A Simple Overview of Drug Resistance in Tuberculosis

The goal of this page is to explain drug resistance in TB simply and clearly.

First we should define tuberculosis. Tuberculosis (TB) is a disease caused by bacteria that are spread from person to person through the air. TB generally affects the lungs, however it is important to note that TB can also affect other parts of the body, such as the brain, the kidneys, or the spine. In most cases, TB is treatable and curable; however, left untreated it is very deadly.  The strain of tuberculosis (TB) that is curable by our current drugs (i.e. not drug resistant) is in fact one of the world’s deadliest communicable diseases.  In 2013, an estimated 9.0 million people developed TB and 1.5 million died from the disease.

Our best drugs to cure this strain of TB take a minimum of 6 months of several tablets daily to treat. Historically, this posed several challenges to successfully curing a patient suffering from TB. Although there are many challenges, some of the key challenges include ensuring an adequate drug supply, ensuring the patient takes their pills each and every day, and ensuring the quality and potency of the drugs. Tuberculosis programs were not always able to meet these challenges, and as a result patients were not treated fully and were not completely cured of the disease. However, this partial treatment killed off the “weak” strains of the bacteria, leaving only the “strong” ones to survive and replicate. These “strong” ones were resistant to one or several of the drugs we use to treat TB, and when they replicated this resistance made it much more difficult to treat the patient.

 This is how drug resistance historically came about, but drug resistant tuberculosis can be transmitted from person to person through the air.  

All non-resistant TB patients are treated with several drugs, called “first line” drugs. The strongest first line drugs are isoniazid and rifampin. Other common first line drugs include ethambutol and pyrazinamide  Resistance can form in any of these drugs, however, when a strain of TB is resistant to at least isoniazid or rifampin, we call that  Multi-Drug Resistant TB (MDR-TB) . MDR-TB must be treated with second line drugs which are not as effective as first line drugs.

 Extensively drug resistant TB (XDR TB)  is a  type of MDR-TB that is not only resistant to isoniazid and rifampin, but also some of the second line drugs. Specifically, XDR-TB is defined when a strain is resistant to any fluoroquinolone and at least one of three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin). Because XDR TB is resistant to the most potent TB drugs, patients are left with treatment options that are much less effective.