Incidence of tuberculosis in Olmsted County, Minnesota, 1990-2001

OBJECTIVE: To describe and compare the incidence and clinical characteristics of tuberculosis in Olmsted County, Minnesota, among US-born and foreign-born persons. PATIENTS AND METHODS: We performed a retrospective cohort study at the Mayo Clinic in Rochester, Minn, of all residents of Olmsted County (2000 population: 124,277) diagnosed as having tuberculosis between January 1, 1990, and December 31, 2001. Potential cases were identified with use of a computerized diagnostic coding database and microbiological laboratory data; all identified medical records were abstracted. Definite cases were those in which Mycobacterium tuberculosis was recovered in culture. Probable cases were those that met predefined clinical or radiographic evidence of tuberculosis and other criteria. Age-specific, sex-specific, and country of origin-specific incidence rates were calculated with use of Olmsted County census data. Variables were compared among risk groups using the Fisher exact test. RESULTS: During a 12-year period, 71 cases of tuberculosis (53 definite, 18 probable) were identified, for an incidence of 5.3 per 100,000 person-years. Of these cases, 54 (76%) occurred during the second half of the study (Incidence: 7.7 per 100,000 person-years). The incidence among US-born persons was similar throughout the study period; however, the incidence among foreign-born persons increased more than 3-fold during the second half of the study period. Twenty-five patients (35%) were former refugees. All Isoniazid-resistant Infections (12% of isolates) and multidrug-resistant infections (6% of isolates) occurred among foreign-born persons. CONCLUSION: The incidence of tuberculosis increased substantially in Olmsted County between 1990 and 2001, primarily because of an increase in the number of cases among foreign-born persons.