Karina Saadi

Bovine tuberculosis (bTB) is mainly caused by Mycobacterium bovis, which affects cattle, leading to significant economic losses. In Chile, the vaccination with the M. bovis Bacillus Calmette-Guérin (BCG) strain has been implemented in dairy herds with high prevalence of bTB. This study evaluated non-specific protection associated with BCG on the detection of pathogen-associated genes (nsp5, stx1, stx2, invA, IS1081) and mortality related to diarrhea and pneumonia in calves. A total of 186 calves from a commercial dairy farm were enrolled and grouped as vaccinated (n = 96) and non-vaccinated (n = 90). The BCG Russia strain (2–5 × 10⁵ UFC) was inoculated subcutaneously within the first 30 days after birth. Animals were monitored through fecal sampling at 3 and 6 months of age for molecular detection of gene sequences. A logistic regression analysis showed differences in detection rates of the stx1 sequence at 3 months, with a higher risk for the non-vaccinated individuals (OR 2.91, CI 1.42–5.94, p = 0.03) and for those born in the cold season (OR 9.55, CI 2.02–45.11, p = 0.004). A Kaplan–Meier survival analysis showed a significant difference in deaths in vaccinated calves compared with non-vaccinated animals (p = 0.018), suggesting that BCG confers non-specific protection during the first 3 months after birth, in field conditions.

Bovine tuberculosis (bTB) is a zoonotic disease caused mainly by Mycobacterium bovis, which is associated with major economic losses for milk and meat producers. The objective of this trial was to assess the efficacy of the BCG Russia strain in a cohort study performed under field conditions, with the vaccination of calves in seven dairy farms from a high prevalence area in central Chile. The trial was performed with 501 animals, subcutaneously vaccinated with 2–8 × 10⁵ colony-forming units of BCG, whilst 441 matched control animals received a saline placebo. Peripheral blood was collected at 6, 12 and 18 months post-vaccination, and infection status was determined using the IFNγ release assay in conjunction with the DIVA (Detecting Infected amongst Vaccinated Animals) antigens ESAT-6, CFP-10 and Rv3615c. The BCG vaccine showed a low but significant level of protection of 22.4% (95% CI 4.0 to 36.4) at the end of the trial. However, diverse levels of protection and a variable duration of immunity were observed between trial herds. This diverse outcome could be influenced by the general health condition of calves and their exposition to non-tuberculous mycobacteria. These results suggest that BCG vaccination of dairy calves in a natural transmission setting confers variable protection to animals against bTB in a high prevalence area.