FIELD-STUDY OF VACCINATION OF CATTLE WITH BRUCELLA-ABORTUS STRAINS RB51 AND 19 UNDER HIGH AND LOW DISEASE PREVALENCE

Objective-To assess humoral and protective immunity in cattle vaccinat ed by 12 months with Brucella abortus vaccine strains RB51 and 19 unde r field conditions of high and low brucellosis prevalence. Animals-450 seronegative female cattle: 330 three to eight months old (calves). a nd 120 ten to twelve months old (heifers). Procedures-Ranch A had high prevalence (39%) of brucellosis, and ranch B had low prevalence (2%), as determined by results of conventional serologic testing: agar gel immunodiffusion and the ring test. Seronegative cattle were vaccinated once OF twice with 5 x 10(9) colony-forming units of B abortus strain RB51 or once with strain 19. After vaccinating 285 cattle with strain RB51 and 165 with strain 19, 74 (26%) and 30 (18%), respectively, wer e bred to seropositive bulls, then were kept within the infected herd of origin. Results-All cattle vaccinated with strain 19 seroconverted 30 days later. All 285 cattle vaccinated with strain RB51 had negative results for all serologic tests, including agar gel immunodiffusion. All RB51-vaccinated cattle that became pregnant had negative results f or the ring test and for conventional serologic tests after their firs t carving. Conclusions-Strain RB51 can be used as a live organism vacc ine without inducing antibody titers that interfere with serodiagnosis , and induced 100% protection against field strain B abortus-induced a bortion in cattle vaccinated at least 1 year before mating to an infec ted bull. Vaccination with strain 19 under similar conditions was less effective than vaccination with strain RB51.