Multidrug-resistant forms of malaria-causing parasites are spreading across southeast Asia leading to "alarmingly high" treatment failure rates of widely used frontline medication, researchers have warned.
In a study published in The Lancet Infectious Diseases journal, they reveal that in parts of Thailand, Vietnam and Cambodia up to 80% of the most common malaria parasite were now resistant to the two most common anti-malarial drugs.
The Plasmodium falciparum parasites have also acquired resistance linked to the failure of treatment in half of cases to one of the newest and most potent frontline drug combinations, they said.
The parasite is transmitted through the bite of the female Anopheles mosquito.
"These worrying findings indicate that the problem of multidrug resistance in P falciparum has substantially worsened in southeast Asia since 2015," said Olivo Miotto from the Wellcome Sanger Institute and University of Oxford, who co-led the study.
"This highly successful resistant parasite strain is capable of invading new territories and acquiring new genetic properties."
He warned of the "terrifying prospect" of the parasite spreading to Africa, where most malaria cases occur.
A similar resistance to a long-time frontline malaria drug, chloroquine, contributed to millions of deaths across Africa in the 1980s.
More than 200 million people are infected with the P falciparum parasite, which is responsible for nine out of ten malaria deaths globally.
A drug combination known as DHA-PPQ was initially effective against the parasite, before doctors noticed signs of resistance in 2013.
The most recent study into DHA-PPQ failure rates showed they have now reached 53% in southwest Vietnam, and as high as 87% in northeastern Thailand.