Letters to the Editor
To the Editors:
Sridhar et al1 contribute to the body of evidence that associates smoking with tuberculosis (TB).2 They report an association between the presence of a smoker in the household and higher rates of childhood infection with Mycobacterium tuberculosis (Mtb), as estimated by interferon-gamma release assay. They assume this to be because of an effect of passive smoking.
Infection with Mtb follows a complex process through which viable bacilli pass from a source case of tuberculosis, transit through the air and enter the distal lungs to evade the immune defenses of a new host. Household smoking may influence this in several ways. Tobacco smoke, whether primary or secondary, may make TB source cases more infectious, either by increasing the aerosolization of bacilli through effects on airway mucus3 and increasing cough frequency,4 or by lengthening the delay in seeking medical attention because of a greater tolerance of respiratory symptoms.5 It is theoretically possible that particulate matter produced by smoking favors successful passage of Mtb through the air, for example, by providing protection from ultraviolet light. Cigarette smoke also increases susceptibility to Mtb infection in the secondary host by effects on the immune response.2
Although household smoking seems to be associated with increased transmission of Mtb, Sridhar et al.’s1 data do not allow us to determine whether this is because of effects on the infectiousness of the TB source case, on the aerial transit of Mtb or on susceptibility to infection from passive smoking. Further insights into TB transmission are required.
Richard Douglas Turner, MB ChB
Graham Henry Bothamley, PhD
Department of Respiratory Medicine
Homerton University Hospital NHS
London, United Kingdom
1. Sridhar S, Karnani N, Connell DW, et al. Increased risk of Mycobacterium tuberculosis
infection in household child contacts exposed to passive tobacco smoke. Pediatr Infect Dis J. 2014;33:1303–1306
2. Bothamley GH. Smoking and tuberculosis: a chance or causal association? Thorax. 2005;60:527–528
3. King M, Wight A, DeSanctis GT, et al. Mucus hypersecretion and viscoelasticity changes in cigarette-smoking dogs. Exp Lung Res. 1989;15:375–389
4. Turner RD, Bothamley GH. Cough and the transmission of tuberculosis. J Infect Dis. 2015;211:1367–1372
5. Bam TS, Enarson DA, Hinderaker SG, et al. Longer delay in accessing treatment among current smokers with new sputum smear-positive tuberculosis in Nepal. Int J Tuberc Lung Dis. 2012;16:822–827