PCL 13

What is Tuberculosis?

Tuberculosis (TB) is an infection caused by a bacteria called the tubercle bacillus or Mycobacterium tuberculosis. Until effective anti-tuberculosis drugs were introduced about 50 years ago, TB was one of the main causes of death. TB is still a major problem in many countries.
It can affect the lungs (pulmonary TB) or other parts of the body, such as the lymph nodes (tuberculous adenitis or scrofula), the skin and the bones. Tubercle bacilli can remain dormant for years before producing active disease. In most cases lung infection is well controlled by the immune system, and shows no symptoms. Active lung disease occurs if the immune system becomes less effective.
Also:
- Latent TB. In this condition, you have a TB infection, but the bacteria remain in your body in an inactive state and cause no symptoms. Latent TB, also called inactive TB or TB infection, isn't contagious.

- Active TB. This condition has clinical symptoms and signs and is contagious

Pathophysiology
Tubercle bacilli initially cause a primary infection, which only rarely causes acute illness. Most (about 95%) primary infections are asymptomatic and followed by a latent (dormant) phase. However, a variable percentage of latent infections subsequently reactivate with symptoms and signs of disease. Infection is usually not transmissible in the primary stage and is never contagious in the latent stage.

Primary infection: Infection requires inhalation of particles small enough to traverse the upper respiratory defenses and deposit deep in the lung, usually in the subpleural airspaces of the lower lung. Large droplets tend to lodge in the more proximal airways and typically do not result in infection. Infection usually begins from a single initial focus.
To initiate infection, tubercle bacilli must be ingested by alveolar macrophages. Tubercle bacilli that are not killed by the macrophages actually replicate inside them, ultimately killing the host macrophage (with the help of CD8 lymphocytes); inflammatory cells are attracted to the area, causing a focal pneumonitis that evolves into the characteristic tubercles seen histologically. In the early weeks of infection, some infected macrophages migrate to regional lymph nodes, where they access the bloodstream. Organisms may then spread hematogenously to any part of the body, particularly the apical-posterior portion of the lungs, epiphyses of the long bones, kidneys, vertebral bodies, and meninges.

In 95% of cases, after about 3 wk of uninhibited growth, the immune system suppresses bacillary replication before symptoms or signs develop. Foci of infection in the lung or other sites resolve into epithelioid cell granulomas, which may have caseous and necrotic centers. Tubercle bacilli can survive in this material for years; the balance between the host's resistance and microbial virulence determines whether the infection ultimately resolves without treatment, remains dormant, or becomes active.

Activation of the disease:
In about 10% of immunocompetent patients, latent infection develops into active disease, although the percentage varies significantly by age and other risk factors. In 50 to 80% of those who develop active disease, TB reactivates within the 1st 2 yr, but it can occur decades later. Any organ initially seeded may become a site of reactivation, but reactivation occurs most often in the lung apices, presumably because of favorable local conditions such as high O2 tension.


Signs and symptoms of active TB include:

- Unexplained weight loss
- Fatigue
- Fever
- Night sweats
- Chills
- Loss of appetite

Tuberculosis usually attacks your lungs. Signs and symptoms of TB of the lungs include:
-Coughing that lasts three or more weeks
- Coughing up blood
-Chest pain, or pain with breathing or coughing

Tuberculosis can also affect other parts of your body, including your kidneys, spine or brain. When TB occurs outside your lungs, symptoms vary according to the organs involved. For example, tuberculosis of the spine may give you back pain, and tuberculosis in your kidneys might cause blood in your urine