Background: We examined the molecular epidemiology of tuberculosis (TB) in San Francisco during a 13-year period encompassing the peak of TB resurgence and subsequent decline to historic low levels.
Objective: To compare rates of TB caused either by rapid progression of recent Mycobacterium tuberculosis infection or by reactivation of latent infection.
Methods: All TB cases reported from 1991 to 2003 were included. Genotyping was performed to identify clustered cases.
Results: The annual TB case rate decreased significantly from 50.8 to 28.8 cases/100000 persons from 1992 to 1999 (P < 0.0001). After 1999, no significant decrease was observed for the population as a whole or in any subgroup examined. Similarly, the rate of clustered cases decreased significantly from 1992 to 1999 (11.4 to 3.1 cases/100000, P < 0.0001). Although the rate of non-clustered cases also declined significantly (25.6 to 17.6 cases/100,000, P < 0.0001), there was a disproportionate reduction in clustered cases (94.7% vs. 50.8%, P < 0.0001). Neither clustered nor non-clustered cases decreased significantly after 1999.
Conclusions: TB case rates reached a plateau despite ongoing application of control measures implemented in 1993. These data suggest that intensification of measures designed to identify and treat persons with latent TB infection will be necessary to further reduce TB incidence.