Association Between Smoking and Tuberculosis

What Is Tuberculosis?

Tuberculosis is a bacterial infection caused by Mycobacterium tuberculosis. It primarily affects the lungs. However, it can also target other parts of the body, like the spine, kidneys, and brain. There are two main types of TB conditions:

Latent TB Infection: In this case, TB bacteria are present in the body but do not cause active illness or symptoms. It remains dormant and non-contagious.

TB Disease (Active TB): This occurs when the TB bacteria become active and lead to illness. It can be a life-threatening condition if left untreated. It is primarily caused by a bacterium called Mycobacterium tuberculosis. The infection spreads through the air when individuals with TB disease in their lungs or throat cough, sneeze, talk, or sing, releasing the bacteria into the surrounding air. Inhalation of the contaminated air can lead to TB infection. TB is not transmitted through touch, kisses, or sharing food or utensils.

The risk of contracting TB is higher when in close proximity to someone with TB disease, such as living or working together for extended periods. Certain individuals are more likely to develop TB disease after being infected. They include:

• People with HIV/AIDS, their weakened immune systems are less able to control TB bacteria.

• Individuals who have recently been infected with TB bacteria (within the last two years).

• People with underlying health conditions, such as diabetes, compromise the body's ability to fight off TB bacteria.

• Those with alcohol use disorder or who inject illegal drugs.

• Individuals who were previously treated for TB but did not complete the full course of medication.

• Children under the age of 5 and older adults may have weaker immune systems.

What Is the Association Between Smoking and Tuberculosis?

Tuberculosis (TB) is a leading cause of mortality, particularly in developing nations. Smoking tobacco is a widespread habit, predominantly prevalent in low- and middle-income countries where the TB burden is also high. It is estimated that around 1.3 billion individuals are tobacco smokers worldwide. The World Health Organization has reported that smoking accounts for approximately 9 % of all deaths globally. Studies have established a connection between tobacco smoking and TB infection, as well as the negative impact of smoking on TB prognosis.

Numerous studies have demonstrated a significantly higher incidence of TB and increased mortality among current smokers compared to non-smokers during follow-up periods. Smoking may also heighten the risk of TB relapse by facilitating the persistence of Mycobacterium tuberculosis infection even after treatment. Additionally, smoking augments the risk of any remaining M. tuberculosis bacilli promoting infection and subsequent disease development.

Of the various organ systems affected by smoking, the lungs are particularly vulnerable to damage. Smoking impairs lung function and weakens the body's immune system, rendering smokers more susceptible to TB infection. The occurrence of TB has been linked to altered immune responses and defects in immune cells such as macrophages, monocytes, and CD4 lymphocytes. Other mechanisms, such as the disruption of cilia function and hormonal effects, may also arise as secondary effects of smoking. Collectively, these factors contribute to an increased susceptibility to TB infection in individuals who smoke.

How Does Smoking Increase the Risk of Developing Tuberculosis?

• Cigarette smoke plays a significant role in the development of tuberculosis by affecting various aspects of the immune response and promoting conditions favorable for Mycobacterium tuberculosis infection.

• One of the effects of cigarette smoke is ciliary dysfunction, which impairs the clearance of bacteria from the respiratory tract. Additionally, smoking leads to reduced immune response and defects in the immune response of macrophages, making individuals more susceptible to M. tuberculosis infection.

• In the lungs, alveolar macrophages bind to the tuberculosis bacillus through complement receptors. Activated lymphocytes release cytokines that recruit macrophages, fibroblasts, and other lymphocytes.

• TNF-α, a major cytokine involved in granuloma formation, is released by macrophages upon exposure to M. tuberculosis antigens. TNF-α activates macrophages and dendritic cells, playing a protective role. However, in smokers, nicotine acts through the α7 nicotinic receptor to reduce the production of TNF-α by macrophages, thereby preventing its protective action and promoting tuberculosis development.

• Another aspect of the immune response against tuberculosis is the production of IFN-γ by natural killer cells induced by IL-12 secretion from macrophages. The Th1 response targets to destroy M. tuberculosis by forming a fibrous granuloma.

• Cigarette smoke selectively impairs the production of interleukin-12 and TNF-α, hindering granuloma formation and allowing the infection to progress towards active tuberculosis in immunocompetent individuals.

• Mortality rates related to tuberculosis are significantly higher in smokers compared to never-smokers. Among individuals without a history of tuberculosis, smokers face a nine-fold higher risk of death due to tuberculosis compared to never-smokers.

• Recent studies have shown that smoking and HIV infection are significant risk factors for mortality in patients with multidrug-resistant tuberculosis (MDR-TB). However, quitting smoking can significantly reduce the risk of death due to tuberculosis. Those who quit smoking experience a 65 % decrease in tuberculosis-related mortality compared to those who continue smoking, emphasizing the importance of smoking cessation in reducing the impact of tuberculosis.

• Both active and passive exposure to cigarette smoke is linked to an elevated risk of Mycobacterium tuberculosis infection and the development of active tuberculosis. A comprehensive qualitative systematic review conducted in 2007 demonstrated a strong association between smoking and active tuberculosis. Additionally, the review found a moderate correlation between passive smoking and active tuberculosis, as well as the need for retreatment. Considering the impact of parental smoking on children's respiratory infections, it has become customary to investigate parental smoking history in cases of such infections.

• Furthermore, a recent study revealed that children residing in tuberculosis-endemic regions faced an increased risk of M. tuberculosis infection, and parental smoking was significantly associated with the risk of developing active tuberculosis in children, even after adjusting for other associated factors. Therefore, the effects of passive smoking are also concerning in relation to active tuberculosis. It is crucial to educate individuals with tuberculosis who smoke about the harm their addiction can cause to others, particularly their close contacts who are at a higher risk of contracting active tuberculosis.

Conclusion:

Extensive research and findings have solidified the link between smoking and tuberculosis (TB). While suspicions of this connection have long existed, new evidence confirms that smoking not only affects the risk of TB infection but also increases the likelihood of developing active TB and experiencing fatal outcomes. Furthermore, smoking has been shown to have detrimental effects on the response to treatment and raises the risk of relapse. The detrimental effects of smoking on lung health and immune function contribute to increased vulnerability to TB.