Aetiology and pathophysiology involved in TB and leprosy

Tuberculosis (TB) and leprosy are both chronic infectious diseases caused by different types of bacteria, but they have distinct aetiologies and pathophysiologies.

Tuberculosis (TB)

Aetiology:

• Pathogen: Mycobacterium tuberculosis
• Transmission: Primarily through airborne droplets from an infected person (e.g., coughing or sneezing). It can also spread through direct contact with infected bodily fluids or tissues.

Pathophysiology:

1. Inhalation of Bacteria: M. tuberculosis enters the lungs through inhalation and is engulfed by alveolar macrophages.
2. Immune Response: The bacteria can evade destruction inside the macrophages. They trigger an immune response leading to the formation of granulomas (tubercles) in the lungs.
3. Granuloma Formation: The immune system attempts to contain the infection by forming a granuloma, which consists of macrophages, lymphocytes, and other immune cells.
4. Caseous Necrosis: Within the granuloma, some bacteria may persist, leading to caseous necrosis (a cheese-like appearance of dead tissue). This can result in tissue damage and loss of lung function.
5. Latency and Reactivation: The infection can remain dormant (latent TB) in the granulomas for years without symptoms. It can reactivate later, particularly if the immune system becomes compromised.

Leprosy

Aetiology:

• Pathogen: Mycobacterium leprae
• Transmission: Likely through prolonged close contact with an infected person, though the exact mode of transmission is not well understood. It may also involve nasal droplets or skin contact.

Pathophysiology:

1. Infection and Incubation: M. leprae infects the skin and peripheral nerves. It has a long incubation period, often several years, before symptoms appear.
2. Nerve Damage: The bacteria primarily target peripheral nerves, leading to nerve damage and loss of sensation. This is because M. leprae has a preference for cooler areas of the body, such as the extremities.
3. Immune Response: The body’s immune response can vary:
   • Tuberculoid Leprosy: Characterized by a strong cell-mediated immune response, resulting in fewer skin lesions and nerve damage.
   • Lepromatous Leprosy: Associated with a weaker immune response, leading to widespread skin lesions, deformities, and severe nerve damage.
4. Clinical Manifestations: Symptoms include skin lesions, loss of sensation in affected areas, and deformities due to repeated injury or infection of the damaged tissues.

Both diseases can lead to significant morbidity if left untreated, but with appropriate antibiotic treatment, they are manageable.