Smoking raises the risk of developing TB disease: study

Published in The Hindu on August 24, 2009

Several studies have shown that smoking is a risk factor for developing the tuberculosis disease (TB). But there was no large-scale study done on the general population to understand this association as most studies looked at high-risk individuals to understand how smoking makes a person develop the disease.

A large-scale study involving 17,699 participants, aged above 12 years, in the general population from a nationally representative sample in Taiwan has shown a clear link between smoking and the TB disease.

The study found that smokers had a two-fold risk of developing the TB disease. The results are published online in the American Journal of Respiratory and Critical Care Medicine.

The subjects of the study had participated in Taiwan’s 2001 National Health Interview Survey (NHIS). They were followed up from 2001 to 2004.

Although smoking per se does not cause the TB disease, those infected with the TB bacteria run a greater risk of developing the disease if they are smokers. Smoking is hence considered as a risk factor for developing the TB disease. In Taiwan, much like in the case of India and other low- or middle-income countries, most people are infected by the TB bacteria (Mycobacterium tuberculosis).

Of the 17,699 participants, about 3,900 were current smokers (people who were smokers at the time of interview) and about 13,250 had never smoked. Active TB occurred in 24 current smokers compared with 30 in those who had never smoked.

The study observed significant dose-response relations for cigarettes per day, years of smoking and pack-years. “Based on our analysis, 17 per cent of incident TB cases in this population were attributable to smoking. When extrapolated to the national population, this translated into 2,841 cases among the 16,580 reported in Taiwan in 2005,” notes the paper.

What is significant in the Taiwan study is that unlike previous observational studies, this took into consideration the cofounding factors, such as alcohol use and socioeconomic status. HIV was not taken in to consideration as its prevalence in Taiwan was only 0.02 per cent at the end of 2003.

According to Dr. Hsien-Ho Lin, the lead author of the study from the Harvard School of Public Health, Boston, younger smokers were more likely to develop the disease than smokers older than 65. But the small number of TB cases prevented the team from examining the risk in the elderly in greater detail.

The study has great significance for India. Most people in India are infected with the TB bacteria. The prevalence of HIV is also higher at 0.34 per cent in the age group of 15-49 (2.3 million) here.

According to the World Health Organisation, the prevalence of TB disease in India in 2008 was over 3.4 million — about one-fifth of the global figure — making it the most TB disease-prevalent country, killing about 2 people every 3 minutes.