The definition of extensively drug-resistant tuberculosis -- TB that is resistant to first- and second-line drugs -- has been updated in the Dec. 20 issue of the Journal of the American Medical Association following an outbreak of the strain in South Africa, SAPA/Independent Online reports. XDR-TB is now defined as TB that is resistant to isoniazid and rifampin -- the two first-line TB drugs -- as well as at least two of the six primary classes of second-line drugs, one being a fluoroquinolone and the other an injectable drug. The second-line TB drugs are aminoglycosides, polypeptides, fluoroquinolones, thioamides, cycloserine and para-aminosalicyclic acid. The previous definition of XDR-TB was TB that is resistant to isoniazid and rifampin, as well as at least three of the six primary classes of second-line drugs. According to the JAMA article, the revised definition follows the emergence of XDR-TB and the high mortality among people living with both XDR-TB and HIV in South Africa and Iran. The emergence and spread of XDR-TB "highlight the urgency of strengthening national" TB and HIV/AIDS "control programs worldwide, particularly in settings with high HIV prevalence," the article says. In addition, CDC is "collaborating with national and international health agencies to provide leadership, technical support, and capacity building to ensure proper action is taken to limit the development and spread of XDR-TB," according to the article. The updated definition of XDR-TB was formulated in October at a World Health Organization global task force on XDR-TB (SAPA/Independent Online, 12/20).