A startling number of tuberculosis cases in Asia, Europe, Africa and Latin America are now resistant to up to four powerful antibiotic drugs.

A new study in The Lancet medical journal looking at at TB in eight countries found 47 per cent of cases were considered “multi-drug-resistant.” That means the patients weren't cured with the two basic antibiotics used as the standard treatment, and also failed to respond to one of the second-line drugs.

The problem is worst in Latvia, the researchers found. That’s where 62 per cent of multi-drug-resistant TB – or MDR TB, as it’s now called – are no longer susceptible to at least one second-line drug.

The study also found much higher than expected levels of extensively drug-resistant, or XDR TB, though its precise prevalence is still not well understood.

XDR-TB is not only resistant to at least two first-line drugs, it also resists a powerful antibiotic called a fluoroquinolone and a second-line injectable antibiotic.

Tuberculosis is a contagious lung disease that infected 8.8 million people in 2010 and killed 1.4 million, according to the World Health Organization. Though the illness occurs in every part of the world, more than 95 per cent of cases and deaths are in developing countries.

The problem of drug-resistance has been growing steadily in recent years, thought to be fuelled by the incomplete treatment of TB patients.

TB patients need to take antibiotics for at least six months and many patients fail to complete their treatment correctly. That allows the bacteria to “learn” to develop resistance before going on to infect others.

“As more individuals are diagnosed with, and treated for, drug-resistant TB, more resistance to second-line drugs is expected to emerge,” Tracy Dalton, the study’s lead author and a TB expert with the U.S. Centers for Disease Control and Prevention (CDC), said in a news release.

“So far, XDR TB has been reported in 77 countries worldwide, but exact prevalence remains unclear.”

For the study, Dalton and a CDC team tested spit samples from 1,278 adults in eight countries with suspected MDR TB. The samples were tested to see how they reacted to 11 first-line and second-line anti-tuberculosis drugs.

They found resistance to any second-line drug ranged from 33 per cent in the patients from Thailand, to 62 per cent in Latvia.

In a fifth of cases, resistance to at least one second-line injectable drug was identified, ranging from 2 per cent in the Philippines to 47 per cent in Latvia.

XDR TB was detected in 6.7 per cent patients overall, with prevalence in South Korea (15.2 per cent) and Russia (11.3 per cent) more than twice the current WHO global estimate, at 5.4 per cent.

Commenting on the paper, Sven Hoffner from the Swedish Institute for Communicable Disease Control says the results show that XDR TB is increasingly a cause for concern, especially in areas where there already a lot of MDR TB.

“Nevertheless, information remains insufficient to give a clear view of the worldwide distribution and true magnitude of XDR TB,” he said.

“Updated information on MDR TB and investigation of the trends are urgently needed... especially since the true scale of the burden of MDR and XDR tuberculosis might be underestimated and seem to be rapidly increasing.”