Malaria parasites in western Cambodia are becoming increasingly resistant to artemisinin-based therapies, say researchers.
Artemisinin-based therapies are the first-line treatment for malaria, which is caused by most deadly form parasite, Plasmodium falciparum.
It is derived from Artemisia annua, also known as sweet wormwood, which had been used in Chinese medicine for centuries under the name Qinghaosu.
Rediscovered in the 1970s, artemisinin was evaluated first in South-East Asia, and eventually accepted as an essential component of antimalarial treatment. These drugs were commonly available in the form of the derivative artesunate.
According to the researchers, although the drugs can be used alone, fears over the possible development of resistance mean that they are usually given in conjunction with one or more other drugs as artemisinin-based combination therapies (ACTs), now recommended by the WHO as the first-line treatment for uncomplicated falciparum malaria in all endemic countries.
During the study, the researchers at the Wellcome Trust-Mahidol University Oxford Tropical Medicine Research Programme, based in Bangkok, studied the susceptibility of P. falciparum parasites to the drugs in 40 patients in each in western Cambodia, and north-western Thailand.
On average, patients in Thailand were clear of parasites in 48 hours; in western Cambodia this took 84 hours - almost twice as long to clear the parasites as it did in Thailand.
In this study, out of the twenty patients treated with the monotherapy in each country, there were recurrences of the infection in six patients in western Cambodia compared to just one person in Thailand.
Moreover, of the twenty patients treated with the combination therapy, infection recurred in two patients in Cambodia compared to one in Thailand.
These results again suggest that artemisinin was less effective on the Cambodian parasites.
"Our study suggests that malaria parasites in Cambodia are less susceptible to artemisinin than those in Thailand," the New England Medical Journal quoted Dr Arjen Dondorp, lead author of the study as saying.
Dondorp added: "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.
"With artesunate losing its potency, ACTs rely much more on the weaker partner drug, increasing the risk that resistance also evolves towards the partner drug. This has very important consequences for the lifespan of ACTs. Losing ACTs would be a disaster for malaria control."
With signs of artemisinin-resistance occurring in other areas of Cambodia and Thailand, Dondorp says swift action is required to contain the spread.