Global efforts to combat malaria are under threat from new strains of drug-resistant malaria, which are cropping up in Southeast Asia.
Over the last decade, the number of malaria deaths around the world has dropped sharply, from just over 1 million in 2000 to roughly 600,000 last year.
Much of that progress is due to the widespread use of drugs containing artemisinin. The new malaria drugs quickly kill the parasite.
But in Southeast Asia doctors are starting to see cracks in artemisinin's armor. The drugs are working more slowly, and sometimes they're failing to wipe out the malaria parasite entirely.
The World Health Organization reported Monday that artemisinin resistance has now been detected in four countries: Cambodia, Myanmar (also known as Burma), Thailand and Vietnam.
Although the resistance is still limited to Southeast Asia, WHO officials worry that it could spill out of the region.
Two hot spots for artemisinin resistance are in Thailand's thickly forested border regions. One is along Thailand's eastern border with Cambodia, and the other is on the country's western boundary with Myanmar.
If artemisinin-based treatments become ineffective in Africa — where malaria remains the leading killer of children under the age of 5 — it could be disastrous. Doctors say they have few other powerful drugs to use against the disease.
Global funding for the fight against malaria rose to $1.71 billion in 2010, or nearly twentyfold since 2000. But it has plateaued in the past few years, the WHO said. Many malaria interventions, like distribution of bed nets and insecticide spraying have also leveled off during this time, the report found.