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Despite high rates of TB in HIV-positives, TB rates stable in HIV-negative S Africans with TB
Michael Carter
2003-10-16
TB rates remained static amongst HIV-negative gold miners, a population at high risk of the disease, during a period when TB prevalence generally has increased four-fold, according to investigators writing in the October 15th edition of the Journal of Infectious Diseases. The reduced infectious period of TB in HIV-positive individuals, due to rapid disease progression, means that there are few cases of secondary infection in HIV-negative
individuals, according to the investigators.
Earlier studies using tuberculin reaction skin tests on children in areas of high HIV prevalence suggested that the incidence of TB was little changed from the pre-HIV era. However, these studies were relatively insensitive to short term changes in TB incidence.
Investigators from London, Zimbabwe and South Africa looked at TB incidence in three cohorts of gold miners who had tested HIV-negative over a ten year period. Gold miners are unusually susceptible to TB because of exposure to silica dust, which is a strong predisposing risk factor for TB. Amongst gold miners as a whole, the HIV epidemic has greatly increased the incidence of TB, in 1990 there were 1143 cases per 100,000 person years, an incidence which rose almost four-fold to 4156 cases per 100,000 person years by 1999.
All three cohorts included in the study were retrospective, and routine investigations were used to diagnose TB including smears, cultures and x-rays.
A total of 5014 individuals were included in the three cohorts. The first cohort looked at TB incidence between 1991 – 97; the second between 1998-99, and the third between 1999 – 2000. TB incidence varied between 962 cases per 100,000 person-years between 1991- 94, to 1589 cases per 100,000 person-years in 1999-00. This increase was not however significant (p=.17) and the increase disappeared when rates were adjusted for the increasing age of the cohort members.
Age and silicosis were predictors of TB disease. Surface workers were significantly less likely to have TB than deep miners as they were not exposed to silica dust.
The investigators comment, “this study has demonstrated remarkably constant age-specific incidence rates of TB among HIV-negative employees in a South African gold-mining workforce that has experienced a severe epidemic of HIV-associated disease since the 1990s”. During the ten year period of the study, however, TB-incidence increased four-fold at a workforce level. This increase was confined to the HIV-positive employees.
The natural history of HIV-associated TB could be one factor explaining the static incidence of TB in the HIV-negative individuals in this study. TB has a reduced infectious period in HIV-positive patients, explained by more rapid TB disease progression. This means that there is less potential for secondary infections to occur. TB control programmes may also have helped to prevent the transmission of TB to HIV-negative people, emphasising, “good TB control programs may…be able to maintain stable incidence rates of TB among HIV-negative individuals living in populations with high prevalence of HIV.” (Source: aidsmap 14 October 2003 http://www.aidsmap.com/news/newsdisplay2.asp?newsId=2347) Related link:http://www.aidsmap.com/
p>TB rates remained static amongst HIV-negative gold miners, a population at high risk of the
disease, during a period when TB prevalence generally has increased four-fold, according to
investigators writing in the October 15th edition of the Journal of Infectious Diseases. The
reduced infectious period of TB in HIV-positive individuals, due to rapid disease
progression, means that there are few cases of secondary infection in HIV-negative
individuals, according to the investigators.
Earlier studies using tuberculin reaction skin tests on children in areas of high HIV
prevalence suggested that the incidence of TB was little changed from the pre-HIV era.
However, these studies were relatively insensitive to short term changes in TB incidence.
Investigators from London, Zimbabwe and South Africa looked at TB incidence in three cohorts
of gold miners who had tested HIV-negative over a ten year period. Gold miners are unusually
susceptible to TB because of exposure to silica dust, which is a strong predisposing risk
factor for TB. Amongst gold miners as a whole, the HIV epidemic has greatly increased the
incidence of TB, in 1990 there were 1143 cases per 100,000 person years, an incidence which
rose almost four-fold to 4156 cases per 100,000 person years by 1999.
All three cohorts included in the study were retrospective, and routine investigations were
used to diagnose TB including smears, cultures and x-rays.
A total of 5014 individuals were included in the three cohorts. The first cohort looked at TB
incidence between 1991 – 97; the second between 1998-99, and the third between 1999 – 2000.
TB incidence varied between 962 cases per 100,000 person-years between 1991- 94, to 1589
cases per 100,000 person-years in 1999-00. This increase was not however significant (p=.17)
and the increase disappeared when rates were adjusted for the increasing age of the cohort
members.
Age and silicosis were predictors of TB disease. Surface workers were significantly less
likely to have TB than deep miners as they were not exposed to silica dust.
The investigators comment, “this study has demonstrated remarkably constant age-specific
incidence rates of TB among HIV-negative employees in a South African gold-mining workforce
that has experienced a severe epidemic of HIV-associated disease since the 1990s”. During the
ten year period of the study, however, TB-incidence increased four-fold at a workforce level.
This increase was confined to the HIV-positive employees.
The natural history of HIV-associated TB could be one factor explaining the static incidence
of TB in the HIV-negative individuals in this study. TB has a reduced infectious period in
HIV-positive patients, explained by more rapid TB disease progression. This means that there
is less potential for secondary infections to occur. TB control programmes may also have
helped to prevent the transmission of TB to HIV-negative people, emphasising, “good TB
control programs may…be able to maintain stable incidence rates of TB among HIV-negative
individuals living in populations with high prevalence of HIV.” (Source: aidsmap 14 October 2003
http://www.aidsmap.com/news/newsdisplay2.asp?newsId=2347)
Related link:http://www.aidsmap.com/
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