Molecular and geographic patterns of tuberculosis transmission after 15 years of directly observed therapy

Context.-Recent studies suggest that one third of tuberculosis cases in urban areas result from recent transmission. Improved tuberculosis control measures such as uniform implementation of directly observed therapy might reduce the proportion of cases resulting from recent transmission. Objective.-To determine patterns of tuberculosis transmission in Baltimore, Md, after 15 years of community-based directly observed therapy. Design.-A 30-month (January 1994-June 1996), prospective, city-wide study of all cases of tuberculosis using traditional contact investigations, geographic information systems data, and molecular epidemiologic comparison of Mycobacterium tuberculosis isolates with 2 DNA probes. Patients.-One hundred eighty-two patients with culture-positive tuberculosis. Main Outcome Measures.-Proportion of disease defined as recently transmitted based on epidemiologic linkage by traditional contact tracing and molecular, linkage by DNA fingerprint analysis of isolates; geographic foci of transmission based on linkage of residences by geographic information systems data. Results.-Of the 182 patients who had isolates of M tuberculosis available, 84 (46%) showed molecular clustering with 58 (32%) defined as being recently transmitted. Only 20 (24%) of 84 cases with clustered DNA fingerprints had epidemiologic evidence of recent contact. Geographic analysis showed significant spatial aggregation of the 20 clustered cases with epidemiologic links (P<.001), occurring in areas of low socioeconomic status and high drug use, The 64 cases with clustered DNA fingerprints but without epidemiologic links shared common risk factors and demographic features with the 20 clustered patients who did have epidemiologic links. Conclusions.-Recently transmitted tuberculosis accounts for a high proportion of tuberculosis cases in Baltimore. Recently transmitted cases occur in geographically distinct areas of Baltimore, and location-based control efforts may be more effective than contact tracing for the early identification of cases.