Researchers have sounded the alarm over drug-resistant tuberculosis, calling it a curse that is rapidly becoming more difficult and costly to treat. In a study of eight countries, scientists found nearly half (43.7 per cent) of TB patients did not respond to at least one second-line TB drug, a strategy used when the most powerful first-line drugs have already failed. "Most international recommendations for TB control have been developed for MDR [multidrug-resistant] TB prevalence of up to around five per cent. Yet now we face prevalence up to 10 times higher in some places," Sven Hoffner of the Swedish Institute for Communicable Disease Control wrote in medical journal The Lancet . The study, which covered Estonia, Latvia, Peru, the Philippines, Russia, South Africa, South Korea and Thailand, found a 6.7 per cent rate for an even more fatal form of resistance called XDR (extensively drug-resistant), whereby a patient does not respond to any second-line drugs. XDR TB requires two years of expensive treatment that often causes side effects with no guarantee of a cure. Usually TB, an airborne disease of the lungs, is treatable with a six-month course of antibiotics. But if patients do not take their medicines as prescribed, the bacteria that causes TB can develop drug resistance. In rare cases, people can be infected with already resistant strains. According to the World Health Organisation, 8.8 million people contracted TB in 2010 and 1.4 million died. Co-infection with the disease causes about a quarter of all deaths among people with HIV. "So far, XDR TB has been reported in 77 countries worldwide, but exact prevalence remains unclear," said study author Tracy Dalton of the US Centres for Disease Control and Prevention.