Resistance to the most-effective anti-malarial drug is rapidly increasing throughout mainland Southeast Asia, and Cambodia is ground zero, according to a new study in the New England Journal of Medicine.

By analysing blood samples from 1,241 malaria patients in 10 Asian and African countries, researchers found artemisinin-resistant Plasmodium falciparum, the deadliest form of the mosquito-borne parasite, has spread throughout Thailand, Cambodia, Myanmar, Laos and Vietnam.

Artemisinin, a wormwood derivative, typically clears parasites faster than other antimalarials. But the Mahidol Oxford Tropical Medicine Research Unit found that up to 73 per cent of sample patients in Pailin continued to have parasites in their blood 72 hours after treatment, a widely used barometer for resistance.

The number represents a surge of drug-resistant infections: in 2012, the National Malaria Center estimated that just 26 per cent of Pailin’s malaria cases were drug-resistant.

This is the third time the parasite has developed a resistance to treatment, and each time resistance emerged from the Thai-Cambodia border.

“Previously [it was] because of the widespread use of sub-standard antimalarials and influx of counterfeits,” said Sonny Krishnan, communications director for the World Health Organization in Cambodia. “For artemisinin, it was the use of artemisinin mono therapies . . . now banned in Cambodia.”

Switching up the treatment, however, only seems to work for so long; the more a treatment is used, the faster its efficacy erodes. In Cambodia, where ACTs have been in use for over 30 years, it now takes six days to clear parasites instead of the standard three, the study found.

“Stepping up existing control measures has not worked – we need a more radical approach,” said Nicholas White, senior author of the study.

Krishnan said that artemisinin derivatives “are still our best first-line treatment for malaria. If we lose it, we will lose the war against malaria.”