Smoking May Be A Risk Factor For Tuberculosis

About one-third of the world's population is infected with the bacteria that cause tuberculosis (TB), but the organism is usually rendered inactive by the immune system, according to background information in the article. Active tuberculosis developed in approximately 8.8 million individuals in 2003 and is responsible for about 1.7 million deaths worldwide each year. "It has long been suggested that tobacco smoking may affect rates of TB morbidity and mortality," the authors write. "This could be a result of increasing the risk of infection with TB mycobacteria, increasing the rate of active TB disease, or increasing the TB mortality rate; plausible mechanisms exist."

Michael N. Bates, Ph.D., and colleagues at the School of Public Health, University of California, Berkeley, performed a meta-analysis of 24 previously published studies that examined the association between smoking and TB. Six of the publications dealt with the risk for TB infection, 13 with active TB disease and five with death from TB.

Across the studies, individuals who smoked had a 73 percent increased risk of becoming infected with tuberculosis and were more than twice as likely to develop active tuberculosis than those who did not smoke. This suggests that smokers are 40 to 60 percent more likely than non-smokers to develop tuberculosis disease after being infected with TB bacteria. The excess risk of death from TB associated with smoking was less than the excess risk of TB disease, suggesting that smoking does not increase the risk of death among those who already have active TB.

There are several potential explanations for the association, although more research is needed to determine the mechanisms involved, the authors note. Smoking could decrease immune response or damage the function of cilia in the airways, increasing the risk for TB. "Tuberculosis control policies should in the future incorporate tobacco control as a preventive intervention," the authors write.

"Potentially, smoking is one of the most modifiable of exposures," they conclude. "In developing countries, where life expectancy is short, highlighting smoking as a risk factor for TB may have greater resonance than advertising its risks for cancer and cardiovascular disease."