Field validation of the use of RB51 as antigen in a complement fixation test to identify calves vaccinated with Brucella abortus RB51

In order to confirm the efficiency of an experimental RB51-based complement fixation (CF) test in identifying cattle vaccinated with Brucella abortus strain RB51, 831 sera from 110 vaccinated and 48 unvaccinated Hereford heif ers of Iowa, collected for studies conducted in different years, were sent to Italy without coding to be tested in a CF test using RB51 as antigen. Mo st of the calves, aged from 3 to 10 months, were vaccinated subcutaneously with the recommended dosage of 10(10) CFU of RB51 commercial vaccine, while only six calves received 10(9) CFU of the same vaccine. Serum samples for serologic testing, collected until 16 postinoculation weeks (PIW), were als o tested by routine surveillance tests for brucellosis such as rose bengal plate and CF tests performed, with B. abortus smooth strain 99 as control a ntigen. RB51 CF test results obtained by testing sera from cattle vaccinate d in 1999 indicate that the sensitivity of the reaction is 97% at 2 to 3 PI W and 90% until 8 PIW and decreases to 65% at 12 PIW, the specificity remai ning at 100%, Collectively, the results of this study confirm that serologi c standard tests fail to detect antibodies to RB51 while the RB51-based CF test is able to monitor antibody responses to RB51 until 15 to 16 PIW with a specificity of 100%. In addition, unlike the RB51-based dot blot assay, w hich is the only test currently used to monitor antibody responses to RB51, the CF test also detected specific responses following vaccination with 10 9 CFU of RB51, although seroconversion was only 50% at 8 PIW, In conclusion , because of high specificity and sensitivity, the CF test described here c an be used to efficaciously monitor serologic responses following RB51 vacc ination in cattle and could also be employed to detect RB51 infection in hu mans exposed to this strain.