Isolation of a field strain of Brucella abortus from RB51-vaccinated- and brucellosis-seronegative bovine yearlings that calved normally.

A study was carried out in Pichucalco, Chiapas (Mexico) to determine whether recently calved cows or those that aborted shed Brucella. Serological diagnosis of brucellosis was made in all animals (209). Six of the cows that calved normally and two that aborted underwent a bacteriological study of milk and vaginal exudate. Brucella abortus was isolated from vaginal exudate samples in two 3- to 4-year-old seronegative first-birth cows that had calved normally. This was confirmed through bacteriological identification and PCR as a field strain and smooth phenotypes. We conclude that seronegative cows vaccinated with RB51 which calved normally and shed B. abortus in the vaginal exudate after calving could be a serious problem because these cows are overlooked in routine diagnoses and are a source of Brucella infection.