Tuberculosis

Tuberculosis in short

• a respiratory disease caused by Mycobacterium tuberculosis
• one-third of humanity is infected with tuberculosis (TB)
• 5–10% of infected individuals develop active TB at some point after infection
• about 2 million people die of TB each year
• in HIV-infected individuals, TB is the leading cause of death

Tuberculosis in detail

Pathogenic agent
The majority of human tuberculosis (TB) cases are caused by Mycobacterium tuberculosis, an obligate aerobic bacterium of the genus Mycobacterium. There are several other closely related species within the so-called Mycobacterium tuberculosis-complex that can cause TB, namely M. bovis, M. africanum, M. canetti and M. microti, but they are far less common. In about 80% of infections, the bacteria affect the lungs (pulmonary TB), but in principle every organ can be attacked. Humans are the only reservoir for M. tuberculosis.

Transmission/pathogenesis
Tuberculosis is spread through the air. Individuals with acute pulmonary TB expel bacteria when coughing, sneezing or speaking. Depending on the environmental conditions, M. tuberculosis can survive for several hours in the air. To be infected, a person needs to inhale only a small number of bacteria. But to be infected doesn’t necessarily mean to be sick. People can carry M. tuberculosis for years without developing tuberculosis; this is called "latent TB".

Once the immune system is weakened (e.g. by another disease), the carrier can become sick. Between 5 and 10% of infected individuals develop active TB at some point in their life, the majority within the first 2 years of infection. They show TB symptoms and become infectious, capable of spreading M. tuberculosis to others.

Symptoms
The first symptoms of TB are unspecific, including fatigue, loss of appetite, fever, unexplained weight loss and night sweats. Coughing for more than 3 weeks, coughing up blood and chest pain are symptoms of a pulmonary TB infection.

Treatment
A combination therapy comprised of several tuberculosis drugs that need to be taken for at least 6 months is commonly used to treat active tuberculosis. After 2–3 weeks of treatment, the infected person is usually no longer able to spread the bacteria to others. The drugs need to be taken regularly and exactly as prescribed. If not taken correctly or for the required period, M. tuberculosis can develop drug resistance.

Drug-resistant bacteria are already causing severe problems in the global battle against TB. According to the World Health Organization (WHO), M. tuberculosis resistant to a single drug have been documented in every country surveyed. What’s more, multi-drug-resistance (defined as resistance to isoniazid and rifampicin, the two most powerful TB drugs) is on the rise, especially in the Newly Independent States of the former Soviet Union. Treatment of drug-resistant TB is possible but requires longer treatment with more expensive drugs.

M. tuberculosis defined as "extensively drug-resistant" (XDR-TB) has also been identified; these bacteria are not only unresponsive to isoniazid and rifampicin, they also resist treatment with any fluoroquinolone and at least one of three injectable second-line drugs. XDR-TB is still relatively rare, occurring most often in settings where many TB patients are also HIV-positive.

Drug-resistant bacteria emerge when patients stop their treatment prematurely or when doctors prescribe the wrong treatment. Especially in remote areas in developing countries, securing a constant supply of TB drugs is a challenge to health care workers and often hampers correct treatment.

Vaccination
The Bacille Calmette-Guérin (BCG) vaccination for TB is composed of a live attenuated strain of M. bovis. It is mainly used in countries with a high incidence of TB, where it remains the standard to prevent life-threatening TB forms in infants and young children. However, its efficacy is highly debated among experts: study results vary greatly, from showing optimal protection to showing no protection at all. Efforts are underway to develop a better vaccine, and several candidates are currently being tested. For more information see the WHO Initiative for Vaccine Research (IVR) (http://www.who.int/vaccine_research/diseases/ari/en/index6.html#vaccine).

Incidence and mortality
One-third of the world’s population - about two billion people - are infected with tuberculosis. According to a recent WHO report on global tuberculosis, 9.2 million new cases occurred in 2006, and 1.7 million people died. However, since 2003 the number of new cases per capita has fallen globally, making it feasible to reach the Millennium Development Goal 6: To have halted and begun to reverse the incidence of TB before 2015. The Stop TB Strategy initiated by the WHO has been crucial to the success in the efforts to reduce the worldwide burden of TB (http://www.who.int/tb/strategy/en/).

HIV and TB, a “duel epidemic”
The majority of TB deaths occur in HIV-infected people. Each disease speeds the progress of the other: as HIV weakens the immune systems, it increases the chances that M. tuberculosis will be acquired and speeds progression to active disease. And M. tuberculosis infection speeds the progression of AIDS in duel-infected individuals. To combat the lethal duo, the WHO and its partners have formed a TB/HIV working group. For more information see: ww.who.int/tb/challenges/hiv/en/

To know or not to know...
"Left untreated, each person with active TB disease will infect on average between 10 and 15 people every year." (WHO)

More information:
- International Union Against Tuberculosis And Lung Disease: www.theunion.org
- WHO: www.who.int/topics/tuberculosis/en/
- WHO Stop TB strategy: www.who.int/tb/strategy/en/
- WHO working group TB/HIV: www.who.int/tb/challenges/hiv/en/
- Robert Koch Institute on tuberculosis (in German only): www.rki.de/cln_091/nn_196658/DE/Content/InfAZ/T/Tuberkulose/Tuberkulose__node.html?__nnn=true
- Centers for Disease Control and Prevention (CDC) on tuberculosis: www.cdc.gov/tb

Literature
- WHO fact sheet on tuberculosis: www.who.int/mediacentre/factsheets/fs104/en/index.html
- WHO Report 2008: Global tuberculosis control – surveillance, planning, financing: www.who.int/tb/publications/global_report/en/
- WHO Initiative for Vaccine Research: www.who.int/vaccine_research/diseases/ari/en/index6.html
- Robert Koch Institute (in German only): www.rki.de/cln_091/nn_196658/DE/Content/InfAZ/T/Tuberkulose/Tuberkulose__node.html?__nnn=true
- Centers for Disease Control and Prevention: www.cdc.gov/tb