Responses of adult cattle to vaccination with a reduced dose of Brucella abortus strain RB51.

The efficacy of a reduced dosage of Brucella abortus strain RB51 (SRB51) was evaluated in adult cattle. Hereford heifers were vaccinated with saline, or three SRB51 dosing regimens (3 x 10(9) colony forming units (cfu) once, 1 x 10(9) cfu once, or 1 x 10(9) cfu twice). Cattle vaccinated with 3 x 10(9) cfu of SRB51 had greater (P < 0.05) antibody responses than non-vaccinates, and greater lymphocyte proliferative responses to SRB51 than all other treatments. Four of six non-vaccinated heifers aborted after mid-gestational challenge with B abortus strain 2308 (S2308) and S2308 was recovered from tissues obtained from all the non-vaccinates. In comparison, S2308 or SRB51 were not recovered from tissues from heifers vaccinated with any of the SRB51 dosage regimens. The data suggest that vaccination with a reduced dosage of SRB51 protects adult cattle against abortion or infection caused by exposure to virulent B abortus during the subsequent pregnancy.