The RB51 vaccine increases false-positives in cattle serology for Brucellosis.

• A. It has no effect on serology
• B. It increases true positives
• C. True
• D. False

Answer: (D)

The main idea is how serologic tests for Brucella detect antibodies. Standard tests look for antibodies against the smooth LPS O-antigen that is present on many Brucella strains. The RB51 vaccine is a rough strain that lacks this O-antigen, so animals vaccinated with RB51 do not produce antibodies that these tests recognize. As a result, RB51 vaccination does not create positive serology results and actually reduces false-positive reactions compared with vaccines that expose the O-antigen. Therefore, saying the RB51 vaccine increases false-positives is not correct. In practice, RB51 is used to avoid serologic interference and keep vaccination from being mistaken for infection, though occasional non-vaccine-related cross-reactions can occur with other antigens, they are not due to the RB51 vaccine itself.