Short communication: Effects of age on the immune response induced by Brucella abortus S19 or RB51 vaccination in calves.

Vaccination of bovine calves is one of the main policies for bovine brucellosis control in endemic areas. However, the effect of animal age on vaccine immunogenicity is still unknown and could help to determine an ideal age for vaccination, in order to maximize immune response. Thus, the objective of this study was to compare the in vitro expression of IFN-γ by stimulated PBMC after vaccination with B. abortus S19 and RB51 strains in calves vaccinated at different ages, between 3 and 8 months. Cell-mediated immune response was assessed through culture of peripheral blood mononuclear cells (PBMC) and quantification of IFN-γ in the supernatant by enzyme-linked immunosorbent assay (ELISA). In addition, serological assays were performed using 2-mercaptoethanol (2-ME), Standard Tube Agglutination (STAT) and Fluorescent polarization assay (FPA) tests. Blood samples and sera were collected in the inoculation day, as well as at 28 and 56 days after vaccination. A generalized linear mixed model was used to evaluate effect of age at vaccination on in vitro production of IFN-γ and no differences were observed comparing the different ages, for both RB51 and S19 vaccines (p > 0.05). A higher percentage of animals vaccinated with S19 at 3-4 months-old [77.28 % (7/9)] returned to the serological negative status at day 56, when compared to 5-6-months [50 % (5/10)] and 7-8 months-old animals [27.28 % (3/11)]. In conclusion, our findings indicated similar levels of IFN-γ in vitro production in animals between 3 and 8 months of age, following vaccination with S19 and RB51 strains.