We report the evolution of an outbreak of bovine brucellosis (Brucella abortus) in the region of Extremadura (Spain) involving more than 1000 herds and nearly 40,000 animals. S19 vaccination of young cattle combined with a test and slaughter strategy did not result in a rapid decrease in herd prevalence and animal incidence; these parameters showed a constant decreasing trend only when a combination of restriction of cattle movements, increased test frequency, S19 vaccination and mass RB51 vaccination (with yearly revaccinations) were applied to all susceptible populations. These measures were applied for 5 years; abortions following RB51 vaccination of pregnant cows were limited to the first inoculation and the involvement of the vaccine strain could only be demonstrated in 78 out of 897 abortions. Our results demonstrate the usefulness - and lack of significant side effects - of RB51 mass vaccination as a complementary tool to control bovine brucellosis outbreaks in areas where the disease cannot be contained using more conservative approaches.
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