Tuberculosis - causes, symptoms, diagnosis, treatment, pathology

Summary

Tuberculosis (TB), caused by Mycobacterium tuberculosis, primarily affects the lungs but can spread throughout the body. The majority of infections remain latent, with symptoms emerging when the immune system weakens. TB is transmitted via inhalation and evades immune defenses, forming granulomas that can calcify over time. If reactivated, TB can cause severe complications. Testing includes the PPD skin test or IGRA blood test, with chest X-rays for active TB. Treatment involves a combination of antibiotics, and multi-drug-resistant TB (MDR-TB) or XDR-TB requires special regimens. New treatments show promise for hard-to-treat cases.

Takeaways

• 😀 TB (tuberculosis) is an infection caused by Mycobacterium tuberculosis, primarily affecting the lungs.
• 😀 About two billion people worldwide are infected with TB, but most are asymptomatic or unaware due to the bacteria being dormant.
• 😀 Mycobacterium tuberculosis is a 'strict aerobe,' meaning it requires oxygen to survive and thrives in areas with high oxygen levels.
• 😀 The bacteria have a waxy cell wall containing mycolic acid, making them acid-fast and resistant to weak disinfectants.
• 😀 TB spreads via inhalation, and once inside the lungs, it can be engulfed by macrophages but often survives and proliferates inside them.
• 😀 Primary TB may cause mild symptoms, but in most cases, the immune system forms a granuloma to contain the infection.
• 😀 A 'Ghon complex' is formed when TB spreads to the lymph nodes, and calcification of the granuloma results in a 'Ranke complex' seen in X-rays.
• 😀 If the immune system is weakened (e.g., by AIDS or aging), latent TB can reactivate, leading to active TB with symptoms like fever, night sweats, and coughing.
• 😀 TB can spread to other parts of the body, causing complications like kidney disease, meningitis, and liver issues, often referred to as systemic miliary TB.
• 😀 TB testing includes the PPD intradermal skin test or IGRA blood test, and active TB can be confirmed via sputum sample analysis, including PCR.

Q & A

What is tuberculosis (TB) and how widespread is it?

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis, primarily affecting the lungs. It is estimated that about two billion people worldwide are infected with TB, although not all show symptoms.

Why do most people infected with TB not show symptoms?

Most people (90-95%) infected with TB do not show symptoms because their immune system is usually able to contain the bacteria, keeping it latent or dormant. This means the bacteria do not multiply or cause disease unless the immune system is compromised.

What makes Mycobacterium tuberculosis unique compared to other bacteria?

Mycobacterium tuberculosis is a slender, rod-shaped, and oxygen-dependent bacterium. It has a waxy cell wall made of mycolic acid, which makes it 'acid-fast' and resistant to weak disinfectants, allowing it to survive on surfaces for months.

How does tuberculosis typically enter the body and what happens when it reaches the lungs?

Tuberculosis is usually transmitted through inhalation of airborne particles containing the bacteria. Once in the lungs, the bacteria are ingested by macrophages, immune cells that try to kill them. However, TB can prevent the fusion of the phagosome and lysosome, allowing it to survive and multiply within the macrophage.

What is a Ghon focus, and what happens during the primary tuberculosis infection?

A Ghon focus is an area of caseous necrosis (cheese-like tissue death) that forms when the immune system walls off the TB infection in the lungs. Primary tuberculosis usually results in mild or asymptomatic illness, but it can form this granuloma in response to the infection.

What is the significance of the Ghon complex in TB diagnosis?

The Ghon complex refers to the combination of a Ghon focus in the lung and caseous necrosis in nearby lymph nodes. It is characteristic of primary TB infection and can be detected on chest X-rays. A calcified Ghon complex is called a Ranke complex and indicates past infection.

How does TB become reactivated, and which parts of the lungs are most affected?

If a person’s immune system weakens (e.g., due to AIDS or aging), the latent TB infection can reactivate. TB bacteria typically re-emerge in the upper lobes of the lungs, which are areas with the highest oxygen concentration, preferred by TB as an aerobe.

What are the symptoms and complications of systemic miliary TB?

Systemic miliary TB occurs when TB spreads through the bloodstream to other organs, leading to complications such as kidney disease (sterile pyuria), meningitis, Pott disease (spinal infection), Addison’s disease, liver hepatitis, and scrofula (cervical lymphadenitis).

What tests are used to diagnose TB and how do they differ?

The Tuberculin Skin Test (TST) or PPD test and the Interferon Gamma Release Assay (IGRA) are both used to detect TB exposure. TST requires a follow-up visit to assess the reaction, while IGRA is a blood test that is more specific to TB and unaffected by the BCG vaccine.

How is active TB treated, and what is the role of directly observed therapy (DOT)?

Active TB is treated with a combination of antibiotics like isoniazid, rifampin, ethambutol, and pyrazinamide over several months. To ensure adherence and prevent resistance, patients are often placed on directly observed therapy (DOT), where a healthcare worker confirms medication intake.