Protection against infection and abortion induced by virulent challenge exposure after oral vaccination of cattle with Brucella abortus strain RB51

Objectives-To determine efficacy of orally administered Brucella abortus va ccine strain RB51 against virulent B abortus challenge exposure in cattle a s a model for vaccination of wild ungulates. Animals-20 mixed-breed beef cattle obtained from a brucellosis-free herd. Procedure-Sexually mature, Brucella-negative beef heifers were vaccinated b y mixing > 10(10) viable RB51 organisms or, diluent with their feed. Heifer s were fed individually and consumed their entire ration. Each heifer recei ved approximately 3 X 10(10) colony-forming units (CFU). Six weeks after or al vaccination, heifers were pasture-bred to brucellosis-free bulls. At app roximately 186 days' gestation, heifers were challenge exposed conjunctivel y with 10(7) CFU of virulent B abortus strain 2308. Results-Vaccination with the rough variant of B abortus RB51 did not stimul ate antibodies against the O-polysaccharide (OPS) of B abortus. After chall enge exposure and parturition, strain 2308 was recovered from 80% of contro ls and only 20% of vaccinates. Only 30% of the vaccinates delivered dead, p remature, or weak calves, whereas 70% of the controls had dead or weak carv es. Conclusions-Cattle vaccinated orally with the rough variant of B abortus st rain RB51 develop significant (P < 0.05) protection against abortion and co lonization and do not produce OPS-specific antibodies. Clinical Relevance-Results encourage further investigation into use of stra in RB51 to vaccinate wild ungulates (elk and bison) orally.