Serological findings obtained in cattle herds immunised with the Brucella melitensis Rev.1 and the B. abortus B19 vaccine in Mongolia.

To reduce the economic losses caused by Brucella abortus infection under the conditions of nomadic cattle breeding, more than half million cattle over 3 months of age (excluding male animals) were immunised with the B. melitensis Rev.1 vaccine in 7 provinces of Mongolia in the first year of a programme (1987). In the second year, only heifer calves over 3 months of age, yearling animals not vaccinated in the first year, as well as heifers over 2 years of age and cows were vaccinated. The vaccine was administered subcutaneously at a dose of 5 x 10(9) viable organisms. In co-operatives that had used the B. abortus B19 vaccine, the existing vaccination programme was maintained, and all cattle over 3 months of age (except male animals) were vaccinated with a dose of 5 x 10(10) viable organisms, using the same vaccination programme as in herds immunised with the Rev.1 vaccine. Before vaccination, two serological tests were carried out at an interval of 20-25 days to determine the incidence of Brucella infection. Serum samples were tested by the Rose Bengal test (RBT), serum agglutination test (SAT) and complement fixation test (CFT). An infection rate of 3.8-35% was found in the herds tested. Animals serologically positive for brucellosis were removed from the herds. Fifteen to 21 days after vaccination, blood samples were taken from 10% of the animals in all immunised herds. The serum samples were tested by SAT and CFT to check the seroconversion rate. In herds immunised with the Rev.1 vaccine 90% of the cows and 92.7% of the heifers were seropositive, while the seropositivity rate of B19-vaccinated cows and heifers was 68.8% and 89.7%, respectively. The serological status of the vaccinated animals was checked not only immediately after vaccination but also 12 months thereafter (in B19-vaccinated cows also 24 months after vaccination). After Rev.1 vaccination, antibodies were present in 7% of the cows, 4.6% of the 3-year-old heifers, 3.1% of the 2-year-old heifers, and 1.6% of the 1-year-old animals. By comparison, 13.2% of the cows vaccinated with the B. abortus B19 vaccine were serologically positive 2 years after immunisation. As regards the B19-vaccinated heifers, 7.9%, 5% and 2.7% of the 3-year-old, 2-year-old and 1-year-old animals, respectively, were serologically positive even 12 months after vaccination.