Malaria parasites resistant to several drugs are spreading rapidly in South East Asia, scientists have said.

A new report in The Lancet Infectious Diseases examined samples collected from patients with malaria from Cambodia, Laos, north-eastern Thailand and Vietnam, between 2007 and 2018.

From 1,673 whole genome sequences, researchers found the new resistant type of parasite in 1,615 cases.

Before 2009, this type of parasite was only found in western Cambodia but, by 2016/17, its prevalence had risen to more than 50% in all of the surveyed countries except for Laos, the study found.

In north-eastern Thailand and Vietnam, the parasites made up 80% of cases.

The researchers, including experts from the Wellcome Sanger Institute in the UK, said: “Our data clearly show that KEL1/PLA1 (malaria strain) has continued spreading out from western Cambodia and is now highly prevalent in multiple regions of Laos, Thailand, and Vietnam, where it has frequently replaced previous indigenous populations of parasites.

“These findings show an evolutionary process in action.”

Experts told the BBC that the findings raise the “terrifying prospect” that drug resistance could spread to Africa, where most malaria cases and deaths occur.

Malaria is treated with a combination of two 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.

Inspecting the parasite’s DNA showed that resistance had spread across Cambodia and was also in Laos, Thailand and Vietnam.

“This strain has spread and has become worse,” Dr Roberto Amato, from the Wellcome Sanger Institute, told the BBC.

Professor Tran Tinh Hien, from the Oxford University Clinical Research Unit, in Vietnam, said: “With the spread and intensification of resistance, our findings highlight the urgent need to adopt alternative first-line treatments.”

Professor Olivo Miotto, from the Wellcome Sanger Institute and University of Oxford, told the BBC: “This highly successful resistant parasite strain is capable of invading new territories and acquiring new genetic properties, raising the terrifying prospect that it could spread to Africa, where most malaria cases occur, as resistance to chloroquine did in the 1980s.”

However, Professor Colin Sutherland, from the London School of Hygiene and Tropical Medicine, said that, while the drug-resistant parasite has undoubtedly spread, it is not necessarily a global threat.

The World Health Organisation (WHO) estimates there were 435,000 deaths due to malaria across the globe in 2017, of which 403.000 were in Africa.