New resistance emerging in SE Asia

A new study shows that artemisinin-resistant P falciparum malaria parasites have been gradually emerging in Western Thailand on the border with Burma over the last ten years.

The study, funded by the Wellcome Trust and the National Institutes of Health, follows reports in 2006 of artemisinin-resistant malaria parasites 800km away in western Cambodia.

Professor François Nosten of the Mahidol Oxford Tropical Medicine Research program, Shoklo Malaria Research Unit, and colleagues followed parasite clearance in over 3,000 patients in malaria clinics along the Northwestern border of Thailand. From 2001 to 2010, when patients were treated with artemisinin-containing drugs, the time for parasite clearance lengthened from 2.6 to 3.7 hours — a clear sign that the drugs were becoming less effective. This half life compares to 5.5 hours in 119 Western Cambodian patients. 

The research team used the Parasite Clearance Estimator tool recently developed by WWARN.

Genotyping data on the parasites did not establish a genetic connection to suggest that recent importation of Cambodian parasites explains the decline in parasite clearance. The researchers provided compelling evidence that the decline in parasite clearance half-life was due to genetic changes making the parasites drug resistant. Together with the Texas Biomedical Research Institute-led international team of scientists, the same researchers have identified a region of chromosome 13 on the P falciparum genome that shows a strong association with the slow parasite clearance rates.

WWARN Director, Philippe Guerin said: “Prof Nosten's study clearly demonstrates the importance of longitudinal detailed monitoring for early detection of resistance. The findings of this study suggest that the changes in susceptibility to artemisinin derivatives in Thailand may have evolved independently from those in Cambodia — making the call for revitalised surveillance in all endemic countries even more urgent. The identification by Anderson's group of a region on the parasite genome that may confer artemisinin resistance is critically important. This information brings us closer to designing molecular tools for the identification of resistant parasites and more efficient surveillance. Validation of those candidate markers will be the next urgent step.”

Professor Nick White, Chairman of the Wellcome Trust’s South-East Asia Major Overseas Programmes and co-author of this manuscript added: “Initially we hoped we might prevent this serious problem spreading by trying to eliminate all P falciparum from western Cambodia. Whilst this could still be beneficial, this new study suggests that containing the spread of resistance is going to be even more challenging and difficult than we had first feared.”

Professor François Nosten, warned: “Malaria already kills hundreds of thousands people a year – if our drugs become ineffective, this figure will rise dramatically.”

For more information, including full citations for both papers, read the Wellcome Trust press release.