Immune responses and efficacy after administration of a commercial Brucella abortus strain RB51 vaccine to cattle.

Brucella abortus strain RB51 (SRB51) is a newly approved live vaccine to protect cattle against brucellosis. The purpose of this study was to evaluate the immunologic responses of cattle to a commercially available SRB51 vaccine and to characterize the efficacy of the vaccine to protect against abortion or infection after midgestational challenge with virulent B. abortus strain 2308 (S2308). All cattle were pasture bred, and pregnancy was confirmed by rectal palpation. Pregnant cattle were intraconjunctivally challenged with 1 x 10(7) colony-forming units (CFUs) of S2308 at 180 days gestation. Serologic responses were monitored in all heifers after vaccination using the standard tube agglutination test and a dot-blot assay using killed SRB51 as antigen. In one study, 3-month-old Hereford heifers were subcutaneously inoculated with 10(9) CFU or 10(10) CFU of a commercially available SRB51 vaccine or 2 mL of 0.15M sodium chloride (saline). In this study, four of eight nonvaccinates, two of four 10(9) CFU SRB51 vaccinates, and two of 14 10(10) CFU SRB51 vaccinates aborted after midgestational challenge with virulent S2308. The challenge strain was recovered at necropsy from maternal or fetal tissue from six of nine nonvaccinates, four of four 10(9) CFU SRB51 vaccinates, and seven of 14 heifers vaccinated with 10(10) CFU of SRB51. In a separate study, 6-month-old Hereford heifers were subcutaneously inoculated with 10(10) CFU of SRB51 or saline. Peripheral blood mononuclear cells proliferative responses to gamma-irradiated SRB51 were monitored. In this study, three of seven nonvaccinates and one of 18 10(10) CFU SRB51 vaccinates aborted. The S2308 challenge strain was recovered from maternal or fetal tissue of five of seven nonvaccinates and seven of 18 SRB51 vaccinates. In both studies, cattle vaccinated with 10(10) CFU of SRB51 had greater (P < .05) antibody responses to SRB51 at 4 and 8 weeks after vaccination than nonvaccinates. SRB51-vaccinated cattle had greater lymphocyte proliferative responses to killed SRB51 at 10, 12, 14, and 16 weeks when compared with nonvaccinates. The data presented in this study indicate that 10(10) CFU of the commercial SRB51 vaccine was highly efficacious (P < .02) in preventing Brucella-induced abortion or fetal infection. Considering that fetal infection and abortion are the predominant mechanisms for transmission of brucellosis, our data suggest that the commercially available SRB51 vaccine will be efficacious in preventing abortion and controlling brucellosis in cattle under field conditions. However, incidence of maternal infection with Brucella at necropsy did not differ (P > .05) among 10(10) CFU SRB51 vaccinates and nonvaccinates.