(CNN) Drug-resistant strains of malaria are spreading across Southeast Asia, raising fears of a "potential global health emergency," two new studies have found.

The reports were published Monday in The Lancet, warning that a multi-drug-resistant strain had evolved and was spreading across Thailand, Cambodia, Laos, and Vietnam.

The new findings come as countries and health experts struggle to fight the parasitic disease. There have been some successes -- Algeria and Argentina were declared malaria-free in May -- but in other places, cases have been rising significantly.

The evolution of the resistant strains in Southeast Asia has had "disastrous consequences," researchers said -- they have rendered a widely-used drug essentially ineffective, leading to treatment failures at "alarmingly high rates."

The drug, dihydroartemisinin-piperaquine (DHA-PPQ), has now reached a 62% failure rate in western Cambodia, 27% in northeastern Cambodia, 53% in southwestern Vietnam, and 87% in northeastern Thailand, researchers said in a statement

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