Scientists have found an alarming number of cases of the lung disease tuberculosis in Africa, Asia, Europe and Latin America that are resistant to up to four powerful antibiotic drugs.
In a large international study published in the Lancet medical journal on Thursday, researchers found rates of both multi drug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) were higher than previously thought and were threatening global efforts to curb the spread of the disease.
"Most international recommendations for TB control have been developed for MDR-TB prevalence of up to around 5 percent. Yet now we face prevalence up to 10 times higher in some places, where almost half of the patients ... are transmitting MDR strains," Sven Hoffner of the Swedish Institute for Communicable Disease Control, said in a commentary on the study.
TB is already a worldwide pandemic that infected 8.8 million people and killed 1.4 million in 2010.
Drug-resistant TB is more difficult and costly than normal TB to treat, and is more often fatal.
MDR-TB is resistant to at least two first-line drugs - isoniazid and rifampicin - while XDR-TB is resistant to those two drugs as well as a powerful antibiotic type called a fluoroquinolone and a second-line injectable antibiotic.
Treating even normal TB is a long process, with patients needing to take a cocktail of powerful antibiotics for six months. Many patients fail to complete their treatment correctly, a factor which has fuelled a rise in the drug-resistant forms.
Researchers who studied rates of the disease in Estonia, Latvia, Peru, the Philippines, Russia, South Africa, South Korea and Thailand found almost 44 percent of cases of MDR TB were also resistant to at least one second-line drug.
Tom Evans, chief scientific officer at Aeras, a non-profit group working to develop new TB vaccines, told Reuters treatment options for XDR-TB patients were "limited, expensive and toxic".
Treatments for drug-resistant TB can cost 200 times more than those for normal TB, he said in an emailed statement. They can also cause severe side effects like deafness and psychosis, and can take two years to complete, he added.
In the United States, MDR-TB treatment can cost $250,000 or more per patient, and in many poorer countries costs can be catastrophic to health systems and patients' families.
"Without a robust pipeline of new drugs to stay one step ahead, it will be nearly impossible to treat our way out of this epidemic," Evans said.