Responses of cattle to two dosages of Brucella abortus strain RB51: serology, clearance and efficacy

A new brucellosis vaccine, Brucella abortus strain RB51 (SRB51), is current ly recommended for use as a calfhood vaccine in the us at dosages between 1 x 10(10) and 3.4 x 10(10) colony-forming units (CFU). The purpose of the s tudy reported here was to compare responses to minimal and maximal recommen ded SRB51 dosages. Eighteen heifer calves were vaccinated subcutaneously wi th 1.6 x 10(10) CFU of SRB51, 3.2 x 10(10) CFU of SRB51, or saline (n = 6 p er treatment). The vaccine strain was recovered from the superficial cervic al lymph node 14 weeks after vaccination in two of six animals that receive d 1.6 x 10(10) CFU SRB51, but not from any cattle vaccinated with 3.2 x 10( 10) CFU SRB51. The higher SRB51 dosage stimulated greater antibody titres. Protection against abortion or infection following B. abortus strain 2308 ( S2308) challenge was similar for both SRB51 dosages and greater than resist ance of non-vaccinates. The vaccine strain was recovered from one heifer an d her fetus at necropsy 1 week prior to estimated parturition. Data from th is study suggests that SRB51 induces similar protective immunity across the recommended dosage range. The SRB51 vaccine may persist in some cattle int o adulthood but the incidence and significance of this persistence remains unknown.