3.7: Pulmonary Tuberculosis II

Tuberculosis, or TB, is a bacterial infectious disease caused by Mycobacterium tuberculosis. While its primary impact is on the lungs, leading to pulmonary tuberculosis, it can also affect various other organs, a condition referred to as extrapulmonary tuberculosis.

Here is a detailed explanation of its pathophysiology:

Transmission: The process begins when a person inhales droplet nuclei containing M. tuberculosis. These are typically released into the air when an individual with pulmonary or laryngeal TB coughs, sneezes, or speaks.

Primary Infection: In this initial phase, inhaled bacteria reach the alveoli in the lungs, encountering alveolar macrophages. The macrophages engulf the bacteria, but the bacteria survive and replicate within instead of being destroyed.

Cell-Mediated Immune Response: Approximately 2-6 weeks post-infection, the body mounts a cell-mediated immune response. T cells recognize mycobacterial antigens presented by the infected macrophages. It triggers cytokine release, activating other immune cells and processes.

Granuloma Formation: Infected macrophages and T cells cluster together to form a granuloma, an immune response that aims to contain the spread of the bacteria. Within the granuloma, the bacteria can remain dormant (latent TB infection) for many years, causing no symptoms and not being contagious.

Progression to Active Disease: In about 5-10% of individuals with latent TB infection, it is asymptomatic and not contagious. In a latent TB infection, the pathogen is not always dormant, as the bacteria can still be alive but controlled by the immune system, and can still cause active TB disease. This commonly occurs when the immune system is weakened, as observed in cases of HIV infection. The bacteria multiply and spread, leading to tissue damage and the symptoms of TB.

Caseation and Cavitation: The caseation process can occur within the granuloma, where the center becomes necrotic and cheese-like, often worsening symptoms. If this caseous material erodes into a bronchus, it forms a cavity, which allows the bacteria to multiply rapidly.

Dissemination: From the lungs, the bacteria can spread through the bloodstream (miliary TB) or the lymphatic system to other organs, causing extrapulmonary TB.

It is worth noting that not everyone who contracts TB will develop an active disease. Many individuals have latent TB infection, which is asymptomatic and non-infectious.

There is a potential risk for them to develop active tuberculosis in the future, particularly if their immune system becomes compromised.

The pathophysiology of tuberculosis begins when a susceptible host inhales aerosolized droplets containing Mycobacterium tuberculosis.

The droplets reach the lungs' alveoli, where the bacteria multiply and encounter alveolar macrophages, which engulf them.

However, instead of destroying the bacteria, the macrophages allow them to survive and replicate within.

After 2-6 weeks, the body initiates a cell-mediated immune response, forming a specialized structure called a granuloma to contain the bacterial spread.

The granuloma comprises immune cells, live and dead bacterial cells, T-cells, and macrophages.

The bacteria can remain dormant within the granuloma, causing no symptoms and being non-contagious. This stage is known as latent tuberculosis infection.

If the immune system is compromised, latent tuberculosis can progress to active tuberculosis disease through the caseation process, where the center of a granuloma becomes necrotized, forming a cheese-like substance called caseum.

This caseous material then erodes into a bronchus, creating a cavity that facilitates rapid bacterial proliferation and the potential spread of the infection through blood or lymph.

• Tuberculosis - A bacterial infectious disease predominantly affecting the lungs.
• Extrapulmonary Tuberculosis - TB that affects body organs other than lungs.
• Pulmonary Tuberculosis - TB primarily impacting lungs, caused by Mycobacterium tuberculosis.
• Miliary TB - Form of TB where bacteria spread through bloodstream affecting multiple organs.
• Latent TB infection - A state where TB bacteria remain dormant but alive in the host's body.

• Define Tuberculosis - Explain its impact on lungs and other organs (e.g., Pulmonary and Extrapulmonary Tuberculosis)
• Contrast Latent TB vs Active TB - Explain differences in disease progression (e.g., immune response and symptom presentation)
• Explore Miliary TB - Discuss its transmission and contagious nature (e.g., dissemination through bloodstream)
• Explain the pathophysiology of TB - Discuss its various stages from transmission to active disease
• Apply in the context of compromised immunity - Discuss why active disease may occur in people with weakened immune systems

• What is Tuberculosis and how does it differently impact the lungs and other organs?
• What is the difference between Latent and Active TB?
• How does Miliary TB spread and is it contagious?

• Medical Students – Understand how TB pathophysiology supports differential diagnosis and treatment plans
• Health Professionals - Detailed understanding aids in patient care and counseling
• Research scientists - Use this information to develop new treatment methods and vaccines for TB
• Public health officials – Insight into disease transmission aids effective control measures