Safety and immunogenicity of Brucella abortus strain RB51 vaccine in pregnant cattle

Objective-To determine the safety and immunogenicity of Brucella abortus strain RB51 as a vaccine in pregnant cattle. Animals-12 Polled Hereford heifers obtained from a brucellosis-free herd and bred on site at 16 months of age to a brucellosis-free bull. Procedure-Pregnant heifers were vaccinated at 6 months' gestation with 10(9) colony-forming units of B abortus strain RB51 (n = 5), 3 x 10(8) colony-forming units of B abortus strain 19 (n = 5), or sterile pyrogen-free saline solution (n = 2). Samples were periodically collected for serologic testing and lymphocyte blastogenesis assays. At full gestation, heifers were euthanatized and specimens were collected for bacteriologic culture, histologic analysis, and lymphocyte blastogenesis assay, using various antigenic stimuli. Results-None of the strain RB51- or strain 19-vaccinates aborted or had gross or microscopic lesions at necropsy that were consistent with brucellosis. Maternal blood mononuclear cells from strain RB51- and strain 19-vaccinates had proliferative responses to gamma-irradiated strain RB51 and strain 19 that were greater than responses by cells from nonvaccinated controls. In contrast, maternal superficial cervical lymph node cells from strain 19-vaccinates had proliferative responses to gamma-irradiated strain RB51 or strain 19 bacteria greater than those of cells from RB51-vaccinates and nonvaccinated controls. None of the heifers vaccinated with strain RB51 developed antibodies detected by use of the standard tube agglutination test, but all developed antibodies to strain RB51 that reacted in a dot ELISA, using irradiated strain RB51 as antigen. Conclusions-Pregnant cattle can be safely vaccinated with strain RB51 without subsequent abortion or placentitis. Furthermore, strain RB51 is immunogenic in pregnant cattle, resulting in humoral and cell-mediated immune responses, but does not interfere with serologic diagnosis of field infections.