Host-parasite relationships in experimental airborne tuberculosis. II. Reproducible infection by means of an inoculum preserved at -70 C

The use of an inoculum preserved at low temperature for the infection of guinea pigs by the respiratory route was evaluated. In a preliminary study with Mycobacterium bovis (BCG), some of the conditions required for maximal recovery of viable cells stored at low temperature were examined. Survival of BCG was decreased by rapid freezing to -70 C and by storage at -20 C, but there was no decrease when BCG was frozen slowly and stored at -70 C or -196 C. In a subsequent study, the effect of storage at -70 C on viability and infectivity of M. tuberculosis (H37Rv) was considered. There was no loss of viability of H37Rv cells suspended in Dubos broth and stored 1 year at -70 C. This suspension showed no loss of infectivity as assessed by the number of primary pulmonary lesions initiated in guinea pigs. Constant viability and infectivity of a suspension stored at low temperature assures the reproducibility of the amount of infection and facilitates comparisons between experiments. This advantage, as well as others, of storage at low temperature are discussed.

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.