| Abstrakt: |
Yearling beef heifers were vaccinated with 2 doses of an experimental Brucella abortus S45/20 bacterin or a reduced dose of strain 19 (S19) vaccine, or were used as nonvaccinated controls. None of the vaccinated heifers was positive by the acidified plate, Rivanol, complement-fixation, or card tests 5 months after inoculation. Sixteen of 21 heifers given S45/20 bacterin with trehalose dimycolate adjuvant had delayed hypersensitivity when skin was tested 7 months after initial vaccination, whereas none of the S19 vaccinates or controls was reactive. After natural breeding, pregnant heifers were challenge exposed at 3.5 to 5 months of gestation with 2.5 X 10(7) viable B abortus S2308 organisms in the conjunctival sac. Abortions occurred as follows: 16/19 (84.2%) of heifers given S45/20 bacterin, 6/12 (50.0%) of heifers vaccinated with S19, and 17/18 (94.4%) of the nonvaccinated controls. Of the 10 heifers that calved normally, only 3 (1 in each group) were seropositive. A correlation was not seen between skin hypersensitivity and acquired resistance. Possible reasons for the poor protection with a reduced dose of S19 vaccine and the failure of S45/20 bacterin to enhance cellular resistance are discussed. |
