HIV infection greatly increases the risk that tuberculosis (TB) infection will lead to active disease. But does it also enhance the spread of TB infection? Research by the London School of Hygiene and Tropical Medicine shows remarkably static TB transmission rates over a decade in a South African mining community devastated by HIV. Good TB control programmes may be able to protect HIV-negative people from TB, even at the height of an HIV epidemic, they conclude.

Levels of active TB disease in HIV-negative adults are an indirect indicator of the spread of infection as most disease is the result of a recent infection. The researchers combined data from three studies of TB among a total of 7 014 HIV-negative men working at the mine. The studies spanned a decade during which the HIV prevalence rose to 30 % of the general workforce.

Their analysis showed that:

• The incidence of TB disease in HIV-negative miners varied between 962 (1991-1994) and 1 589 (1999-2000) cases per 100 000 person-years.
• This rise disappears when the age of the workforce is taken into account. Age-specific TB rates are surprisingly constant.
• Risk factors for TB include silicosis, age, previous TB treatment and working underground rather than on the surface.

This result suggests that the rate of spread of TB infection has not changed. As the case load of patients with TB increased markedly during the 1990s, this must mean that each person has passed the infection on to fewer people. One possible explanation is that HIV-positive people with TB are actually less infectious than others and for a shorter time.

Another key factor is the quality of TB control efforts. The programme at the mine is based on the DOTS strategy promoted by the World Health Organisation (WHO) with extra features, such as diagnostic TB cultures and active case-finding. Cure rates are higher than the WHO target of 85 %. Drug resistance is uncommon and there is routine testing for drug-sensitivity and tailored treatment for resistant cases. However, overall TB cases and deaths have increased due to HIV, showing that this approach is less successful with HIV-infected patients. Tackling this requires early diagnosis of the majority of HIV infections and reducing the progression and spread of TB among HIV-positive miners. To achieve this, the researchers recommend that TB control programmes should increase access to:

• HIV testing
• drugs to prevent TB
• anti-retroviral drugs.

Source(s):
‘Stable incidence rates of tuberculosis (TB) among human immunodeficiency virus (HIV)-negative South African gold miners during a decade of epidemic HIV-associated TB’, Journal of Infectious Diseases 188: 1156-1163, by E. Corbett et al., 2003

Funded by: Wellcome Trust; Aurum Health Research; Safety in Mines Research Advisory Committee

id21 Research Highlight: 30 September 2004

Further Information:
Elizabeth Corbett
London School of Hygiene and Tropical Medicine and
Biomedical Research and Training Institute
University of Zimbabwe Main Campus (Churchill Road Entrance)
Mount Pleasant
PO Box CY 1753
Causeway Harare
Zimbabwe

Contact the contributor: elc1@mweb.co.zw

London School of Hygiene and Tropical Medicine, UK

Other related links:
'Diagnosing TB in Africa: the quest for cheaper methods'

'Battling the bugs – cutting death rates among HIV-positive TB patients'

'Occupational hazard – protecting healthworkers from TB in Malawi'

'Charity begins at home - community care for HIV and TB patients in Zambia'

'Coughing up for TB tests - cost-effective diagnosis in Zambia'

'Care in the community? TB treatment in rural Swaziland'