Plasmodium falciparum is responsible for the most virulent form of malaria

WHO/TDR/Taylor-Robinson

[BANGKOK] The growing resistance of malaria parasites in western Cambodia to artemisinin-based antimalarial drugs has been documented in the first study of the issue.

Researchers writing in the New England Journal of Medicine last week (30 July) confirm that it takes longer to clear the parasites in the blood of Cambodian patients.

Malaria parasites first began showing signs of resistance in January (see Resistance to key malaria drug emerges).

In the new study, researchers studied 40 patients in both Pailin, western Cambodia and Wang Pha, north-western Thailand.

While it took around 48 hours for the parasites to be cleared from Thai patients, clearance took 84 hours in the Cambodian patients, says Kamolrat Silamut, a researcher at the Faculty of Medicine, Mahidol University, Thailand, and co-author of the study.

Infection recurrence rates were also higher in the Cambodian patients.

These results confirm fears that the parasites are becoming resistant to artemisinin, the researchers say.

"In western Cambodia, malaria parasites have become much less sensitive to artemisinin. It's not 100 per cent resistance, but [the parasites] are much less sensitive or 'partly resistant'," said Arjen M. Dondorp, a researcher at the Wellcome Trust–Mahidol University Oxford Tropical Medicine Research Programme in Thailand and lead author of the study.

"That is very worrying because that is the big step to complete resistance".

Artemisinin drugs are vital for the treatment of Plasmodium falciparum malaria. The big problem, Dondorp says, is that there are no other drugs available to treat this most virulent form of the disease.

"If we lose [artemisinin] and its combination therapy, that will be a disaster for malaria control in the whole world."

Dr Charles Delacollette, coordinator of the WHO-Mekong Malaria Programme, told SciDev.Net that the study further highlights the urgent need to develop and implement a containment project to limit the spread of such parasites (see WHO cracks down on new malaria resistance).

He adds that no-one knows the mechanism of resistance and says that additional studies to clarify the situation and provide additional insights are underway.

The researchers wrote that their findings would help decide how to enhance medical treatment against the disease. Dondorp says they are also looking at whether there are any genetic changes that could help explain the reduced sensitivity of the parasites.

Link to full paper in New England Journal of Medicine