Serologic and bacteriologic test results after adult vaccination with strain 19 in three dairy herds infected with brucellosis.

Milk culture data and serologic test results were evaluated after adult vaccination with Brucella abortus strain 19 in cattle of 3 large California dairy herds infected with brucellosis. Strain-19 organisms were isolated by culture of milk from 1.9% of the vaccinated cows. Isolation of field strain of B abortus varied directly with magnitude of complement-fixation (CF) and rivanol titers. At time of milk culture, 74% of cows from which field strain was isolated had CF titer greater than or equal to 160, compared with 58% of cows from which strain 19 was isolated. Cows with CF titer greater than or equal to 160 at 2 months or greater than or equal to 80 to 4 months after adult vaccination were more likely to be correctly classified as reactors (on the basis of subsequent milk culture results and/or persistently high serologic titer) than were cows with lower CF titer at these times. Cows from which B abortus strain 19 was isolated from milk were more likely to maintain persistent serologic titer than were cows from which neither strain of B abortus was isolated.