Epidemiological basis of tuberculosis eradication. 6. Tuberculin sensitivity after human and bovine infection.

The morbidity from pulmonary tuberculosis and the frequency of pulmonary calcifications have been found to be very much lower among persons infected from bovine sources than among persons infected from human sources. This difference might be wholly or partly due to an attenuated virulence of the infecting bovine tubercle bacilli; the purpose of the study reported was to examine this question. The problem was attacked indirectly by means of a comparison between the tuberculin sensitivity among human-source and bovine-source reactors. The study shows that the sensitivity is stronger among the bovine-source reactors, thus affording no support for the hypothesis that bovine infection may be of low virulence. The alimentary route of infection therefore probably accounts for the low frequency of pulmonary calcifications and possibly also for the low morbidity among the reactors infected from bovine sources.It would be of great significance for tuberculosis control programmes if it were possible to distinguish the low-risk group of bovine-source reactors from the high-risk group of human-source reactors. However, although the former have larger tuberculin reactions, the difference is still so small (of the order of a few millimetres) that the tuberculin reaction cannot be used in practice to differentiate the two groups.