Facts about tuberculosis

It is estimated that two billion people worldwide are infected with Mycobacterium tuberculosis - and every tenth person will go on to develop the disease. Approximately two million people die of tuberculosis (TB) every year. The bacteria are coughed up from the lungs and transmitted in microscopically small drops to the lungs of others where they are assimilated by immune system scavenger cells. Thanks to an ingenious mechanism they are not digested by the immune cells but remain locked inside the cells for a very long time. In this way, they avoid attack by the immune system.

Tuberculosis bacteria (green) within a phagocyte of the immune system. Lysosomes are depicted in red, lysosomes containing parts of the bacteria are depicted in yellow.

Different types of immune cells form, so-called granulomas, in which M. tuberculosis can survive for years (latent infection). They fall into a permanently inactive state with reduced metabolic activity and can thus not be completely eliminated by the immune system. As long as the immune system holds the pathogens in check in this way, latently infected individuals don't notice anything. They are also not infectious. However, if the immune system becomes weakened, it is no longer able to control the infection and the pathogens are released from the granulomas. The infected person then becomes ill and spreads the bacteria.

Other infections that weaken or make demands on the immune system can play an important role in the outbreak of TB. The significant increase in the number of TB patients since the 1980s is also a consequence of HIV infection. Many TB patients are also infected with HIV, and TB is currently the main cause of death in these patients. Worm parasite infections, which are particularly widespread in poor countries, can cause an imbalance in the immune system and thus also promote the development of TB.

The lack of effective vaccines and the increasing resistance of M. tuberculosis to drugs traditionally used to treat the illness are another reason for the spread of TB. The only vaccine available today, which has been in use since the early 20th century (BCG), protects newborn and young children and only shields them against severe progression of the disease. The effect of BCG also diminishes over time. The same vaccination, when administered to adults, is completely ineffective. Twelve vaccine candidates currently tested in clinical trials aim to increase the effectiveness of the 90-year-old vaccine or replace it entirely. Although they cannot stop infection, they can prevent the outbreak of the illness.

Over 20 drugs are available today for the treatment of TB - however, treatment is very complicated. The patients must initially take a cocktail of four drugs for two months, and then continue with two other drugs for a further period of four to seven months.

Because it is often impossible to follow this treatment plan precisely or it is frequently abandoned prematurely, more and more resistant strains of M. tuberculosis have developed in recent years. Some are resistant to the best available medication and only respond to second-choice drugs which, however, have stronger side effects. Other strains cannot be treated using available drugs.

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