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 heifers 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. Most 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 also tested by routine surveillance tests for brucellosis such as rose bengal plate and CF tests performed with B. abortus smooth strain 99 as control antigen. RB51 CF test results obtained by testing sera from cattle vaccinated in 1999 indicate that the sensitivity of the reaction is 97% at 2 to 3 PIW and 90% until 8 PIW and decreases to 65% at 12 PIW, the specificity remaining at 100%. Collectively, the results of this study confirm that serologic 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, which 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 can be used to efficaciously monitor serologic responses following RB51 vaccination in cattle and could also be employed to detect RB51 infection in humans exposed to this strain.

为验证基于RB51株的补体结合试验（Complement Fixation, CF）在鉴定接种流产布鲁氏菌（Brucella abortus）RB51株疫苗的牛只时的有效性，本研究收集了来自艾奥瓦州110头接种疫苗、48头未接种疫苗的赫里福德小母牛的共831份血清样本。这些样本采集自不同年份的相关研究，未进行编码便送往意大利，以RB51作为抗原开展补体结合试验检测。多数受试犊牛的月龄为3至10月龄，以推荐剂量的10^10 CFU的RB51商品化疫苗进行皮下接种，仅6头犊牛接种了10^9 CFU的同款疫苗。截至接种后16周（postinoculation weeks, PIW）采集的用于血清学检测的血清样本，同时采用布鲁氏菌病常规监测试验进行检测，包括以流产布鲁氏菌光滑菌株99作为对照抗原的虎红平板凝集试验（rose bengal plate test）与补体结合试验。对1999年接种疫苗的牛只血清开展RB51补体结合试验的结果显示，该检测方法在接种后2至3周的敏感性为97%，在接种后8周前的敏感性维持在90%，至接种后12周时敏感性降至65%，而特异性始终保持为100%。综合来看，本研究结果证实，常规血清学检测无法检出针对RB51株的抗体，而基于RB51株的补体结合试验可在接种后15至16周内有效监测针对RB51株的抗体应答，且特异性达100%。此外，与当前唯一用于监测RB51株抗体应答的RB51斑点印迹试验（dot blot assay）不同，该补体结合试验还可检出接种10^9 CFU RB51疫苗后的特异性应答，尽管在接种后8周时的血清阳转率仅为50%。综上，凭借优异的特异性与敏感性，本研究所述的补体结合试验可有效监测牛只接种RB51疫苗后的血清学应答，同时也可用于检测接触该菌株的人类的RB51感染情况。