Bovine brucellosis vaccine strain S19 detected in calves before vaccination
Bovine brucellosis is a zoonotic disease that has considerable economic impact on the market of products such as meat and milk. As Brazil has one of the largest cattle herds in the world, this disease is a significant agricultural and public health concern. In 2001, Brazil launched the National Program of Control and Eradication of Animal Brucellosis and Tuberculosis. This program included the vaccination of 3-8 month old calves using the smooth and attenuated S19 strain, as well as monitoring with serological tests. There have been reports of excretion and persistence of S19. In this context, the objective of this study was to monitor the persistence of the S19 vaccine strain in blood samples from vaccinated calves. Seroconversion was investigated to evaluate the vaccine immunogenicity. PCR assays of blood and serum were run at at: "day zero" (before vaccination), 1 to 15 days after vaccination and every month until 12 months, totaling 28 collections for each of 10 calves. The vaccine strain was detected throughout the study, even at day zero, before the calves were vaccinated. Sequencing analysis confirmed the presence of the S19 strain. Results from serological tests did not show agglutination in “day zero” samples, meaning the immune response was negative. An immune response was first detected in two samples on the fourth day. One hypothesis that could explain this positivity before vaccination is that the already vaccinated mothers could have transmitted the vaccine to the calves, as the cows presented excretion of the S19 strain in milk and/or urine. This is the first report of the finding of this bovine brucellosis vaccine strain in the blood of calves before vaccination.