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About TB

 

Tuberculosis is a preventable & curable airborne infectious disease mainly caused by the bacterium Mycobacterium tuberculosis. Approximately 1/3rd of the world’s population is infected with the TB bacillus.¹  Infection occurs through inhalation of airborne TB bacilli with repeat exposure generally necessary for infection to take place.

 

Once infected in most cases (90%) the immune system “walls off” the bacteria and stops it from growing.²  In these cases the infection lies dormant, asymptomatic and is termed as latent TB infection. Those with latent TB infection feel well and cannot pass the infection onto others. There is a 10% lifetime chance that a latent TB infection will progress to TB disease (Active TB).³

 

Active tuberculosis occurs when the TB bacilli overcome the immune system defences and begin to multiply.  Active TB is described in two forms: pulmonary (80-90% of cases) and extrapulmonary (10-20% of cases).¹

 

Pulmonary Tuberculosis

 

Pulmonary TB is localized in the lungs and is the most common form of active TB disease. Patients with untreated pulmonary TB are highly infectious. On average, each person with untreated pulmonary TB will infect 10-15 people a year.⁴  Without treatment by the end of 5 years: 50% of pulmonary TB patients will be dead, 25% will be healthy (self-cured by strong immune defence) and 25% will remain ill with chronic infectious TB.²

 

Extrapulmonary Tuberculosis

 

At infection, tuberculosis bacilli may spread through the blood, lymphatic or GU systems to parts of the body outside the lung.  As in the lungs, the TB infection can lie dormant for months or years. Extrapulmonary TB occurs when active tuberculosis disease occurs at body sites other than the lung.  Where extrapulmonary TB and pulmonary TB are present a patient is usually classified as pulmonary TB.  Symptoms are related to the organs in which the disease occurs and may manifest as a condition e.g. fever or fatigue.

 

Tuberculosis & HIV

 

TB is a leading cause of death in HIV infected patients. TB and HIV have a mutually negative relationship.  As HIV progresses and CD4+ cell counts drop the co-infected patient suffers from immunodeficiency which leads to increased susceptibility to active TB (up to 10x more likely than someone that is HIV negative).²  HIV is driving the TB epidemic in many countries such as Sub-Saharan Africa (up to 50% of TB patients in African countries are also HIV positive), Asia and South America.⁵  Extrapulomonary TB is more common in those co-infected with HIV as the immune system becomes less able to prevent growth and local spread.²

 

Detection

 

In many parts of the world, the primary diagnostic test for the detection of TB is the sputum smear test developed more than a century ago. Microscopy sensitivity ranges between 40%-60% and falls to as low as 20% in patients who are also HIV infected.¹

 

References

 

1. Diagnostics for Tuberculosis: Global Demand and Market Potential. World Health Organization, 2006.
2. TB/HIV A Clinical Manual 2nd edition: WHO/HTM/TB/2004.329
3. WHO TB/HIV http://www.who.int/tb/challenges/hiv/qa.pdf (May 2008)
4. WHO Fact Sheet #104 (revised March 2007)
5. WHO TB/HIV Facts at a glance: (June 2008) 

 

 

Useful links

 

• Stop TB Partnership
• Centers for Disease Control and Prevention (CDC) - The Connection between TB and HIV
• Global Fund for HIV/AIDS, Tuberculosis and Malaria (GFATM)
• PEPFAR (The President's Emergency Plan for AIDS Relief)
• UNAIDS
• WHO tuberculosis fact sheet
• CREATE