Seroconversion attributable to vaccination of heifers with the strain B-19 of Brucella abortus

Abstract

Bovine brucellosis is transmitted by Brucella abortus and causes considerable economic losses to agribusiness. To determine the seroconversion rate attributable to vaccination of heifers with B. abortus strain B-19, 330 blood serum samples from 110 heifers aged three to eight months from five properties in the municipality of Araguaína, Tocantins, Brazil, were analyzed. On day zero (D-0), blood samples were collected, and 55 heifers from the intervention group were vaccinated. The remaining blood samples were collected on days seven (D-7), 14 (D-14), and 21 (D-21). The serum samples were analyzed using the buffered acidified plate antigen (BAPA) test. The rate of seroconversion attributable to vaccination was calculated by subtracting from the seroconversion incidence rate in the intervention group the incidence rate of the control group. On D-0, 100% (110/110) of the heifers were negative in the BAPA test (95% CI [96.70 - 100%], p < 0.05). On D-7, the seroconversion rate was 94.55% (52/55) (95% CI [84.88 - 98.84%], p < 0.05) in the intervention group and 0% (0/55) (95% CI [0.00 - 6.49%], p < 0.05) in the control group. On D-14, the rate was 98.18% (54/55) (95% CI [90.28 - 99.95%], p < 0.05) in the intervention group and 0% in the control group. Therefore, seroconversion attributable to vaccination varied from 78.39% (84.88 - 6.49%) to 92.35% (98.86 - 6.49%) on D-7 and from 83.79% (90.28 - 6.49%) to 93.46% (99.95 - 6.49%) on D-14, respectively. On D-21, the seroconversion rate in the 55 heifers from the control group vaccinated on D-14 was 100% (55/55) (95% CI [93.51 - 100%], p < 0.05) but was not significantly different (p > 0.05) from that of heifers from the intervention group vaccinated on D-7. The results of this study allow making recommendations to the Official Veterinary Service on establishing a vaccination audit, with random collection of blood samples starting on day seven after vaccination in heifers considered vaccinated with B. abortus strain B-19 using the BAPA test as a diagnostic test and considering a minimum seroconversion rate of 75% to evaluate vaccination against brucellosis.