TB case detection likely to prove more effective than HAART in limiting TB spread

Detecting and treating cases of active tuberculosis in countries with high HIV rates is more effective at reducing TB incidence and death than providing HAART, treating latent TB infection, or preventing HIV infection, according to a statistical model developed by researchers at the University of Southampton and the World Health Organization and published in the November 21st edition of AIDS.

The apparent failure of conventional TB control methods in sub-Saharan Africa has led some researchers to question whether additional measures are needed in areas where high HIV rates drive TB epidemics. Figures from 2000 indicate that over a quarter of Africa’s estimated 2 million cases of active TB were among people with HIV, who accounted for almost 40% of the 535,000 TB deaths in Africa.

The researchers present a mathematical projection to simulate the effect of different interventions on TB control in areas of high HIV prevalence. Bayesian statistical methods were used to adapt the model to available data on TB and HIV rates.

A set of assumptions on the impact of TB and HIV interventions guided the mathematical modeling. The researchers postulated a baseline TB case detection rate of 50% and a starting cure rate of 70% for treated TB patients, figures consistent with available data from TB control programmes in sub-Saharan Africa. Adherence to HAART was calculated at either 80% or 100%, and treatment of latent TB infection, over six months or for life, was considered 70% effective at preventing active TB disease.

The effect of each intervention on reducing the incidence of active TB disease and deaths from TB over 10 and 20 years was projected based on current data from Uganda, Kenya, and South Africa. According to the model, a 1% increase in the detection rate of active TB cases would prevent 2,000 TB deaths in Uganda, 4,900 deaths in Kenya, and 22,000 deaths in South Africa over the next decade. Improving the cure rate for TB treatment by 1% would have a smaller impact on reducing TB deaths, but was nearly as effective as increased case detection in decreasing the incidence of new TB cases.

In contrast, a 1% increase in HAART coverage had a much smaller impact, preventing only about a quarter as many TB deaths as increased case detection. However, HAART’s impact on TB incidence and death rivaled improvements in TB case detection when coverage of the intervention reached 100%, suggesting that HIV treatment could achieve significant reductions in TB mortality if antiretroviral therapy were universally available. Previous reports from Brazil and South Africa have found that TB incidence falls by approximately 80% among people with HIV receiving antiretroviral treatment.

Improvements in preventing HIV transmission and treating latent TB infection were projected to have relatively little impact on TB incidence over the next ten years. A reduction in HIV transmission rates would only have a substantial effect on TB control over a twenty-year period.

The authors conclude that TB control programmes in high HIV prevalence areas should focus their efforts on intensifying TB case detection and cure rates. They acknowledge that their model does not address the broader impact of HAART on reducing HIV-related mortality.

This paper appears as national TB programmes are struggling to find ways and resources to collaborate with HIV prevention and treatment initiatives, while maintaining their focus on conventional TB control. A symposium held in late October in Paris brought together researchers and TB control programme officers to discuss the role of TB programmes in WHO’s 3 by 5 initiative to scale up access to antiretroviral treatment.

WHO officials proposed incorporating voluntary HIV counselling and testing for all TB patients, with people testing HIV-positive evaluated for eligibility for antiretroviral therapy. TB programmes could become a point of entry into HIV care for up to 536,000 people with HIV.

However, as yet unresolved is the question of who will deliver antiretrovirals to TB patients — TB or HIV programmes? TB control officers fear that TB clinics would be overwhelmed by the demands of dispensing HIV drugs; even if they only provided antiretrovirals during the course of TB therapy, patients would need to be referred to other health care services to continue HIV treatment. These issues will frame the prospects of collaboration between TB and HIV programmes in the near future.