What is Tuberculosis?

Tuberculosis (TB) is an infectious disease caused primarily by the bacterium Mycobacterium tuberculosis. It predominantly affects the lungs, although it can target various other body parts such as the kidneys, spine, and brain. Understanding TB’s biological basis can provide insights into its prevention, symptoms, and forms.

Biological Basis of Tuberculosis

The Bacterium: Mycobacterium tuberculosis

Mycobacterium tuberculosis is a slow-growing, aerobic bacterium that thrives in oxygen-rich environments, making the lungs its preferred habitat. This bacterium has a unique cell wall structure that is rich in lipids, particularly mycolic acids, rendering it resistant to the effects of many antibiotics and contributing to its virulence. The slow replication rate of TB bacteria means that the disease can develop gradually, leading to a protracted incubation period that can last from weeks to years.

Transmission

Tuberculosis is primarily spread through airborne droplets when an infected person coughs, sneezes, or talks. When these droplets are inhaled, the bacteria can settle in the lungs and begin to multiply. Potentially anyone can contract TB, but certain populations are at greater risk, including those with weakened immune systems, such as individuals with HIV/AIDS or those undergoing immunosuppressive therapies.

Immune Response

Once inhaled, Mycobacterium tuberculosis is engulfed by immune cells known as macrophages. The bacteria can survive and multiply within these cells, which is a key factor in their ability to cause disease. The body's immune response often leads to the formation of granulomas, which are clusters of immune cells that attempt to wall off the bacteria. This can contribute to the asymptomatic latent TB infection stage, where an individual carries the bacteria without displaying symptoms.

Latent vs. Active TB

  • Latent TB Infection (LTBI): Individuals with LTBI do not exhibit symptoms and are not contagious. However, they harbor live bacteria, which can reactivate and lead to active TB, particularly if the immune system becomes compromised.
  • Active TB Disease: When the bacteria overcome the immune response, active TB develops, leading to a range of symptoms. This can occur soon after infection or many years later.

Symptoms of Tuberculosis

The symptoms of tuberculosis can vary depending on which part of the body is affected, but the most common manifestations involve the respiratory system. General symptoms of active TB include:

Respiratory Symptoms

  1. Chronic Cough: A persistent cough lasting three weeks or longer is a hallmark of pulmonary TB.
  2. Hemoptysis: Coughing up blood or sputum that has a blood-tinged color is a serious symptom.
  3. Chest Pain: Discomfort or pain in the chest can occur when the lungs are affected.

Systemic Symptoms

Active TB can also present with systemic symptoms that affect the whole body:

  1. Fever: A low-grade fever that can escalate is common in individuals with active TB.
  2. Night Sweats: Profuse sweating, particularly at night, can be quite pronounced in those with tuberculosis.
  3. Weight Loss: Unexplained and significant weight loss is often reported.
  4. Fatigue: A general feeling of tiredness that does not improve with rest.

Extrapulmonary Tuberculosis

When TB spreads outside the lungs, it can lead to various complications in different parts of the body:

  1. Tuberculous Meningitis: Involvement of the brain and spinal cord, leading to neurological symptoms such as headaches, confusion, and seizures.
  2. Skeletal TB: Infection of bones or joints, particularly in the spine (Pott's disease), can lead to severe pain and disability.
  3. Lymphatic TB: Infection of the lymph nodes can cause swelling and discomfort, particularly in the neck.

Forms of Tuberculosis

Tuberculosis can present in different forms based on its location, the severity of the infection, and whether it is drug-resistant.

Pulmonary Tuberculosis

This is the most common form, affecting the lungs. It can be either primary TB, where the infection occurs for the first time, or secondary TB, which refers to reactivation of a previously dormant infection.

Extrapulmonary Tuberculosis

As previously mentioned, when TB spreads beyond the lungs, it can affect various organs. This can be classified as:

  • Lymphatic Tuberculosis: Involving lymph nodes, often leading to swelling and discomfort.
  • Genitourinary Tuberculosis: Affecting the kidneys, bladder, or reproductive organs, potentially causing symptoms of urinary tract infections.
  • Miliary Tuberculosis: A form of disseminated TB, where the bacteria spread throughout the body via the bloodstream, leading to small lesions in multiple organs.

Drug-Resistant Tuberculosis

The emergence of multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) poses significant challenges for treatment. MDR-TB is resistant to at least two of the most effective anti-TB drugs, isoniazid and rifampicin, while XDR-TB is resistant to a broader range of antibiotics. Factors contributing to drug resistance include incomplete treatment, inappropriate prescribing, and poor healthcare practices.

Diagnosis of Tuberculosis

Prompt diagnosis is crucial for effective management and to curb the spread of TB. Common diagnostic tools include:

  • Tuberculin Skin Test (TST): A classic method where a small amount of purified protein derivative (PPD) is injected under the skin, with a subsequent assessment of the skin reaction after 48 to 72 hours.
  • Interferon-Gamma Release Assays (IGRAs): Blood tests that measure the immune response to specific TB proteins.
  • Chest X-rays: To identify abnormalities in the lungs indicative of TB.
  • Sputum Tests: Microscopic examination and culture of sputum can confirm the presence of Mycobacterium tuberculosis.

Treatment of Tuberculosis

Effective treatment is paramount in managing TB. The standard regimen for drug-susceptible TB typically involves a combination of four antibiotics taken for at least six months:

  1. Isoniazid
  2. Rifampicin
  3. Ethambutol
  4. Pyrazinamide

For drug-resistant forms of TB, prolonged treatment with second-line drugs is necessary and may involve longer treatment durations and more severe side effects.

Conclusion

Tuberculosis remains a significant public health concern around the world. Understanding its biological basis, symptoms, and different forms is key to prevention, early detection, and effective treatment. Continued awareness and education remain crucial tools in combating this resilient disease.