One of the hypothesis put forth for the failure of BCG vaccine to show protection against bacillary pulmonary tuberculosis in Chingleput trial was the interference from non-tuberculous mycobacteria that were prevalent in the trial area. In order to test this, a study was conducted with the following objectives: to investigate (1) Protection given by BCG and M.avium intracellulare (MAI) which is the most prevalent species, against the challenge with high and low virulent strains of M.tuberculosis in sensitised guineapigs. (2) Whether M.avium Intracellulare (MAI) interferes with the protective effect of BCG against challenge with both high and low virulent strains of M.tuberculosis. Sensitization was done with MAI in guineapigs using both oral and intradermal routes. Groups of species were immunized with BCG/Placebo and later challenged with high/low virulent strains of M.tuberculosis. Colony counts of M.tuberculosis bacilli from spleens of the animals were done to measure the protective effect.
The findings were: (1) BCG showed protection against both high and low virulent challenges. (2) MAI in both oral and intradermal routes had no effect against low virulent challenge. (3) There was no significant interaction between BCG and MAI against low virulent challenge. (4) MAI when given orally, showed a significant protection against high virulent challenge. The same was not seen with intradermal route. (5) MAI orally, interfered with the protective effect of BCG against high virulent strains of M.tuberculosis.