Tuberculosis Pathophysiology

Updated: May 24

Tuberculosis (TB) is an airborne infection caused by a tiny pathogen called Mycobacterium Tuberculosis. In most cases, the TB infection targets the lung parenchyma but the mycobacterium can sometimes travel to the meninges, kidneys, bones and lymph nodes.


In these notes, we’ll be going over the Tuberculosis Pathophysiology and all the manifestations of TB. But before getting into that make sure that you’ve understood the Anatomy and Physiology of the Respiratory System


 

Tuberculosis Pathophysiology

Tuberculosis infections start off when an individual inhales a Mycobacterium that would have been transmitted through air particles. Once inhaled, the bacterium travels down the airways and settles into the alveoli. Here, the Mycobacterium starts to multiply, creating identical bacteria that travel through the lymphatic and circulatory systems and find their way into other areas of the lung or other organs in the body.


As the infection starts to spread, it triggers an inflammatory reaction which releases Phagocytes and TB-specific lymphocytes to fight it off. The Phagocytes capture the TB bacteria and stop them from multiplying further, and the TB-specific lymphocytes break down the bacilli. But, in doing so they also damage the normal tissue and the body is left with the remains of the dead bacilli and damaged healthy tissue. Eventually, the remains start to pile up and form an exudate inside the alveoli, which then leads to bronchopneumonia.


As the inflammation continues, the exudate from the dead bacilli and the live bacteria start to stick together and they create Granulomas. As soon as they develop, the granulomas are immediately surrounded by Macrophages which create a protective wall around them. From there, the Granulomas and Macrophages fuse and transform into a mass of fibrous tissue, with the inner part referred to as Ghon Tubercle.


If the infection is left untreated, the bacteria and macrophages inside the granuloma become necrotic. They break down and create a cheesy mass that preserves the bacteria and temporarily stops the active disease. If the patient has a weak immune system or is exposed to TB again the dormant bacteria are activated and the infection repeats itself. Moreover, apart from the patient getting reinfected, they also become contagious and they can spread the airborne bacteria onto others around them.


Tuberculosis Epidemiology

Because TB is so easily transmitted, the number of worldwide cases are very high. In 2020, roughly 10 million individuals contracted TB, and nearly 1.5 million of them died from the infection. TB infection can be detected all over the globe, however, the majority of the cases are located in developing countries within South-East Asia and Africa.


Efforts are currently being made to lower the risk of infection with TB. In fact, between 2015 and 2020, the rate of infection with TB fell by 11%, which means that the infection is declining at a steady rate of 2% per year. Moreover, the United Nations Sustainable Development Goals (SDG) are continuously striving to end the pandemic by 2030.


The Symptoms of Tuberculosis

When a patient first comes in contact with Mycobacterium Tuberculosis, they would likely not notice it. The infection can take anywhere from 2 to 10 weeks to start showing manifestations, and once it does the initial symptoms are usually mild. In most cases, patients would have

  1. Low-grade fever

  2. Cough

  3. Night sweats

  4. Fatigue

  5. Weight loss

The cough differs from one patient to another, some might present with a dry and non-productive cough while others can have mucopurulent sputum, or Haemoptysis (coughing up blood).


The Risk Factors of Tuberculosis

The biggest risk factor to contract TB is being in close contact with an individual who has active TB because the bacteria from their lungs travel out during exhalation. This makes it very likely for those around them to inhale the infected air particle and harbour the infection.


Individuals that have a compromised immune system such as those with HIV, cancer patients and organ transplant patients have a higher risk of acquiring the infection and developing severe symptoms. Similarly, people with substance abuse or alcohol addiction are more likely to get infected.


TB is also considered to be more common in overcrowded and underdeveloped countries, where health care is not adequate. And lastly, health care workers who are in contact with TB patients and those administering aerosol treatments are also at an increased risk.

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