Abstract: [Objective] To investigate the dynamic changes in antibody levels and bacterial shedding in Simmental calves following vaccination with the Brucella A19 vaccine. [Methods] Sixty 3-month-old Brucella antibody-negative calves were selected, with 30 calves subcutaneously vaccinated with the A19 vaccine and the other 30 injected with an equal volume of physiological saline as a control. A total of 960 serum samples were collected from both the experimental and control groups 1 day before immunization and on days 2, 4, 6, 8, 14, 21, 28, 56, 84, 112, 140, 168, 196, 224, and 260 post-immunization. Antibody levels of the calves were dynamically monitored using the Rose Bengal Plate Test （RBT）, Standard Tube Agglutination Test （SAT）, Competitive Enzyme-Linked Immunosorbent Assay （cELISA）, and Brucellosis Fluorescence Polarization Assay （FPA）. Additionally, fluorescence quantitative PCR was employed to detect Brucella nucleic acid in blood samples. The health status of the calves was closely monitored and recorded during a 7-day observation period following A19 vaccination. [Results] Within 6 days after the initial vaccination, Brucella nucleic acid was detectable in the blood of vaccinated calves, but no Brucella nucleic acid was detected beyond 6 days. Five calves in the experimental group exhibited mild symptoms such as elevated body temperature and reduced appetite after vaccination, which resolved spontaneously within 72 hours. From days 2 to 21 post-vaccination, the Brucella antibody-positive detection rate in calves gradually increased, with cELISA and FPA achieving a 100% detection rate on day 14. On day 28, the antibody-positive detection rate reached 100% across all four detection methods. Over time, the antibody-positive detection rate gradually declined, and on day 260, the positive detection rate of Brucella antibodies in the calves in the experimental group decreased to 0. [Conclusion] The A19 vaccine demonstrates highly safe. After the first vaccination, bacteremia induced by Brucella A19 vaccine strain in calves is of short duration, and the vaccine rapidly stimulates antibody production in the calves, with the antibody-positive detection rate exhibiting a trend of initial increase followed by a decrease.

Key words: Brucellosis A19 vaccine, calves, serological testing, fluorescence quantitative PCR, antibody-positive detection rate