Isolation of Brucella organisms from the milk of seronegative cows.

During an investigation of bovine brucellosis in Iran, conducted by the Razi Institute over a twelve-month period, samples of serum and milk were collected simultaneously from 6,472 cows in eight infected herds for serological and bacteriological testing. A total of 1,056 cows were serologically positive and 1,632 of 6,472 milk samples were positive to the milk ring test (MRT). Culture of the positive milk samples yielded 397 isolates of Brucella, 119 of which came from the 5,686 seronegative cows. The isolates belonged to Brucella abortus biotypes 2 (one isolate), 3 (356 isolates) and 9 (40 isolates).