HIV has led to an increase in new tuberculosis (TB) infections, not only in individuals with HIV, but also in HIV-negative individuals, according to a cohort study involving South African gold miners published in the March 5th edition of AIDS.
The findings of this study stand in contrast to a similar study published last year, which was conducted in three South African gold mines and found a stable incidence of TB amongst HIV-negative miners, despite huge increases in incidence of TB amongst their HIV-positive colleagues (see link below).
The newly published retrospective cohort study was conducted in four South African gold mines between 1991 and 1997, and involved 27,874 individuals whose HIV status was known.
A total of 3,371 miners were infected with HIV at entry to the study, and a further 2,737 tested HIV-positive during the study period.
Annual chest X-rays were used to screen for TB, but contact tracing and presentation with symptomatic TB disease also contributed TB case reports. All miners from four mines who had regular HIV testing between 1991 and 1997 were included in the study, but individuals with a past history of TB were excluded. No patient received either antiretroviral therapy for TB or isoniazid prophylaxis for TB during the course of the study.
The incidence of new TB cases increased dramatically at the four mines between 1991 and 1997, from 0.53% to 1%, with preliminary data showing that this incidence continued to increase to 1.2% in 1998 and 2% in 2000. In addition, the investigators found that the proportion of TB cases involving HIV-positive individuals increased from 10% in 1991 to 64% in 1997.
It was also established that TB incidence increased in HIV-negative miners from 0.50% in 1991 - 92, to 1.28% in 1995 - 97. During this period, TB incidence in HIV-positive individuals increased to 3% per year.
TB incidence also increased with age (incidence risk ratio [IRR] for under-30s of 1, increasing to 4.8 for over-50s), but even after adjustment for increasing age, the investigators still found that the annual incidence of TB in both HIV-positive and HIV-negative miners increased over time (p<0.001).
Furthermore, the investigators found that the proportion of TB cases attributable to HIV increased from zero in 1991 to 41% in 1997.
“In this population we have demonstrated an increase in the incidence of tuberculosis during a period of rapidly rising HIV prevalence,” comment the investigators, who note that TB incidence increased not only in HIV-positive miners, but also in HIV-negative individuals. “The increased incidence of tuberculosis in HIV-negative miners is likely to be caused by onward transmission from the increased number of (mainly HIV-positive) tuberculosis cases.”
The investigators warn that even in mines, where there is active TB case finding and good diagnostic techniques, and where treatment outcome is above the target set by the World Health Organisation, the “effect of HIV on the incidence of tuberculosis cannot be adequately contained”.
Further information on this website
Despite high rates of TB in HIV-positives, TB rates stable in HIV-negative S Africans with TB risk - news story
TB - overview
TB - factsheet
Sonnenberg P et al. HIV and pulmonary tuberculosis: the impact goes beyond those infected with HIV. AIDS 18: 657 - 662, 2004.