The in vitro study had the objectives of monitoring the sensitivity of Plasmodium vivax to chloroquine and artemisinin, and to assess its baseline sensitivity to mefloquine in northwestern Thailand in an area near the border to Myanmar. The investigations were carried out in 2004 at the malaria clinics of Mae Sot, Chedi Ko and Mae Ka Sa, all in the district of Mae Sot, Province of Tak. The in vitro tests followed the method of Tasanor. Successful tests were obtained with 45 fresh isolates of P. vivax. The EC(50) and EC(90) values for chloroquine were 120.9 nM and 655.7 nM, respectively, the GMCOC was 1699.7 nM. There was a significant decrease of the chloroquine sensitivity since 1998/1999. However, results of parallel investigations continue to indicate clinical-parasitological sensitivity to chloroquine. With mefloquine the EC(50) and EC(90) the (baseline) values were 131.6 nM and 972.6 nM, respectively, the GMCOC was 1987.0 nM. For artemisinin the EC(50) and EC(90) values were 8.7 nM and 105.2 nM, respectively, the GMCOC was 310.5 nM. As compared to 2002, the sensitivity of P. vivax to artemisinin has shown a slight but not significant increase.

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