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Multidrug-resistant tuberculosis is a form of tuberculosis infection caused by bacteria that are resistant to treatment with at least two of the most powerful first-line anti-TB medications , isoniazid and rifampin. Some forms of TB are also resistant to second-line medications, and are called extensively drug-resistant TB.
Tuberculosis is caused by infection with the bacteria Mycobacterium tuberculosis. Almost one in four people in the world are infected with TB bacteria. Only when the bacteria become active do people become ill with TB. Bacteria become active as a result of anything that can reduce the person's immunity, such as HIV, advancing age, diabetes or other immunocompromising illnesses. TB can usually be treated with a course of four standard, or first-line, anti-TB drugs.
However, beginning with the first antibiotic treatment for TB in 1943, some strains of the TB bacteria developed resistance to the standard drugs through genetic changes Currently the majority of multidrug-resistant cases of TB are due to one strain of TB bacteria called the Beijing lineage. This process accelerates if incorrect or inadequate treatments are used, leading to the development and spread of multidrug-resistant TB. Incorrect or inadequate treatment may be due to use of the wrong medications, use of only one medication , not taking medication consistently or for the full treatment period. Treatment of MDR-TB requires second-line drugs , which in general are less effective, more toxic and much more expensive than first-line drugs. Treatment schedules for MDR-TB involving fluoroquinolones and aminoglycosides can run for 2 years, compared to the 6 months of first-line drug treatment, and cost over US$100,000. If these second-line drugs are prescribed or taken incorrectly, further resistance can develop leading to XDR-TB.
Resistant strains of TB are already present in the population, so MDR-TB can be directly transmitted from an infected person to an uninfected person. In this case a previously untreated person develops a new case of MDR-TB. This is known as primary MDR-TB, and is responsible for up to 75% of cases. Acquired MDR-TB develops when a person with a non-resistant strain of TB is treated inadequately, resulting in the development of antibiotic resistance in the TB bacteria infecting them. These people can in turn infect other people with MDR-TB.