Brucella abortus is a facultative intracellular Gram-negative bacterium of the Brucellaceae family that is the causative agent of bovine brucellosis. According to the Food and Agriculture Organization (FAO), World Health Organization (WHO), and World Organization for Animal Health (OIE) reports, bovine brucellosis has been reported worldwide and is still one of the widespread bacterial zoonoses in the world.Brucella abortus is a major cause of abortion in cows, especially in countries without a national control program. Brucellosis reduces fertility and milk production, and is capable of causing abortion storm in susceptible herds. The disease is called undulant fever (Malta fever) in humans, and livestock and their products are considered as the main sources of the infection.Vaccination is one of the main methods for controlling and preventing the disease. In Iran, Recommended doses of IRIBA (RB51) vaccine full dose (FDIRIBA) containing 10-34 × 109 CFU/ml bacterial cells per 2 ml; and a reduced dose (RDIRIBA) containing 1-3.4 ×109 CFU/ml bacterial cells per 2 ml. This vaccine may result in bacteremia in the vaccinated animals. Consequently, the present study was carried out to investigate the possibility and duration of Brucella abortus bacteremia after vaccination. Fifty cows were randomly selected from 500 heads of an imported Simmental fleckvieh adult cows with no history of brucellosis vaccination and/or infection, divided into two equal random groups (n=25), and vaccinated with IRIBA vaccines. The first group received a full dose vaccine and the second one was vaccinated with a reduced dose. Blood samples were taken on days 3, 7, 14, 21, 28, 35, 42, 49, 56 and 64 after vaccination followed by DNA extraction using a commercial kit. DNA samples were then subjected to a Brucella genus-specific PCR assay. The statistical comparison of the two groups was made using chi-square test, and the study of each group in different periods of the experiment was done by the C
