WASHINGTON, Aug 24 (Reuters) - Poor treatment may be fueling the rise of an especially hard to treat form of tuberculosis called extensively drug resistant or XDR TB, doctors reported on Sunday.

But they said their project, based at a remote Siberian prison, showed they could successfully treat nearly half the patients with XDR TB. It also suggested that better treatment of patients with other forms of multiple-drug-resistant TB would help prevent the infection from worsening into the XDR form.

"While early studies suggested that XDR TB is untreatable, our report indicates that while it may be difficult, it is possible to treat these patients through the use of aggressive regimens," Dr. Salmaan Keshavjee of Brigham and Women's Hospital and Harvard Medical School said in a statement.

"A cure rate of 48.3 percent is promising in a disease that has been touted as untreatable."

Keshavjee is part of the Partners in Health program that has been treating prisoners and civilians in Tomsk, Siberia, who are infected with tuberculosis.

They reported on 608 patients with multi-drug resistant tuberculosis who were treated between September 2000 and November 2004. Four percent or 29 of patients were diagnosed with XDR TB.

They said 48.3 percent of patients with XDR TB and 66.7 percent of the other patients were cured. Earlier this month another team reported similar results in patients with MDR and XDR TB in Peru.

Most of the patients with XDR TB had been treated, incompletely, for MDR TB, the doctors found.

Nearly a third of the world's population is infected with the bacterium that causes tuberculosis, although active TB only develops in a fraction of those cases.

MDR TB can resist treatment with the antibiotics normally used to treat TB. The World Health Organization estimates 490,000 new cases of MDR TB arise worldwide every year.

XDR TB resists the second-line drugs and must be treated for many months or even years with a cocktail of antibiotics and, often, surgery to remove the most badly infected lung tissue.

The WHO identified XDR TB as a global threat in 2006 and it has so far been confirmed in more than 45 countries. In Africa it is often fueled by the AIDS virus but the researchers said few of the Russian patients were infected with HIV.

"Through aggressive management of XDR TB cases, including by ensuring that patients are correctly diagnosed as early as possible and put on appropriate treatment for the correct length of time, it may be possible to slow the rise of XDR TB deaths around the world and reduce further transmission of the most drug-resistant strains of TB," said Keshavjee.

(Reporting by Maggie Fox, Editing by Sandra Maler)

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