefore, (R) is not the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are correct, but (R) is not the correct explanation of (A), which corresponds to option (B).