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 performed on Mycobacterium tuberculosis isolates from culture-positive pulmonary tuberculosis 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 number of culture-positive pulmonary tuberculosis cases generated by a single source case and calculated for subgroups.

RESULTS

The transmission index was higher in US-born (0.59) than in foreign-born groups (0.21), and was highest in blacks, in particular those aged under 35 years. The increased transmission index among blacks was not explained by smear-positivity, human immunodeficiency virus infection, or increased 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 transmission. These results suggest the need to develop interventions targeted towards this risk group.