"Our study is the first to quantify the HIV epidemic’s effect on TB outbreaks and to show how high levels of TB treatment could actually halt this synergistic effect," said Dr. Sally Blower, principal investigator and professor of biomathematics at the UCLA AIDS Institute.
Blower’s group created a computer-simulation model to calculate how HIV shaped TB outbreaks over two years. The scientists found that HIV epidemics substantially increase the frequency and severity of TB outbreaks by compressing TB’s stages of development and accelerating its transmission to others.
"We modeled how HIV speeds up TB’s progression from infection to full-blown disease," Blower said. "This telescoping effect is very important, because only people in the disease stage of TB can transmit it to others. Our predictions show that HIV’s telescoping of TB causes the HIV epidemic to significantly amplify the TB epidemic."
Researchers estimate that one-third of the world is currently infected with TB. Normally, only 10 percent of these people advance to the contagious stage of the disease, while the rest of the infected population’s immune systems keep the bacteria in check. This all changes, however, when someone infected with TB also contracts HIV.
"It’s like lighting a fire," Blower said. "You get HIV and boom! Suddenly your TB is killing you."
"An immune system weakened by HIV can no longer fight TB infection," said co-author Dr. Peter Small, associate professor of infectious diseases at Stanford University. "This permits the TB bacteria to flourish and overwhelm the lungs."
While HIV exacerbates many diseases, TB poses a particularly dangerous problem because it is so widespread. World health professionals predict that more than one-quarter of the AIDS patients in the world will die from TB this year.
Offering a light at the end of the tunnel, Blower’s model revealed that HIV’s influence falls directly in proportion to the rate of TB treatment. In places with strong TB control programs — such as San Francisco, where doctors treat 95 percent of the city’s TB cases — HIV epidemics are less likely to increase the number or size of TB outbreaks.
In contrast, a modest HIV epidemic can nearly double the size of TB outbreaks in developing countries, which treat only a moderate or low number of their TB cases.
Blower and colleagues advocate treating more HIV, because this will indirectly improve the TB epidemic. Their recommendations contradict world health professionals who argue that HIV is out of control and too expensive to treat.
"HIV epidemics do not amplify TB in regions with well-implemented TB treatment programs," Blower said. "The problem occurs in developing countries where little or no treatment exists for either disease."
"The World Health Organization’s calculations do not reflect HIV’s amplification effect on TB," Small said. "Our findings suggest that WHO’s target treatment levels for TB underestimate the rate necessary for world control of drug-resistant TB."
Because high TB treatment rates can curb HIV’s effect, the authors recommend that world health professionals invest more resources in TB treatment programs, particularly in developing countries burdened by both diseases.
Secondarily, the authors found that large TB outbreaks can happen by chance. "Dramatic TB outbreaks are not necessarily caused by virulent TB strains," Small said.
As the rate of TB incidence falls and the United States moves toward eliminating TB, the authors warn that outbreaks of TB will continue to dominate the epidemiology of the disease.
"Public health professionals should expect to see lots of blips up and down in the number of cases of TB disease — not a continuous downslide," Blower cautioned. "Sporadic outbreaks are to be expected. We shouldn’t conclude that TB controls are not working."
Postdoctoral fellow Travis Porco, Ph.D., formerly in Blower’s research group and now a mathematical epidemiologist at the San Francisco Department of Health, also co-authored the study. Grants from the National Institute of Allergy and Infectious Diseases and the National Institute on Drug Abuse supported the research.