Susceptibility to chloroquine, mefloquine and artemisinin of Plasmodium vivax in northwestern Thailand

Abstract

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.

Sensitivity of Plasmodium vivax to chloroquine, mefloquine, artemisinin and atovaquone in north-western Thailand

“…Th e main malaria season extends from May to September, followed by a secondary rise in November, but malaria transmission is also possible outside the main peaks. Until now, no clinically relevant resistance of P. vivax has been reported from the area, although the in vitro sensitivity is decreasing [29]. Currently, 60% of all malaria cases in the district of Mae Sot are caused by P. vivax.…”

“…It borders in the west over a length of 200 km on Myanmar. Th e area has currently the highest incidence of malaria in Th ailand, marked by the extensive population movement across the border [29].…”

Synergism between pyronaridine and retinol in Plasmodium vivax in vitro

“…In the rainy season, between May and August/September, malaria transmission has its main peak, followed by a secondary peak in November. Until now, no clinically relevant resistance of P. vivax to chloroquine has been reported, altough the in vitro sensitivity is decreasing [16]. About 60% of the malaria cases in the area are caused by P. vivax.…”

“…Th e eastern archipelago of Indonesia, particularly Papua, is aff ected by the highest risk of resistance, with about 45% of resistant isolates [14]. Th ere is no evidence of chloroquine resistance in Th ailand, but in vitro observations have shown a signifi cant decrease of the chloroquine sensitivity of P. vivax in the last decade [16].…”

In vitro drug sensitivity testing in Plasmodium vivax

“…Graphic processing is easy, with the log concentration scale on the [19] x-axis and the response probit scale on the ordinate. Examples are shown in Fig.…”