Comparative study of the in vitro sensitivity of Plasmodium falciparum to artemisinin in two border areas of Thailand.

Artesunate was introduced in Thailand in 1995 for the treatment of falciparum malaria in areas of multidrug resistance, where it is used in combination with mefloquine. The studies were conducted between May and August 1999, 2000 and 2001 in the provinces Mae Hong Son and Tak (Mae Sot District) in northwestern Thailand, both on the border to Myanmar. The province of Mae Hong Son is still largely unaffected by multidrug resistance and infections with Plasmodium falciparum are treated with mefloquine alone. In the district of Mae Sot, 350 km southwards, more than 60% of the Plasmodium falciparum isolates were found to be resistant to mefloquine. Between 1999 and 2001, a total of 227 fresh isolates of Plasmodium falciparum were successfully tested for their sensitivity to artemisinin using the WHO standard in vitro microtest. The weighted mean EC5o and EC90 values for 1999-2001 were 9.20 nM and 34.37 nM in Mae Hong Son and 11.18nM and 71.63nM in Mae Sot, respectively. The comparison of the sensitivity to artemisinin between Mae Hong Son and Mae Sot showed no significant difference in 1999, but significant differences in 2000 (p<0.05) and in 2001 (p<0.01). This phenomenon could be a consequence of different drug pressure. Furthermore, the lower sensitivity of Plasmodium falciparum to mefloquine in Mae Sot may play a minor (but amplifying) role, as the activities of artemisinin and mefloquine show a significant correlation.