LONDON - Scientists warn the most effective drug used to treat malaria is becoming ineffective in parts of Southeast Asia — and unless rapid action is taken, it could lead to a global health emergency.

Writing in the Lancet journal, researchers from Thailand’s Mahidol University and Britain’s Oxford University say parasites that carry malaria are developing resistance to a key drug combination across multiple regions of Cambodia, Laos, Thailand and Vietnam.

The report warns that the parasite Plasmodium falciparum — which causes the most lethal form of human malaria — is becoming resistant to the first-choice drug, DHA-piperaquine, in parts of Southeast Asia, with patients seeing a failure rate of 50 percent or more.

The situation is so critical that scientists say the treatment should not be used in Cambodia, Vietnam and northeast Thailand, because it is ineffective and contributes to increased malaria transmission.

New treatments must be considered, says Sterghios Moschos of the University of Northumbria.

“It might be opportune at this point in time to explore whether or not we should bring together different new classes of medications so that when the problem starts becoming more substantial, there is a solution potentially that works at the multi-drug level,” said Moschos.

The report says urgent action is now needed to eliminate falciparum malaria from the region — otherwise the resistant strains of the parasite could further spread to other parts of Asia and Africa, potentially causing global health emergency.

“All it takes is a ship with infected individuals, or a pool of water where mosquitoes are, getting into Africa and then slowly that parasite establishing a foothold," he added. "The likely scenario, however, will be that improvement of health care on a day-to-day basis in Africa will create the opportunity for the parasite to evolve resistance.”

Currently, malaria vaccine trials are under way in several African countries. But drug combinations like DHA-piperaquine remain vital in treating malaria — especially in countries with poor health systems.

Since 2014, global progress against malaria has stalled. There were an estimated 219 million cases and 435,000 related deaths in 2017, most of them children under the age of five in sub-Saharan Africa.