| Sumario: | With the objective of analyzing the behavior of two Brucella abortus rough vaccines, a brucellosis infected herd in which the prevalence of the disease was 8.7% was worked with. The animals were randomly distributed in three groups: Group vaccinated with RB51 had 392 cows; Group vaccinated with rfbK had 237 cows, and non-vaccinated Group had 35 cows. Bacteriological and serological studies were performed in those animals for a period of one year and an experimental longitudinal comparative and prospective epidemiological study was carried out. There were abortions and stillborn calves in the three groups, and Brucella abortus biotype 1 was isolated from the milk of two cows in the control group. Using the criteria that animals vaccinated with rough strains, positive to card test, were infected, the rate of monthly incidence for the vaccinated cows with RB51 was 1.65 per each 100. For the cows that were immunized with rfbK this value was of 2.7, with a significant difference between groups (P < 0.001). If the results are analyzed using the criterion by which, animals that present equal or higher titers to 1:100 remain as reactors, and those that have titers of 1:50 or less present a transitory answer becoming negatives, the rate of monthly incidence for the cows vaccinated with RB51 was of 0.21 per each 100 immunized cows. For the cows that were immunized with rfbK this value was of 0.35, not showing a significant difference between groups (P > 0.01). It is concluded that in vaccinated herds with intermediate prevalence of brucellosis, the presence of post-vaccinated reactors is due to a secondary type response to contact with B. abortus field strains, which does not necessarily indicate infection and can be discerned by using the test of Rivanol; under this criterion, both vaccines confer good protection.
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