Effect of vaccination with Brucella abortus strain RB51 on heifers and pregnant cattle

Thirteen cows, which had been vaccinated as calves with strain 19, were rev accinated twice or three times as adults with 1 x 10(9) cfu of B. abortus s train RB51. Their serological responses following adult vaccination remaine d negative to conventional brucellosis surveillance tests. Vaccination with strain RB51 during the eighth month of pregnancy did not induce abortion, although strain RB51 was recovered from milk for up to 69 days after vaccin ation. In a parallel study, thirteen 8- to 10-month-old heifers were vaccin ated as calves with 10(9) cfu of strain RB51. The heifers remained seronega tive to conventional brucellosis surveillance tests but antibody responses to RB51 could be demonstrated using an indirect ELISA. This study showed th at multiple vaccination with strain RB51 did not induce seroconversion to b rucellosis surveillance tests. In addition, this study suggests that 10(9) cfu of strain RB51 is safe for use in pregnant cattle.