Epidemiology of emergence and spread of drug-resistant falciparum malaria in Southeast Asia.

In Southeast Asia the medicated salt project of Pailin, on the Kampuchea-Thai border, demonstrated that drug resistance, especially chloroquine resistance, can develop when a large population of P. falciparum parasites is exposed to intense transmission under intense drug pressure. The selection of resistant parasites being activated by the introduction of non-immune groups. Emergence of drug resistance was the result of continuous and prolonged mass exposure of P. falciparum to pyrimethamine and chloroquine resulting in the selection of resistant mutants. This selection was associated with multiple exposures of the parasites to much higher drug doses, during repeated passages through the non-immune hosts, increasing the degree of resistance. Resistances spread to the receptive areas of Kampuchea and other neighbouring countries through the movements of the temporary migrants who, by then, had become carriers infected with drug resistant falciparum parasites. The rapid and early spread of chloroquine resistance in A. balabacensis areas was not a coincidence but the result of the biological advantages of this species complex in relation to malaria transmission. In Australasia the medicated salt project carried out in Irian Jaya, on the border with Papua New Guinea, also resulted in the development of drug resistance in P. falciparum.