Drug‐resistant tuberculosis: Past,present, future

In a population of Mycobacterium tuberculosis, random chromosomal mutation that results in genetic resistance to anti‐tuberculosis (TB) drugs occurs at a relatively low frequency. Anti‐TB drugs impose selection pressure so that mycobacterial mutants gradually outnumber susceptible bacilli and emerge as the dominant strains. Resistance to two or more anti‐TB drugs represents cumulative results of sequential mutation. The fourth report on global anti‐TB drug resistance provides the latest data on the extent of such problem in the world. The median prevalence of multi‐drug‐resistant TB (MDR‐TB) in new TB cases was 1.6%, and in previously treated TB cases 11.7%. Of the half a million MDR‐TB cases estimated to have emerged in 2006, 50% were in China and India. The optimal duration of any given combination of anti‐TB drugs for treatment of MDR‐ and extensively drug‐resistant TB (XDR‐TB) has not been defined in controlled clinical trials. Standardized treatment may be feasible for MDR‐TB patients not previously treated with second‐line drugs, but a different strategy needs to be applied in the treatment of MDR‐TB patients who have received second‐line drugs before. Unfortunately, the reliability of drug susceptibility testing of most second‐line anti‐TB drugs is still questionable. Drug‐resistant TB is not necessarily less virulent. Findings from modelling exercise warned that if MDR‐TB case detection and treatment rates increase to the World Health Organization target of 70%, without simultaneously increasing MDR‐TB cure rates, XDR‐TB prevalence could increase exponentially. Prevention of development of drug resistance must be accorded the top priority in the era of MDR‐/XDR‐TB.