A super-strain of malaria is spreading across Asia and could cause havoc if it reaches Africa.
British scientists discovered the new variant of the malaria parasite, carried by female anopheles mosquitoes, has evolved to resist key drugs.
Researchers collected parasite samples from patients across East Asia from 2007 to 2018 and unveiled their findings in the Lancet journal.
It is feared the deadly strain could kill thousands if it arrives in sub-Saharan Africa where most of the almost half a million annual malaria deaths occur.
The global battle against malaria relies on a combination of the drugs artemisinin and piperaquine.
The researchers said that by 2013 these drugs were failing to clear malaria infection in 46% of patients treated in western Cambodia.
An analysis of DNA in the parasite samples showed the resistant strain had spread across Cambodia and was also in Laos, Thailand and Vietnam.
Dr Roberto Amato, from the Wellcome Sanger Institute, said: “This strain has spread and has become worse.”
Professor Tran Tinh Hien, from the Oxford University Clinical Research Unit, in Vietnam, added: “With the spread and intensification of resistance, our findings highlight the urgent need to adopt alternative first-line treatments.”
The World Health Organisation estimates malaria killed 435,000 worldwide in 2017, with 403,000 of the deaths in Africa.
Researchers analysed the genome of 1,673 samples from Cambodia, Laos, north-east Thailand and Vietnam.
They found the resistant parasite in 1,615 cases.
Before 2009, this type of parasite was only found in western Cambodia but by 2016/17 it made up more than 50% in all surveyed countries except Laos.
In north-eastern Thailand and Vietnam, the resistant parasites made up 80% of cases.
A report in The Lancet Infectious Diseases concluded: “Our data clearly show that KEL1/PLA1 (malaria strain) is now highly prevalent in Laos, Thailand, and Vietnam, where it has frequently replaced indigenous parasites.
“These findings show an evolutionary process in action.”