Small Numbers Of Patients With Drug-resistant TB May Account For High Proportion Of New Infections

Every year, more than nine million people develop tuberculosis—a contagious infection usually involving the lungs—and nearly two million people die from the disease. The bacteria that cause TB are spread in airborne droplets when people with the disease cough or sneeze.

Adapting a detection system used in classic 1950s experiments, Rod Escombe of Imperial College London, and colleagues in Perú and the UK investigated airborne infectious tuberculosis in a hospital in Lima. By venting air from an HIV-tuberculosis ward through a guinea-pig enclosure on the hospital roof, and genetically matching the strains of TB infecting the guinea pigs to those cultured from patients, the investigators were able to determine which patients and strains accounted for most of the guinea pig infections.

They found that only 8.5% of the 118 admissions to the HIV-TB ward during the 505-day study accounted for 98% of the guinea pig TB infections. Ninety percent of the infections were traced to patients with TB that was resistant to multiple antibiotics, and a small number of such patients were found to have coughed out very high numbers of infectious bacteria.

This study demonstrates the potentially high infectiousness of inadequately treated patients with multiply drug-resistant tuberculosis (MDR-TB) and HIV infection, and suggests that rapid, routine testing for antibiotic susceptibility should improve TB control by ensuring that patients with drug-resistant TB are identified and treated effectively and quickly. The results also confirm the importance of implementing TB infection control measures (for example, provision of isolation rooms, and adequate natural or mechanical ventilation of tuberculosis wards, crowded waiting rooms or emergency departments where tuberculosis patients may be found) to prevent airborne TB transmission in health-care facilities, particularly in areas where many patients are HIV positive and/or where MDR-TB is common.