Extensively drug-resistant tuberculosis: new strains, new challenges.

Extensively drug-resistant (XDR)-TB, defined as TB with resistance to at least isoniazid, rifampin, a fluoroquinolone and either amikacin, kanamycin or capreomycin, is a stark setback for global TB control. Overburdened public-health systems with inadequate resources for case detection and management and high HIV coinfection rates in many regions have contributed to the emergence of XDR-TB. Patients with XDR-TB have poor outcomes, prolonged infectious periods and limited treatment options. To prevent an epidemic of untreatable XDR-TB, improvements in XDR-TB surveillance, increased laboratory capacity for rapid detection of drug-resistant strains, better infection control and the development of new therapeutics are urgently needed.