Tuberculosis

Tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis, and can affect any part of the body, but most commonly the lungs (pulmonary).

Only TB of the lung or of the throat can be infectious.  This infection is generally transmitted by prolonged and/or frequent contact with an infected person.  The bacteria enters the body through the lungs by breathing.  It can however travel through the blood and infect other organs causing non-pulmonary forms of TB. Diagnosis of these non-pulmonary forms can be more difficult, as the person will not display the cough so commonly associated with TB.

A few years ago TB was thought to be a disease of the past, particularly in the developed world. However,even in the United States the disease continues to be a significant challenge due to:

• Immunosuppressive conditions (HIV, diabetes, hematological disorders, chronic renal disease, immunosuppressive therapy)
• Immigration
• Increasing non-compliance with testing and treatment protocols

To view the relationship between active disease and latent infection click on the thumbnail.

It is important to define the difference between tuberculosis infection (latent TB infection or LTBI) and tuberculosis disease:

Tuberculosis infection (latent TB infection) occurs when people carry small numbers of dormant Mycobacterium tuberculosis bacilli in their body. Dormant bacteria are normally controlled by the immune system and do not cause disease or any TB symptom.

People with latent TB infection are not infectious. They would be negative to most TB tests including culture, Nucleic Acid Amplification Tests (NAAT), and smear microscopy. Small nodules are occasionally seen on chest x-ray. Until recently the only in vitro test capable of identifying LTBI has been the tuberculin skin test (TST).

Tuberculosis disease (active TB disease) occurs when the bacterial load is increased and overcomes the body's immune defense. Tuberculosis disease presents with clinical signs and symptoms and people with the disease may be infectious.

Active TB disease can often (but not always) be identified by culture, smear microscopy, NAAT or chest x-ray. The number of people with active TB at a given time is just the tip of the iceberg, as many more are infected with TB and are therefore at a risk of developing the disease.

Diagnostics for Tuberculosis:  Global Demand and Market Potential/TDR, FIND SA.  WHO  2006: p. 21.