Western Cambodia has become a hotspot for a new form of drug-resistant malaria that threatens to overrun the world's front-line defences against the disease.
Scientists in the region have documented an alarming spread of malaria that resists the drug artemisinin, Until now, artemisinin was considered a wonder cure for the disease because it was fast-acting, had few side effects and was almost 100 per cent effective.
The treatment, derived from an ancient Chinese herbal remedy, was only introduced as the world's preferred malaria treatment in the past decade. It is the drug of choice for dealing with a parasitic infection that kills about a million people each year.
However, a study has found that malaria patients in the province of Pailin in western Cambodia taking far longer than normal to recover, and a significant proportion failing to rid their bodies of the malarial parasite. said Arjen Dondorp, the leader of a World Health Organisation study.
The survey, the first detailed analysis of artemisinin-resistance in the field, compared how long it took to treat 40 malaria patients in Cambodia with a comparable set of patients in neighbouring north-west Thailand.
"Our study suggests that malarial parasites in Cambodia are less susceptible to artemisinin than those in Thailand. This means it takes longer to kill the parasites," Dr Dondorp said. "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."
Artemisinin, which is most commonly given as the drug artesunate, was originally derived from the sweet wormwood tree, Artemisia annua, which was used in Chinese medicine for centuries under the name Qinghaosu.
Its use as a malaria treatment was rediscovered in the 1970s and the Chinese gave it to the Khmer Rouge regime in Cambodia, where it has been used ever since. This, and the fact it is possible to buy artemisinin privately from street vendors, with little or no control over how it is used, has probably aided the development of resistant strains in Pailin province, which was once famous for its gemstones and dense forests.
"We do not see 100 per cent resistance, but the parasite is much less susceptible to artemisinin than we are used to. If used in combination with other drugs we can still cure malaria but it takes a few days longer," said Dr Dondorp, who reports his findings in the latest edition of The New England Journal of Medicine.
"It is a very worrying trend because instead of being able to kill the parasite in 48 hours it now takes about 84 hours. This longer time means that in some patients the parasite is not completely eradicated from the body, allowing the few parasites that are left to grow and reappear weeks later."
An estimated 250,000 people in the epicentre of the outbreak in Pailin are infected with malaria and more than half may be infected with resistant strains of the parasite. A further 10 or 20 per cent of infected patients in a zone outside the epicentre may also be infected, Dr Dondorp added.
The WHO, whose study was part-funded by a British medical research charity, the Wellcome Trust, has begun trying to contain the spread of the drug-resistant parasite by issuing Cambodian families with mosquito nets for their beds and screening with blood tests. It is also treating infected people with a combination of anti-malarial drugs in the hope of completely eliminating the disease from the region. It is the first time the WHO has attempted to contain drug-resistant malaria from spreading globally, according to a report in the British medical journal The Lancet.
"If malaria parasites develop full-blown resistance to artemisinin derivatives... there will be nothing in the malarial drug pipeline to replace these compounds for five years, or possibly more," it says.
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