Humoral immune response in cross-bred heifers immunized with brucella abortus strain rb51 vaccine in military dairy farm of Bangladesh

Background: Brucellosis is a chronic zoonotic disease with negligible mortality rate that might be the reason not to attract the concerned authority to prevent and eradicate it in low income endemic countries. Recently, it has been recognized as a re-emerging zoonotic disease not only in low income countries but also its eradicated developed world. Objective: The main objective was to determine the humoral immune response (HIR) in crossbred dairy heifers immunized with Brucellaabortusstrain RB51 vaccine by using indirect ELISA Materials and Methods: Each of the 20 randomly selected B. abortussero-negative crossbred (Holstein- Friesian  Local) dairy heifers aged between 4 to 8 months old at the Military Dairy Farm received 2.0 ml imported commercial B. abortus SRB51 strain vaccine subcutaneously in the neck region at day 0 and then booster dose at 60 days after first vaccination with similar dose and route during the period from June to October 2020. Each of the collected serum samples of 20 heifers at day 0, 7, 14, 21, 28, 60, 90, 120 and 150 was tested to detect the antibody status by using commercial indirect ELISA kit. Results: The humoral immune response (HIR) in terms of antibody levels detected by OD values in the serum of immunized cross-bred dairy heifers by using B. abortus strain RB51 commercial vaccine resulted 0.097  0.0032 (mean  SE) OD value at 0 day (i.e. pre-immunization) and 0.108 ± 0.0032 at 7th day. After that, the OD value started to rise from day 14 (OD value 0.124 ± 0.0032) and reached to a peak level at 60 days (OD value 0.223  0.0032) with the initial vaccination. Booster vaccination inoculated at 60 days resulted peak antibody level in terms of OD value (0.313  0.0032) at the day 90 and then the antibody level started to decline from 120 days (OD value 0.242  0.0032) to 150 days (OD value 0.199 0.0032) in cross-bred dairy heifers. Conclusions: This study suggests that the commercial B. abortus RB51 strain vaccine has induced satisfactory HIR with initial inoculation and significantly higher HIR produced with a booster dose in crossbred heifers by using commercial I-ELISA. The presence of Brucella antibodies have importance on sero-diagnosis whereas the cell mediated immunity (CMI) plays major role in protection against brucellosis which needs further investigation in cross-bred heifers in Bangladesh.