Transmission of tuberculosis in San Francisco and its association with immigration and ethnicity

OBJECTIVE: To determine tuberculosis transmission dynamics in San Francisco and its association with country of birth and ethnicity. METHODS: Restriction fragment length polymorphism (RFLP) typing was perform ed on Mycobacterium tuberculosis isolates from culture-positive pulmonary t uberculosis patients in San Francisco (1991 through 1996), using IS6110 as a probe. Patients were assigned to clusters based on mycobacterial isolates with identical DNA fingerprints. Clusters were assumed to have arisen from recent transmission. A transmission index was defined as the average numbe r of culture-positive pulmonary tuberculosis cases generated by a single so urce case and calculated for subgroups. RESULTS: The transmission index was higher in US-born (0.59) than in foreig n-born groups (0.21), and was highest in blacks, in particular those aged u nder 35 years. The increased transmission index among blacks was not explai ned by smear-positivity, human immunodeficiency virus infection, or increas ed susceptibility to disease progression. CONCLUSION: US-born tuberculosis cases generated more secondary cases than immigrants. Young blacks appear to be a high-risk group for tuberculosis tr ansmission. These results suggest the need to develop interventions targete d towards this risk group.