The role of living vaccines in prophylaxis

Although strain 19 vaccine has been widely recognized as the most effective vaccine for the control of bovine brucellosis, its use in the United States has declined in the last ten years. When the national level of infection was reduced to 1%, it was believed possible to move towards complete eradication of bovine brucellosis. The continued use of strain 19 vaccine was thought to interfere with the goal of complete eradication because: 1. persistence of serum agglutination titers in some animals caused confusion in the serological diagnosis, 2. occasional persistence of strain 19 infection in adult animals caused reactions in surveillance tests which required follow-up herd tests, 3. vaccination was reported to "mask" virulent infection in a herd by prolonging the incubation period. These perceived disadvantages of strain 19 are discussed in relation to the benefits derived from its use in infected areas. In the last ten years the number of calves vaccinated with strain 19 has steadily declined, whereas the national average of infected cattle has remained stationary. The level of infection in some areas has increased however. The date for complete eradication of bovine brucellosis in the United States has had to be postponed. A retrospective examination of the brucellosis eradication program shows that in general those states which vaccinated 40% or more of their eligible calves with strain 19 were able to eradicate brucellosis. States which still have a bovine brucellosis problem, in general, have never employed wide-scale vaccination. Other factors contributing to these differences in the progress of eradication will be discussed. The important role of strain 19 in the successful eradication of bovine brucellosis in some areas of the United States has been the reduction in the level of infection to the point where eradication was economically feasible.