Epidemiological study of Brucellosis in cattle, immunized with Brucella abortus RB51 vaccine in endemic zones.

In this study the behavior of the Brucella abortus RB51 vaccine was evaluated in bovine herds, with different prevalence of Brucellosis. A prospective longitudinal study was made, in two dairies, one of low prevalence (9%) with 538 cows, and the other of high prevalence (15%) with 612 cows. The cattle were vaccinated twice 90 days apart with RB51 at a dose of 1×10(9)cfu/ml. The monthly incidence was determined during 660 days of observation. In the low prevalence dairy, all positive animals were eliminated as soon as they were diagnosed as positive and in this herd the number of new cases decreased to less than 1% between days 120, and day 660. In the dairy with high prevalence, positive cows were not eliminate resulting in the herd increasing its incidence by the end of the first year. Once positive animals were eliminated the incidence diminishes by day 660 to less of 1%. The odds ratio (OR) in the group of cows with abortion history, in the low prevalence dairy, was of 4.5 (1.2; 16.6), in the dairy ranch with high prevalence it presented an OR of 3.6 (1.5; 8.5). The conclusion from this study was that in brucellosis endemic zones, vaccination with RB51 by itself is not enough to control disease. It is mandatory that the initial elimination of all positive cows at the time of vaccination, the continued elimination of all new positive animals be adhered to for long periods of time.