A STRAIN of tuberculosis that resists almost all of the drugs used to fight it is appearing around the world, including the US, the World Health Organisation and the Centres for Disease Control and Prevention have warned.

The strain, known as "extensively drug-resistant tuberculosis", or XDR-TB, has the potential to return TB treatment "to the pre-antibiotic era" in which the only treatment was cutting out parts of the lungs.

"XDR-TB is cause for concern because it is widely distributed and renders patients virtually untreatable," said Kenneth Castro, director of the division of TB elimination at the Centres for Disease Control. If the strain went unchecked, he added, deaths from TB - which affects 9 million people annually and kills 2 million - would rise.

TB is also the biggest killer of people with HIV, causing about 13 per cent of AIDS deaths.

But while cases of the debilitating and potentially fatal disease are subsiding in the US, the subset of infections resistant to some or most of the drugs used against TB is burgeoning, the WHO said. Between 2000 and 2004, 20 per cent of bacteria recovered from TB patients and tested at WHO-affiliated labs worldwide were resistant to the two main drugs, isoniazid and rifampin, used to treat the disease, earning them the label "multi-drug resistant".

Health authorities fear the development of drug resistance in tuberculosis because it transforms treatment of the disease from a six-month course of daily doses of the two main drugs into a regimen that stretches up to two years.

The regime uses a menu of six "second-line" drugs that are less effective, more expensive and more toxic to patients - the drugs can fail and lead to the death of patients.

Because TB-control programs around the world were complaining of patients who did not recover even with those lengthy treatment programs, the WHO undertook a second analysis to determine how resistant TB had become, said Marcos Espinal of the WHO's Stop TB Partnership.

When they rechecked their samples, the labs found that extensively drug-resistant TB - defined as a TB strain that resists not only the two main drugs, but at least half of the secondary drugs - had existed at least since 2000 and had been growing.

Overall, the WHO said, 347 cases of extensively drug-resistant TB had been found since 2000. By 2004 this had risen to 11 per cent of all drug-resistant cases in industrialised countries.

With 500,000 people worldwide suffering from multi-drug-resistant TB, the true rate of the extensively resistant form is likely to be higher than the labs have recorded, because many countries do not have the resources for TB surveillance and analysis, Dr Espinal said.

The study noted that while the most-resistant TB was identified in all regions, it was most common in Eastern Europe and western Asia.

Factors contributing to increases in drug resistance include interruption in treatment among TB-infected people, lack of testing and the absence of infection-control measures in large settings such as hospitals and prisons, researchers said.

An analysis done just in the US found 74 cases of XDR-TB between 1993, when the Centres for Disease Control began recording any case of drug resistance, and 2004.