3.6 : Pulmonary Tuberculosis I

Tuberculosis, often called TB, is a contagious illness primarily caused by Mycobacterium tuberculosis. It mainly affects the lung parenchyma but can also impact other body parts.

Causative Organism

The primary infectious agent causing tuberculosis is Mycobacterium tuberculosis, a slow-growing, acid-fast, aerobic rod that exhibits sensitivity to heat and ultraviolet light. Instances of Mycobacterium bovis and Mycobacterium avium contributing to the development of TB infection are rare.

Mode of Transmission

Mycobacterium tuberculosis primarily spreads through respiratory droplets generated when infected individuals breathe, talk, sing, sneeze, or cough. As the droplets evaporate, tiny nuclei measuring 1 to 5 µm remain suspended in the air for varying durations, ranging from minutes to hours. Another person can acquire the infection by inhaling these droplet nuclei. It is important to note that TB is not highly contagious, and transmission generally occurs with close, frequent, or prolonged exposure. Brief contact with a few tubercle bacilli rarely leads to infection. It is worth mentioning that TB cannot be transmitted through touch, sharing of food utensils, kissing, or any other form of physical contact.

Various factors increase the transmission probability, such as the number of microorganisms released into the air. The concentration of these microorganisms is higher in small and poorly ventilated spaces, like prisons and homeless shelters. The length of time of exposure also influences transmission. Additionally, the individual's immune system can impact the likelihood of transmission.

While anyone can contract TB, certain risk factors significantly increase the likelihood of transmission, including the following.

• Close contact with an individual with active TB increases the risk of inhaling airborne nuclei. The level of risk depends on the duration of exposure, proximity, and ventilation conditions.

• For instance, Healthcare workers are involved in high-risk activities, such as administering aerosolized medications, bronchoscopy, suctioning, caring for immunocompromised patients, and conducting anesthesia-related procedures like intubation and suctioning.

• Immunocompromised status encompasses individuals with HIV infection, cancer, organ transplant recipients, and those undergoing prolonged high-dose corticosteroid therapy. These conditions render the immune system vulnerable, necessitating special care and attention.

• Individuals who abuse alcohol or use intravenous drugs are at an increased risk of developing substance use disorder, which impairs their immune function and weakens their ability to fight infections, such as tuberculosis. Intravenous drug use, particularly through needle sharing, significantly increases the risk of contracting HIV. When left untreated, HIV causes a substantial reduction in CD4+ T-cells—cells that play a critical role in the immune response. This severe immunosuppression makes individuals far more vulnerable to opportunistic infections, including TB.

• Certain medical conditions or specialized treatments can heighten vulnerability. These include diabetes, chronic kidney disease, malnourishment, specific malignancies, hemodialysis, organ transplantation, gastrectomy, and jejunoileal bypass.

• Additionally, living in overcrowded and substandard housing, institutionalization, immigration from or recent travel to countries with a high prevalence of TB, and limited healthcare access are all factors that contribute to a higher risk of tuberculosis.