Low oral BCG doses fail to protect cattle against an experimental challenge with Mycobacterium bovis

Studies were undertaken to determine whether a dose of oral Mycobacterium bovis bacillus Calmette-Guérin (BCG) which did not induce skin test reactivity could protect cattle against bovine tuberculosis (TB). Groups of calves (n = 9) were vaccinated by administering 10 ⁸, 10 ⁷ or 10 ⁶ colony forming units (CFU) of BCG orally or 10 ⁶ CFU subcutaneous (s.c.) BCG. A control group (n = 10) was not vaccinated. All animals were challenged with M. bovis 18 weeks after vaccination and euthanized and necropsied at 16 weeks following challenge. Positive responses in the single cervical tuberculin skin test (severe interpretation) at 15 weeks post-vaccination were only observed in the s.c. BCG and 10 ⁸ CFU oral BCG groups (four of nine animals/group). Following experimental challenge with M. bovis, both these BCG-vaccinated groups had significant reductions in lesion scores and bacterial counts whereas there was no protection in calves vaccinated with oral doses of 10 ⁶ or 10 ⁷ CFU of BCG. In conclusion, low oral doses of BCG did not induce skin test responses, IFN-γ responses or protection against TB, however, in the BCG vaccine groups where protection was observed, there was no correlation between protection and skin test responses or IFN-γ responses.