Malaria parasites resistant to key drugs have spread rapidly in South East Asia to reach Laos, Thailand and Vietnam.
The parasites were first detected in Cambodia in 2008, and in less than ten years have spread to neighbouring countries.
Experts fear the ‘terrifying prospect’ of the parasites potentially reaching Africa, where the killer mosquito-borne disease is most fatal.
Malaria is caused by the Plasmodium parasite, passed to humans through the bites of infected mosquitoes.
KEL1/PLA1 – proven to be resistant to first-line malaria treatments – is just one type of the parasite.
Experts examined blood samples collected from malaria patients in Cambodia, Laos, north-eastern Thailand and Vietnam, between 2007 and 2018.
They found before 2009, KEL1/PLA1 was only found in western Cambodia, but is likely to have been there for several years.
It is believed the parasites started moving in 2015, and by 2016/17, its prevalence had risen to more than 50 per cent in all the countries except Laos.
Inspecting the parasites DNA showed that in some regions of Thailand and Vietnam, the parasites made up 80 per cent of malaria cases.
The Wellcome Sanger Institute research was led by Dr William Hamilton, who said: ‘These findings show an evolutionary process in action.’
He added: ‘Our data clearly show that KEL1/PLA1 has continued spreading out from western Cambodia.
‘[It] is now highly prevalent in multiple regions of Laos, Thailand, and Vietnam, where it has frequently replaced previous indigenous populations of parasites.’
The parasites have spread by evolving more genetic features, suggesting they have enhanced fitness to survive.
Malaria, which kills around 430,000 people worldwide each year, is treated with a combination of two drugs – artemisinin and piperaquine.
But these first-choice drugs were failing to treat almost half of patients sick with malaria in Cambodia, Thailand and Vietnam by 2017.
The report said by 2013 these drugs were failing to clear malaria infection in 46 per cent of patients treated in western Cambodia.
Half of patients were cured with alternative drugs, according to a second study published in the same journal – The Lancet Infectious Diseases.
Malaria is one of the world’s biggest killers, claiming the life of a child every two minutes, according to the World Health Organization (WHO).
Most of these deaths occur in Africa, where 250,000 youngsters die from the disease every year.
Experts told the BBC the findings raise the ‘terrifying prospect’ that drug resistance could spread to Africa, where most malaria cases and deaths occur.
Professor Olivo Miotto, from the Wellcome Sanger Institute, described the resistant parasite strain as being ‘highly successful’.
He added: [It] is capable of invading new territories and acquiring new genetic properties, raising the terrifying prospect that it could spread to Africa.’
Other experts warned the findings highlight the urgent need to adopt alternative first-line treatments.
‘This strain [KEL1/PLA1] has spread and has become worse,’ Dr Roberto Amato, from the Wellcome Sanger Institute, told the BBC.