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