Summary
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 ≥ 160, compared with 58% of cows from which strain 19 was isolated. Cows with cf titer ≥160 at 2 months or ≥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.
