The bacterium Mycobacterium tuberculosis is the direct cause of tuberculosis (TB), but smoking is a behavioral risk factor that increases a person’s risk of developing the disease. Tuberculosis is an infectious disease that primarily affects the lungs and spreads through airborne droplets from coughs or sneezes. Smoking does not directly cause TB, but it creates a favorable environment for the bacteria to establish an infection and progress into active disease.
The Connection Between Smoking and TB Infection
Inhaling tobacco smoke compromises the lung’s first lines of defense. The respiratory tract is lined with cilia, tiny hair-like structures that sweep mucus and pathogens out of the airways. Chemicals in cigarette smoke paralyze and destroy these cilia, impairing this clearance mechanism and allowing inhaled TB bacteria to settle deeper into the lungs.
This structural damage is compounded by immunological harm. Alveolar macrophages are specialized immune cells residing in the lung’s air sacs, or alveoli, that engulf and digest foreign invaders like bacteria. Components of cigarette smoke interfere with the function of these macrophages, reducing their ability to destroy Mycobacterium tuberculosis. This gives the bacteria a better opportunity to survive and multiply.
The quantity and duration of smoking have a direct relationship with the level of risk. Studies show a dose-response relationship, meaning that the more cigarettes a person smokes per day and the more years they have smoked, the higher their likelihood of acquiring a TB infection. This increased susceptibility is a result of cumulative damage to the lung’s physical barriers and immune cells.
From Latent TB to Active Disease
Upon successful infection, the body’s immune system often contains the bacteria, leading to a condition known as latent TB infection. In this state, the bacteria remain alive but are walled off in small capsules, unable to multiply or cause symptoms. A person with latent TB is not contagious, but this containment relies on a functioning immune system.
Smoking weakens the immune system systemically, not just locally in the lungs. Chemicals in tobacco smoke circulate through the bloodstream, impairing immune cells like T-cells, which are important for controlling TB. Smoking can also reduce the production of interferon-gamma (IFN-γ), a molecule needed to activate macrophages and control the mycobacteria.
This systemic immune suppression tips the balance in favor of the bacteria, making it more difficult for the body to maintain the dormant state of the infection. As a result, smokers with latent TB have a higher risk of the infection progressing to active TB disease, where the bacteria break free, multiply, and cause lung damage and symptoms like coughing, fever, and weight loss.
Impact on Tuberculosis Treatment and Outcomes
For individuals who develop active TB and begin treatment, continuing to smoke has negative consequences. The standard treatment for TB is a months-long course of antibiotics, and its success depends on clearing the bacteria. Smoking interferes with this process, often leading to a slower rate of bacterial clearance from the sputum.
This delayed response translates to poorer treatment outcomes. Smokers undergoing TB therapy are more likely to experience treatment failure, where the antibiotics do not fully eradicate the infection. They also face a higher probability of the disease recurring after the treatment course is completed, a condition known as relapse.
Furthermore, smokers with TB tend to experience more severe forms of the disease. They are more likely to develop cavities, or holes, in their lung tissue, a sign of advanced disease associated with a higher bacterial load. The combination of more severe disease and less effective treatment leads to a greater risk of death among TB patients.
The Role of Quitting and Secondhand Smoke
Quitting smoking can substantially improve a person’s health and TB-related risks. Ceasing tobacco use allows the lungs to begin repairing themselves. The function of cilia can gradually recover, and the performance of immune cells like macrophages can improve. This restoration of the body’s defenses can lower the risk of a latent TB infection reactivating and increases the chances of a successful outcome for those undergoing treatment.
The dangers of tobacco smoke are not limited to the person smoking. Exposure to secondhand smoke, also known as passive smoking, also damages the respiratory tract and weakens the immune system. This puts non-smokers, particularly children and other household members of smokers, at an elevated risk of being infected with TB if exposed to the bacteria. Children are especially vulnerable due to their developing immune systems and smaller airways.