The best treatment for malaria may be in jeopardy, the World Health Organization warned on Thursday, because of the way some pharmaceutical companies market the drug in poor countries.

The fear is that improper use of artemisinin will spur the malaria parasite to evolve to resist treatment with the compound, something that has happened to all previous malaria therapies.

Resistance "will be a major disaster," WHO malaria specialist Dr. Pascal Ringwald said. "If this occurs, we will have no drugs that can cure malaria anywhere for the next 10 years."

Malaria sickens up to half a billion people a year, and kills more than 1 million, many of them young children.

Forms of artemisinin, developed in China from the wormwood plant, have become the leading treatment — but only when used together with older medications. This artemisinin combination therapy, known as ACT, is 95 percent effective in curing malaria.

Taking artemisinin drugs alone, or so-called monotherapy, will make resistance appear more quickly, Ringwald said. Thus, many groups that fund malaria treatments in developing countries now pay only for ACTs.

But some private doctors in developing countries dispense artemisinin monotherapy because it's cheaper for patients who can't afford even the roughly $2 cost of ACT, WHO said.

Worse, monotherapy often comes with manufacturer-written instructions to take it for only five days, WHO found. Resistance aside, five-day treatment with artemisinin alone isn't enough to cure anybody, Ringwald warned.

WHO can't stop what doctors do in private practice. But Thursday, it issued an unprecedented public call to drug makers to stop selling artemisinin monotherapy.

WHO cited more than a dozen manufacturers, most based in China, India or Vietnam. Also listed was the French-based drug company Sanofi-Aventis; a spokeswoman didn't return phone calls seeking comment.

Artemisinin resistance hasn't been documented yet, but there is laboratory evidence that the parasite is becoming less sensitive to the drug, Ringwald said.

ACTs until recently actually were in short supply in many countries. But one leading ACT manufacturer, Swiss-based Novartis, announced Wednesday that it could more than triple production this year, to 100 million ACT courses, if malaria-ridden countries order that much.

Also, simpler ACT formulations at half of today's cost should be available later this year. Currently, ACTs come in blister packs that package artemisinin and an older medicine together. The Drugs for Neglected Diseases Initiative, a public-private partnership, funded research to combine the two medicines into one pill.