July 29 (HealthDay News) -- Malaria parasites in western Cambodia have become resistant to artemisinin-based drugs, a development that could put millions of lives at risk, researchers say.

Artemisinin-based drugs are the first-line treatment for malaria, which kills more than a million people a year, mainly children and pregnant women. Artemisinin causes fewer side effects than other anti-malaria drugs and, until recently, malaria parasites had no resistance against it, according to information in a news release from The Wellcome Trust.

In a new study, researchers at the Wellcome Trust-Mahidol University Oxford Tropical Medicine Research Program, based in Bangkok, compared the effects of artemisinin drugs in 40 malaria patients in western Cambodia and 40 patients in northwestern Thailand. On average, the patients in Thailand were clear of malaria parasites within 48 hours, compared to 84 hours for the Cambodian patients.

"Our study suggests that malaria parasites in Cambodia are less susceptible to artemisinin than those in Thailand," lead author Dr. Arjen Dondorp said in the news release. "This means that it takes longer to kill the parasites. Artemisinin should clear the parasites at an early stage, preventing them further maturing and reproducing. When the drug's action is impaired, it becomes more difficult to eliminate the parasites from the body."

The study appears in the July 30 issue of the New England Journal of Medicine.

"Artemisinins are essential weapons in our war against malaria," study co-author Nick White said in the news release. "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."

SOURCE: Wellcome Trust, news release, July 29, 2009