Synonyms in the broadest sense
consumption, Koch’s disease (after the discoverer Robert Koch), Tbc
Tuberculosis is an infectious disease caused by bacteria of the class of mycobacteria. The most important representatives of this group are the mycobacterium tuberculosis, which is responsible for over 90% of the diseases, and the mycobacterium bovis, which is responsible for the majority of the remaining 10%. The latter is important in that it is the only mycobacterium capable of surviving in an animal host.
There are about two billion (!) people worldwide who are infected with the bacterium, with the main focus on Africa and the former Eastern Bloc countries. Tuberculosis is thus the most common infectious disease. Approximately eight million people die of tuberculosis every year, which is a small number compared to the number of people infected (low mortality). In Germany, there are currently fewer than 10,000 sick people, although the number of infected persons has been decreasing continuously for several years.
Causes of tuberculosis
The bacterium is usually (in more than 80% of all cases) transmitted from person to person by droplet infection (saliva). Other transmission routes via the skin (only if the skin is injured), urine or faeces are possible, but are the exception. If cows are infected with the pathogen mycobacterium bovis, they can infect humans via their raw milk.
However, in western countries the tuberculosis disease of cattle has been eradicated and thus the danger of contracting tuberculosis through milk consumption has been averted. If a healthy person has contact with the bacteria, he can ward off the disease in about 90% of the cases. In other words: the infectivity of the pathogens is low.
In people with immunosuppression (a worsened immune system, for example, AIDS patients, alcoholics, severe diabetes mellitus disease, malnourished people) the risk of infection is significantly increased. Tuberculosis is the main cause of death of HIV-infected persons! Mycobacteria are characterized by the fact that they are surrounded by a thick layer of wax in addition to the normal structure of a bacterium with cell wall.
This wax layer is the reason for the numerous special features: The human immune system fights the bacteria in a special way. If the body’s defense system does not manage to kill all bacteria when they enter the body, the defense cells try to wall in the pathogens. This has the advantage that the bacteria cannot spread any further, but also the disadvantage that they cannot be fought further within this structure.
On the contrary, the pathogens can survive for years in this structure, also known as granuloma or tubercle, and if the body’s defenses deteriorate, they can trigger a new surge of the disease (endogenous reinfection, secondary infection). Over time, a calcification of these granulomas occurs, which can be seen in the X-ray thorax (X-ray image of the thorax). In principle, tuberculosis bacteria can attack all human organs.
Since the main route of infection of tuberculosis is inhalation, the lungs are also affected in more than 80% of cases. Other more frequently affected organs are the pleura, the brain and the liver. If several organs are affected, one also speaks of miliary tuberculosis, because pea-like nodules in the affected organs can be detected with the naked eye (for example during operations or an autopsy).
A detailed overview of all tropical diseases can be found under the article: Overview of tropical diseases
- The exchange of nutrients with the environment (diffusion) is strongly limited. This is the reason why tuberculosis is difficult to combat with antibiotics (special drugs that act quite selectively against bacteria), because they too must first enter the cell in order to be effective.
- Mycobacteria divide extremely slowly. While some bacteria, such as Escherichia coli, which is found in the intestine, have a generation time of 20 minutes (i.e. doubling every 20 minutes), the pathogen causing tuberculosis needs about one day.
This means that there is a long period of time (approx. six weeks) between infection with the pathogen and the outbreak of the disease
- The immune cells (defense cells) of the human body can hardly recognize the bacteria once they have infected the body and thus can hardly fight them. On the contrary, mycobacteria can even survive in certain defence cells, so-called phagocytes, and spread throughout the body.
- Due to their wax layer, they can survive even in a very acidic environment (for example in the gastric juice).