HIV infection and silicosis: the impact of two potent risk factors on the incidence of mycobacterial disease in South African miners

Objective To investigate the combined effects of HIV infection and silicosis on mycobacterial disease Design and setting A retrospective cohort of 1374 HIV-positive and 2648 HIV-negative miners who attended a South African gold mining hospital and primary health clinics. Participants Miners who had been tested for HIV, with consent, at primary health clinics during 1991–1996, predominantly because of a symptomatic sexually transmitted disease. Results Tuberculosis (TB) incidence was 4.9 and 1.1 per 100 person–years in HIV-positive and HIV-negative miners respectively. The incidence of Mycobacterium kansasii disease was also high (0.32 and 0.10 per 100 person–years, respectively). Silicosis was highly prevalent, implying inadequate dust control, and was a significant TB risk factor among both HIV-positive and HIV-negative men (adjusted incidence rate ratios 1.4–2.5 according to radiological severity). The data were consistent with the risks of silicosis and HIV combining multiplicatively, but did not fit an additive model. The incidence of HIV-associated TB increased significantly during the study, with no corresponding change in HIV-negative rates, to reach 16.1 per 100 person–years among HIV-positive silicotics. Conclusions The risks of silicosis and HIV infection combine multiplicatively, so that TB remains as much a silica-related occupational disease in HIV-positive as in HIV-negative miners, and HIV-positive silicotics have considerably higher TB incidence rates than those reported from other HIV-positive Africans. The increasing impact of HIV over time may indicate epidemic TB transmission with rapid disease development in HIV-infected miners. Similar but currently unrecognized interactions may be contributing to TB control problems in other industrializing countries affected by the HIV epidemic.