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To assess the antimalarial sensitivity of Plasmodium falciparum in vivo and in vitro in a highly endemic area of southern Viet Nam, a field study was conducted (in 1999) at a rubber plantation in Binh Phuoc Province north of Ho Chi Minh City. Fifty patients were treated with either artesunate (4 mg/kg on day 0, then 2 mg/kg on day 1 to 4) or mefloquine (10 mg/kg at 0 h, then 5 mg/kg at 6 h), and their progress was followed for 28 days under standard WHO protocols. Blood spots were taken at baseline from all patients, as well as from those who redeveloped parasitaemia during follow-up, for polymerase chain reaction (PCR) determination of parasite genotypes to assist differentiation of re-infection from recrudescence. Both treatments cleared parasites within 5 days. Of the 25 mefloquine-treated patients, 2 (8%) re-presented with probable re-infections. For artesunate, 4 patients (16%) had re-infections and 5 (20%) had recrudescences. Sensitivity tests in vitro of pre-treatment P. falciparum isolates showed geometric mean IC50 values of 29, 38, 209 and 15 nmol/L for chloroquine (n = 32), mefloquine (n = 33), quinine (n = 31) and artemisinin (n = 31), respectively. There were significant correlations between IC50s for artemisinin and mefloquine (r = 0.72, P = 0.004), and chloroquine and quinine (r = 0.44, P = 0.05). These data show that, although mefloquine has been used for 10 years in Binh Phuoc Province, it remains fully effective, perhaps because an artemisinin derivative is commonly given at the same time. The recrudescence rate for artesunate is similar to those reported in other epidemiological contexts. The present in-vitro data imply that quinine remains effective and that reduced drug pressure has been associated with increased sensitivity of local strains of P. falciparum to chloroquine. Although from one hyperendemic area, these results may have implications for antimalarial prophylaxis and treatment strategies for residents and travellers to southern Viet Nam.

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Plasmodium falciparum is the main cause of human malaria and is one of the important pathogens causing high rates of morbidity and mortality. The total number of malaria patients in Vietnam has gradually decreased over the last decade. However, the spread of pathogens with drug resistance remains a significant problem. Defining the trend in genotypes related to drug resistance is essential for the control of malaria in Plasmodium falciparum has long been one of the most important pathogens, causing severe illness and large numbers of deaths worldwide. In the 1990s, more than 1 million people living in Vietnam suffered from P. falciparum infections, resulting in thousands of deaths per year. Since then, the National Institute of Malariology, Parasitology, and Entomology (NIMPE) and the government of Vietnam have focused a great deal of time and effort on a malaria control program. As a result,

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