Smoking increases the risk of contracting TB, scientists have found.
An analysis of 24 studies showed that smokers had a 73 per cent greater chance of being infected by tuberculosis than non-smokers.
For those already infected, the chances of developing active TB disease were 50 per cent greater for smokers.
Overall, a smoker had 2.5 times a non-smoker’s risk of contracting active TB.
Researchers pooled together data from the previously published studies to reveal an underlying trend that was previously hidden.
Dr Michael Bates, from the University of California at Berkeley, US, who led the research, said: ‘Our study is the first systematic, quantitative assessment of TB risks from smoking.
‘There have been mixed opinions on whether smoking has any relevance to TB. Our review and analysis of the research in this area indicates that there is a connection, and that smoking is a major risk factor for TB.’
Nearly two billion people – one third of the world’s population – are infected with Mycobacterium tuberculosis, the bacteria that cause TB.
The bug can remain dormant in the body for decades, kept in check by the immune system. But if the immune system is compromised, it can multiply and trigger active disease.
In 2003, almost 9 million people worldwide developed active tuberculosis. Each year an estimated 1.7 million people die from TB.
Smokers are susceptible
Professor Kirk Smith, from the University of California at Berkeley, a senior author of the paper published today in the journal Archives of Internal Medicine, said: ‘It could be that smoking suppresses the respiratory immune system, allowing latent infections to blossom. Smoking also seems to make people more susceptible to becoming infected in the first place.
‘Because it increases the number of active TB cases, we estimate that smoking is related to half a million of the 1.7 million TB deaths each year.’
Getting people to quit smoking could play an important role in efforts to control TB, said Prof Smith.
‘Currently smoking cessation is not part of TB control programmes,’ he said. ‘The evidence from this study suggests that it should be.’
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