The parasite that causes the most deadly form of malaria is becoming more resistant to the most effective drug used to treat it, scientists in Southeast Asia are reporting.

Their study, in The New England Journal of Medicine, adds urgency to the effort to develop alternatives to artemisinin, a drug that has led to huge advances in the war against malaria.

“Since the turn of the century, we have been winning, and we think we’ve saved about three million lives,” said the senior author, Dr. Nick White, a professor of tropical medicine at Mahidol University in Thailand and Oxford University in England. But he added:

“This paper shows that resistance has spread across Southeast Asia, which is a much worse situation than we previously thought, and it’s really knocking on the doors of India.”

To track the spread of the problem, the researchers did a randomized trial to detect artemisinin resistance in 1,241 adults and children in 10 countries in Southeast Asia and Africa with the malaria parasite Plasmodium falciparum.