Brucella abortus strain RB51 vaccination in elk. II. Failure of high dosage to prevent abortion.

Brucella abortus strain RB51 is used as a vaccine because it induces antibodies that do not react on standard serologic tests for brucellosis allowing differentiation between vaccination and infection. Strain RB51 was evaluated in captive elk (Cervus elaphus) to determine if vaccination protected against abortion following experimental challenge. Thirty elk were vaccinated intramuscularly with 1.0 x 10(10) colony-forming units (CFU) of strain RB51 in March 1998. Fourteen of these were given a booster dose of 1.13 x 10(10) CFU exactly 1 yr later. All vaccinated elk seroconverted via a modified dot blot assay to strain RB51 with the booster group having higher titers (P < or = 0.001). Seventeen other elk served as unvaccinated controls. All elk were bred and determined pregnant using pregnancy-specific protein B analysis. Elk were challenged in March 2000 with 1.1 x 10(7) CFU of B. abortus strain 2308 administered intraconjunctivally and all elk seroconverted to strain 2308. Fifteen of 17 control elk aborted; 16 of 16 elk given a single vaccination aborted (P = 0.44); and 13 of 14 elk given a booster aborted (P = 0.86). There were two viable calves in the control group and one in the booster group. Strain 2308 was recovered from fetuses and nonviable calves in all groups. Based on the results of this and other studies, the use of strain RB51 to prevent abortion in elk cannot be recommended.