Field evaluation of the efficacy of Mycobacterium bovis bacillus Calmette-Guerin against bovine tuberculosis in neonatal calves in Ethiopia

Abstract

In developing countries, the conventional test and slaughter strategy for the control of bovine tuberculosis is prohibitively expensive, and alternative control methods such as vaccination are urgently required. In this study, the efficacy of Mycobacterium bovis bacillus Calmette-Guérin (BCG) for protection against bovine tuberculosis (bTB) was evaluated in Holstein calves under field conditions in Ethiopia. Thirteen neonatally vaccinated and 14 control calves were exposed for 10 to 23 months to skin test reactor cows. Gamma interferon (IFN-γ) testing, comparative intradermal tuberculin testing, postmortem examination, and bacteriological culture were used for the evaluation of BCG efficacy. The overall mean pathology score was significantly (P < 0.05) higher in control calves than in vaccinated calves. Culture positivity for Mycobacterium bovis was higher in the control calves than in the vaccinated calves, and significantly more BCG-vaccinated animals would have passed a standard meat inspection (P = 0.021). Overall, the protective efficacy of BCG was between 56% and 68%, depending on the parameters selected. Moreover, by measuring gamma interferon responses to the antigens ESAT-6 and CFP-10, which are present in M. bovis but absent from BCG, throughout the experiment, we were able to distinguish between vaccinated animals that were protected against bTB and those animals that were not protected. In conclusion, the present trial demonstrated an encouraging protective effect of BCG against bTB in a natural transmission setting in Ethiopia.

FIG. 1.

In vitro IFN-γ responses following vaccination and challenge. (A) Responses of BCG vaccinates; (B) responses of control animals. Results are expressed as mean ΔOD₄₅₀ responses (OD₄₅₀ values induced by PPD-A subtracted from PPD-B values). Time is shown as months postvaccination. VL, animals with visible lesions, NVL, animals with no visible lesions. Please note that NVL BCG-vaccinated animals and controls were also M. bovis culture negative.*, P < 0.05 between BCG-vaccinated VL and BCG-vaccinated NVL (Student's t test).

FIG. 2.

Association between the severity of pathology of bTB in experimental calves and the duration of exposure to the infected herd. Circles, BCG-vaccinated calves; squares, control animals.

FIG. 3.

IFN-γ responses to mycobacterial antigens in vaccinated and control neonatal calves with visible lesions (VL) or no visible lesions (NVL). IFN-γ response was measured upon slaughter just immediately before completion of the exposure time. IFN-γ responses to bovine PPD and ESAT-6-CFP-10 peptide cocktail were greater (P < 0.05) in control calves than in vaccinated calves. E6C10 is a cocktail of ESAT-6 and CFP-10 recombinant proteins. The abbreviations of the different animal groups are defined in the legend to Fig. 1.