Background: Directly observed therapy (DOTS) is the main strategy for prevention and control of tuberculosis worldwide. However, its effect on tuberculosis transmission in populations with moderate rates of drug-resistant disease is not known. Methods: This population-based prospective study in southern Mexico between March, 1995, and February, 2000, was based on passive case finding and detection of acid-fast bacilli in sputum samples to diagnose pulmonary tuberculosis. We also used cultures, drug-susceptibility testing, bacterial genotyping, and monitoring of treatment outcomes. Findings: We enrolled 436 patients; the HIV seroprevalence rate was 2%. We used three indicators to monitor continuing tuberculosis transmission: the incidence rate of pulmonary tuberculosis, which decreased by 54·4% between 1995 and 2000, from 42·1 to 19·2 per 105 population (p=0·00048); the percentage of clustered pulmonary tuberculosis cases, which decreased by 62·6% from 22% to 8% (p=0·02); and the rate of primary drug resistance, which decreased by 84·0% from 9·4 to 1·5 per 105 population (p=0·004). Rates of multidrug-resistant (MDR) tuberculosis also decreased (p<0·0001). The case-fatality ratio was 12% for MDR tuberculosis (five of 41), 7% for strains resistant to at least one drug after exclusion of MDR (four of 55), and 3% for pansusceptible strains (nine of 272). There were 13 treatment failures (11%) in 1995 and one (2%) in 2000 (p=0·012). Interpretation: Even in settings with moderate rates of MDR tuberculosis, DOTS can rapidly reduce the transmission and incidence of both drug-susceptible and drug-resistant tuberculosis. However, further interventions, such as drug-susceptibility testing and standardised or individualised treatment regimens, are needed to reduce mortality rates for MDR tuberculosis.
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