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Scientists have developed a way to track the spread of a dangerous form of malaria that cannot be treated with the main therapy.

Doctors in Cambodia reported the complete failure of artemisinin and piperaquine - the key drugs taken to kill malaria - this year.

The discovery of resistance markers, reported in the Lancet, will allow scientists to track the threat.

Experts said the study was a big step forward.

Artemisinin resistance has been known about for years, but a recent rise in resistance to piperaquine as well means the main malaria treatment, taking both together, is starting to fail.

International groups of researchers analysed the DNA from hundreds of malaria parasites to find out how they learned to shrug off piperaquine.

They uncovered genetic signatures unique to the parasites that were drug-resistant.

Dr Roberto Amato, from the Wellcome Trust Sanger Institute, told the BBC News website: "Resistance is quite widespread, there has been almost complete failure in one province in the western part of the country, and it is spreading quiet fast in the north of the country.

"The problem with complete failure is this might accelerate the spread of drug-resistance to other countries and ultimately to Africa."

Resistance to the drugs would be catastrophic in Africa, where 88% of all malaria cases happen.

Get ahead

Dr Amato added: "The good news is we're starting to get clues on which treatment to use."

Curiously, these resistant parasites appear to still be susceptible to an older drug - mefloquine.

One theory is that the malaria parasites cannot resist both mefloquine and piperaquine, so doctors may be able to rotate which drugs are used.

And for Dr Amato, the long-term aim is to be able to keep one step ahead of the parasite.

He said: "They evolve every single day to escape the human immune system and the insect immune system - they're extremely good at it - and we need to understand that.

"Understanding how and in which direction is crucial, if we understand the process from most basic level then we could at some point predict the direction it is going to evolve."

Prof David Conway, from the London School of Hygiene and Tropical Medicine, said: "These studies are a big step forward in our understanding.

"This evolving parasite resistance is a major threat for malaria control internationally."

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