# Investigation of the Seroprevalence of Brucella Antibodies and Characterization of Field Strains in Immunized Dairy Cows by B. abortus A19

## TL;DR

This study shows that cows vaccinated with Brucella abortus A19 can retain antibodies for over 12 months without being infected, suggesting that antibody tests alone should not be used to cull cattle.

## Contribution

The study introduces a PCR method to distinguish between vaccine-induced and natural Brucella infections and demonstrates the persistence of A19 antibodies beyond 12 months.

## Key findings

- A significant proportion of vaccinated cows retained antibodies for over 12 months without brucellosis infection.
- A PCR method was developed to differentiate Brucella A19 from field strains, detecting 10 field strains in dairy cows.
- Using the Mathematical Expectation strategy reduced testing workload by 80.1% while maintaining detection efficiency.

## Abstract

Brucella abortus strain 19 vaccines, including A19 and S19 vaccines, are widely used to prevent bovine brucellosis. Vaccine-induced antibodies are usually considered to last for no more than 12 months. After this period, individuals testing positive are excluded because they are presumed to be infected with Brucella field strains. In Ningxia, China, immunization with the Brucella abortus strain A19 vaccine is being implemented as part of a brucellosis eradication program. We conducted a serological survey of cattle immunized with the A19 vaccine for over 12 months in Lingwu, Ningxia, from 2021 to 2023. It was found that there was a certain proportion of cows with a persistent antibody titer who were not infected with brucellosis. Therefore, cattle with positive antibodies cannot be eliminated through simple serological detection. Otherwise, a large number of healthy cattle would be mistakenly culled, resulting in significant economic losses. We have also developed a PCR method to distinguish Brucella strain 19 from non-19 Brucella strains and successfully detected 10 Brucella abortus field strains from five dairy farms.

(1) Background: One method of eradicating brucellosis is to cull cattle that test positive for antibodies 12 months after being vaccinated with the 19-strain vaccine. Variations in immunization regimens and feeding practices may contribute to differences in the rate of persistent antibodies. We conducted this study to investigate the real positive rate of Brucella antibody in field strains of Brucella spp. after immunization over 12 months in dairy cows. This research aims to provide data to support the development of strategies for preventing, controlling, and eradicating brucellosis. (2) Method: We employed the baseline sampling method to collect samples from cows immunized with the A19 vaccine for over 12 months in Lingwu City from 2021 to 2023. Serological detection was conducted using the RBPT method. An established PCR method that could distinguish between 19 and non-19 strains of Brucella was utilized to investigate the field strains of Brucella on 10 dairy farms based on six samples mixed into one using the Mathematical Expectation strategy. (3) Results: We analyzed the rates of individual seropositivity and herd seropositive rates in dairy cattle in Lingwu City from 2021 to 2023 and revealed that antibodies induced by the Brucella abortus strain A19 vaccine persist in dairy herds for more than 12 months. We established a PCR method for identifying both Brucella A19 and non-A19 strains, resulting in the detection of 10 field strains of Brucella abortus from 1537 dairy cows. By employing a Mathematical Expectation strategy, we completed testing of 1537 samples after conducting only 306 tests, thereby reducing the workload by 80.1%. (4) Conclusions: There was a certain proportion of cows with a persistent antibody titer, but there was no evidence that all of these cattle were naturally infected with Brucella. The established PCR method for distinguishing between Brucella abortus strain 19 and non-19 strains can be specifically utilized for detecting natural Brucella infection in immunized cattle. We propose that relying solely on the detection of antibodies in cattle immunized with the A19 vaccine more than 12 months previously should not be solely relied upon as a diagnostic basis for brucellosis, and it is essential to complement this approach with PCR analysis to specifically identify field Brucella spp. Brucella abortus was the predominant strain identified in the field during this study. Detection based on the Mathematical Expectation strategy can significantly enhance detection efficiency.