Risk factors for transmission of Mycobacterium tuberculosis in a primary school outbreak: lack of racial difference in susceptibility to infection.

Recent data have suggested that there are racial differences in the susceptibility to infection by Mycobacterium tuberculosis. An opportunity to test this suggestion was afforded by an outbreak of tuberculosis in a racially mixed elementary school in St. Louis County, Missouri. A physical education teacher was discovered to have cavitary pulmonary tuberculosis. Of 343 students in the school, 176 (51 percent) were found to be tuberculin skin test positive (> or = 5 mm induration by Mantoux method); 32 children had abnormal chest radiographs. More frequent contact with the physical education teacher was associated with infection (p < 0.001). Black children were no more likely to be infected than were white children (relative risk (RR) = 0.98, 95% confidence interval (CI) 0.78-1.22). However, black children who were tuberculin positive had larger skin reactions than did white children (mean, 18.9 vs. 16.6 mm, p < 0.001) and were more likely to have abnormal chest radiographs (RR = 2.76, 95% CI 1.44-5.27). Among tuberculin-positive children, low body mass index (less than 10th percentile) was associated with active disease (RR = 2.90, 95% CI 1.45-5.80). The analysis of race was unchanged after controlling for sex, body build, and level of contact with the physical education teacher. Widespread tuberculous infection resulted from contact with a highly infectious staff person. Thin body build was a risk factor for active disease. Black children were no more susceptible to infection than were white children, although they more commonly developed radiographic evidence of active disease.