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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The authors are grateful to the Spanish Ministry of Environment, Rural and Marine Affairs and to the European Commission for their unconditional support, and to the National Reference Laboratories in Santa Fe (Granada) and Algete (Madrid), for their collaboration. They are also grateful to the veterinary staff of the Animal Health Service (Consejería de Agricultura y Desarrollo Rural, Junta de Extremadura), and especially to the staff of the Local Veterinary Units of Coria and Plasencia. Thanks also to the personnel of TRAGSEGA involved in the fieldwork. Kind help on text proofing and valuable suggestions by Dr. Emir Hodzic at the University of California, Davis, and on figure editing by Mr. Sergio Gonzalez at VISAVET (UCM) is greatly acknowledged. Finally, authors are indebted to the editor and referees who have participated in the revision of the manuscript for their thorough work.