BRUCELLA ABORTUSSTRAIN RB51 VACCINATION IN ELK I. EFFICACY OF REDUCED DOSAGE

Bovine brucellosis is a serious zoonotic disease affecting some populations of Rocky Mountain elk (Cervus elaphus nelsoni) and bison (Bison bison) in the Greater Yellowstone Area, USA. The fear that elk and/or bison may spread Brucella abortusto livestock has prompted efforts to reduce or eliminate the disease in wildlife. Brucella abortusstrain RB51 (RB51) vaccine has recently been approved for use in cattle. Unlike strain 19 vaccine, RB51 does not cause false positive reactions on standard brucellosis serologic tests. If effective, it may become the vaccine of choice for wildlife. In February 1995, 45 serologically negative female elk calves were trapped and taken to the Sybille Wildlife Research and Conservation Education Unit near Wheatland, Wyoming, USA. In May 1995, 16 of these elk calves were hand-vaccinated with 1 × 10⁹ colony forming units (CFU) of RB51, 16 were vaccinated with 1 × 10⁸ CFU RB51 by biobullet, and 13 were given a saline placebo. The elk were bred in fall of 1996 and they were challenged with 1 × 10⁷ CFU of B. abortusstrain 2308 by intraconjunctival inoculation in March 1997. Thirteen (100%) control elk aborted, 14 (88%) hand-vaccinated elk aborted, and 12 (75%) biobullet vaccinated elk aborted or produced nonviable calves. These results suggest that a single dose of 1 × 10⁸ to 1 × 10⁹ CFU RB51 does not provide significant protection against B. abortusinduced abortion in elk. However, the vaccine appears to be safe at this dose and additional study may reveal a more effective RB51 vaccine regimen for elk.