Tuberculosis Pathogenesis

Introduction

Tuberculosis is a serious, infectious and often fatal disease of the lungs. It can also affect other parts of the body. But the most common type of Tuberculosis is pulmonary Tuberculosis. It is more common in women than in men. The causative agent is a Gram-positive bacterium (Mycobacterium tuberculosis).

Types Of Tuberculosis

TB manifests as:

• Latent TB
• Active TB

Latent Tuberculosis

It is estimated that about 2 billion people worldwide are affected with Mycobacterium Tuberculosis. That’s a huge number. But 90-95% of this population isn’t even aware that they have it. This means that the Immune system has contained it such that symptoms don’t appear. A dormant or latent stage is such a condition of the body where the microorganism resides within the system but doesn’t manifest its effects. This condition is termed as Latent Tuberculosis.

Active Tuberculosis infection

Sometimes, the immune system debilitates due to AIDS or progressing age. A weakened immune system can’t fight off the Tuberculosis bacterium. The pathogen reactivates and multiplies rapidly within the system, most often in the lungs. This is active TB – the form of pathogenic infection that manifests itself clearly.

Mycobacterium Pathogenesis

• Owing to its strength and integrity, the mycobacterium deserves a brief introduction. Mycobacterium tuberculosis and its related members(which also cause Tuberculosis but to a lesser extent) have a hard, waxy cell wall. This wall is resistant to unfavorable conditions. It is due to its hardiness that the Mycobacterium can survive nearly anything. It goes into a resting stage. In this way, it can stay alive for years and is only activated in an unhygienic environment.
• The bacterium gains entry into the body through the respiratory tract. Another strengthening factor for TB is that it can easily evade the mucous membrane of the air passageways. This mucous membrane can easily bind the other foreign particles and get rid of them later. But not TB. TB is a stubborn traveler least affected by the traps in its path. It reaches the deep airways and alveoli.

Formation Of Gohn Focus

• Once within the lungs, the alveolar macrophages recognize the foreign proteins (of TB) and phagocytose them. Another mode of defense is the formation of a phagosome. A phagosome is formed when the macrophage packages the Tuberculosis into a close space. The phagosome then fuses with a lysosome.
• Lysosomes are rich in hydrolytic enzymes that can break down any biochemical molecule. TB is a clever organism. It secretes a substance that inhibits fusion with a lysosome, thus leading to its survival. It doesn’t just survive, though. This smart guy proliferates by taking over the cellular machinery of the host. A localized infection is caused. At this point, the patient has primary TB.
• After initial exposure, the patient shows mild symptoms. At first, he develops a mild flu-like illness.
  Three weeks after initial infection, cell-mediated immunity kicks in. The cells of the immune system surround the bacterial cells completely, separating it from the surrounding tissue and forming a granuloma. This is done to avoid further spread of infection. Unfortunately, the tissue at the site of granuloma formation dies. This process is called Caseous Necrosis (Caseous meaning cheese and referring to the soft, degenerated tissue) and the area where it occurs is designated as the Gohn focus.

Dissemination Of Tuberculosis

• The Bacterial cells travel to nearby lymph nodes either directly or via an extension of the Gohn focus and infect them as well. The nodes combined with the Gohn focus are collectively called a Gohn complex. The Gohn complexes are usually subpleural and form in the lower part of the lungs. The tissue of the lungs also undergoes fibrosis and calcification. This calcified region of the lungs displays as a scar tissue on the chest Xray.
• The TB can crawl to the upper lobes of the lungs if or when the conditions are favorable. It is an aerobe and the upper lobes have a higher concentration of oxygen. In case of previous exposure, the memory cells quickly come into action. They release cytokines which form more areas of caseous necrosis.
• These granulomas can travel to other parts of the lungs by forming cavities. In this case, the patient will have bronchopneumonia. At other times, it disseminated by the vascular route and tours the other parts of the body. It is called Systemic Miliary Tuberculosis if it moves to other body tissues.

Infection Of Other Organs

When TB spreads to other tissues, it causes complications related to the organs affected.

Renal Tuberculosis

In renal TB, the bacteria travel to the kidneys and urinary tract. The patients have sterile pyuria accompanied by microscopic hematuria. There occurs an increase in the level of blood cells.

TB of Cranial Meninges

When active infection occurs in the meninges(the protective layers of the cranium that contain the CSF), then it causes meningitis.

TB Of The Vertebrae

The TB infection of lumbar vertebrae is called Pott’s disease after the British surgeon, Percivall Pott. It causes arthritis of the intervertebral joints. The lower thoracic and upper lumbar vertebrae are the areas of the spine most affected.

Hepatitis:

In the liver, TB causes hepatitis.

TB Of The Adrenal Glands

In case the TB spreads to adrenal glands, it manifests as Addison’s disease. Addison’s disease is such a disorder in which the secretion of the adrenal gland is decreased. The body goes low on cortisol and aldosterone. The patient experiences extreme fatigue and weight loss

Scrofula in The Neck

Scrofula or King’s Evil is the infection of the lymph nodes by TB or non-TB mycobacteria. It is also called Lymphadenitis of the neck. A fatal but painless mask expands in the neck which grows over time. It is characterized by fever, chills, malaise and weight loss.

Conclusion

This is how the TB infection spreads and exerts its effect over the principal regions of the body. It disseminates either the through the lymphatics or the vascular supply and can lead to death if the treatment is not strictly administered. TB has a wide range of wicked and cruel symptoms. Doctors use a Mantoux Test for its diagnosis in addition to AFB and Chest X-ray. For a look at the detailed Symptoms and Cure of TB, read our detailed article Tuberculosis Symptoms And Cure.

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