Malaria parasites in western Cambodia have become resistant to artemisinin-based therapies, the first-line treatment for malaria, according to a study by Oxford University researchers.
The study, conducted by the university's experts based in Thailand, has been published in the New England Journal of Medicine.
Signs of artemisinin resistance have been reported in the region, but this new research is the first detailed study of the problem, a university release said.
Resistance to the drugs could eventually render them obsolete, putting millions of lives at risk.
Professor Nick White, co-author of the study, believes the implications of the findings are potentially huge: "Artemisinins are essential weapons in our war against malaria. If they become ineffective, we have no immediate replacement. The consequences could be devastating. Elimination of malaria will not be possible and millions of lives could be lost."
Malaria kills more than a million people each year, mainly young children and pregnant women. It is caused by malaria parasites, which are injected into the bloodstream by infected mosquitoes. The most deadly form, Plasmodium falciparum, is responsible for nine out of ten deaths from malaria.
Artemisinin is the most effective anti-malarial drug so far. Artemisinin derivatives have the advantage over other anti-malarial drugs, such as chloroquine and mefloquine, in having few side effects and - until now - malaria parasites had no resistance against it.
Although the drugs - most commonly in the form of the derivative artesunate - can be used on their own as a monotherapy, fears over the possible development of resistance mean that they are usually given in conjunction with one or more other drugs.
These artemisinin-based combination therapies (ACTs) are now recommended by the WHO as the first-line treatment for uncomplicated falciparum malaria in all endemic countries.
The researchers studied 40 patients in Pailin, western Cambodia, and 40 patients in Wang Pha, north-western Thailand. Each was given either the artemisinin-derivative artesunate or a combination of artesunate and mefloquine.
On average, patients in Thailand were clear of parasites in 48 hours; in western Cambodia this took 84 hours - in other words, it took almost twice as long to clear the parasites in Cambodia as it did in Thailand.
Out of the 20 patients treated solely with artesunate in each country, there were recurrences of the infection in six patients in western Cambodia compared to just one person in Thailand. Of the 20 patients treated with the combination therapy, infection recurred in two patients in Cambodia compared to one in Thailand. These results again suggest that artemisinin was less effective on the Cambodian parasites.