Resistant and sensitive strains of Mycobacterium tuberculosis found in repeated surveys among a South Indian rural population.

The findings in a highly selected group of patients, such as those attending clinics or sanatoria, cannot be used as the basis for assessing the true prevalence of strains of Mycobacterium tuberculosis with acquired or primary resistance or of sensitive strains in a community. The present report describes the prevalence of such strains as found in 3 successive surveys in a sizeable random sample of villages in a South Indian district. Changes in the status of cases with such strains from an earlier survey to a later one and the status at an earlier round of cases found at a later one are also described.The prevalence of tuberculous infection among household contacts of cases with acquired resistance to isoniazid was significantly higher than that among contacts of cases with primary resistance or of those with sensitive cultures. This is probably due to the longer duration of sputum positivity of the former at the time of diagnosis. But infectivity, as judged by the incidence of new infections among household contacts, was generally less for cases with acquired or primary resistance than for cases with sensitive cultures, though the difference was not statistically significant.A large number of culture-positive cases, especially those with primary resistance, had no radiological evidence of active pulmonary tuberculosis. The prevalence of primary resistance was very high among certain categories of cases, and the differences between cases with primary resistance and those with acquired resistance were many and large. It is suggested that this could be due to some of the primary resistant cultures being those of atypical mycobacteria, despite positivity in the niacin test.There was a significant increase in the number of cases with acquired resistance to isoniazid at the third survey round owing to irregular treatment with that drug after the second round. The prevalence of primary resistance at the 3 rounds was almost the same.