Following are excerpts from the article, which was published recently in the open access journal PLoS Medicine. "Every year, more than nine million people develop tuberculosis -- a contagious infection usually of the lungs -- and nearly two million people die from the disease. Tuberculosis is caused by Mycobacterium tuberculosis. These bacteria are spread in airborne droplets when people with the disease cough or sneeze."

TB ward in Lima, Peru.

"Most people infected with M. tuberculosis never become ill -- their immune system contains the infection. However, the bacteria remain dormant within the body and can cause tuberculosis years later if host immunity declines. The symptoms of tuberculosis include a persistent cough, weight loss, and night sweats. Diagnostic tests for the disease include chest X-rays, the tuberculin skin test, and sputum cultures (in which bacteriologists try to grow M. tuberculosis from mucus brought up from the lungs by coughing). Tuberculosis can usually be cured by taking several powerful antibiotics daily for several months."

"Researchers have found that a minority of patients actually transmit tuberculosis, that the infectiousness of transmitters varies greatly, and that effective antibiotic treatment decreases infectiousness. Since the 1950s, however, multidrug-resistant (MDR) and more recently extensively drug-resistant (XDR) strains of M. tuberculosis have become widespread."

"Treatment of drug-resistant tuberculosis is much more difficult than normal tuberculosis, requiring even more antibiotics, and for long periods, up to 2 years and beyond. In addition, HIV weakens the immune system and as a result, HIV-positive people are much more likely to develop active tuberculosis (and to die from the disease, which also speeds the development of HIV/AIDS) than people with a healthy immune system."

"Have these changes altered tuberculosis transmission between people? The answer to this question might help to optimize the control of tuberculosis infection, particularly in hospitals. In this study, the researchers investigated current patterns of tuberculosis infectiousness among HIV-positive patients by recreating the 1950s guinea pig model for tuberculosis transmission."

"During the 505 study days, although 97 HIV-positive patients with tuberculosis were admitted to the ward, just ten patients were responsible for virtually all the characterized cases of tuberculosis among the guinea pigs. Six of these patients had MDR tuberculosis that had been suboptimally treated. The average patient infectiousness over the entire study period was 8.2 quanta per hour-six times greater than the average infectiousness recorded in the 1950s. Finally, the three most infectious patients (all of whom had suboptimally treated MDR tuberculosis) produced 226, 52, and 40 quanta per hour."

"These findings show that a few inadequately treated HIV-positive patients with MDR tuberculosis caused nearly all the tuberculosis transmission to guinea pigs during this study. They also show for the first time that tuberculosis infectiousness among HIV-positive patients is very variable. The increase in the average patient infectiousness in this study compared to that seen in the 1950s hints at the possibility that HIV infection might increase tuberculosis infectiousness. However, studies that directly compare the tuberculosis infectiousness of HIV-positive and HIV-negative patients are needed to test this possibility."

"More importantly, this study demonstrates the potentially high infectiousness of inadequately treated MDR TB patients and their importance in ongoing TB transmission. These findings suggest that rapid, routine testing of antibiotic susceptibility should improve tuberculosis control by ensuring that patients with MDR TB are identified and treated effectively and quickly."

Finally, the study authors re-emphasized the importance of implementing environmental control measures (for example, adequate natural or mechanical ventilation of tuberculosis wards, crowded waiting rooms, or emergency departments where tuberculosis patients may be found) to prevent airborne tuberculosis transmission in health-care facilities, particularly in areas where many patients are HIV positive and/or where MDR tuberculosis is common.

8/18/09

Reference
AR Escombe, DA Moore, RH Gilman, and others. The Infectiousness of Tuberculosis Patients Coinfected with HIV. PLoS Medicine 5(9): e188. September 2008. (Free full text).