Tuberculosis in Mexican-born persons in San Francisco: reactivation, acquired infection and transmission.

San Francisco, California. To determine the relative contributions of infection acquired in San Francisco and reactivation of tuberculous infection acquired elsewhere in Mexican-born persons who developed tuberculosis in San Francisco, and to determine the frequency of transmission leading to secondary cases of tuberculosis in other persons. The study population consisted of all Mexican-born tuberculosis patients reported in San Francisco from 1991 through June 1995. All patients had positive cultures for Mycobacterium tuberculosis and DNA fingerprinting of isolates using IS6110 with more than two bands. Patients were classified as infected in San Francisco or infected elsewhere based on pre-defined criteria that included a second DNA fingerprinting technique (polymorphic guanine-cytosine-rich sequence), chart reviews, and selected patient interviews. Of the 43 Mexican-born patients studied, nine (21%) met the definition of infection acquired in San Francisco and 34 (79%) met the definition of reactivation of infection acquired elsewhere. Only one of the 43 cases resulted in two secondary cases in US-born persons. One-fifth of the Mexican-born patients who developed tuberculosis in San Francisco acquired their tuberculous infection in San Francisco; transmission from Mexican-born persons leading to tuberculosis in other persons is uncommon.