In vitro susceptibility of Plasmodium falciparum isolates from Myanmar to antimalarial drugs

In vitro drug susceptibility profiles were assessed in 75 Plasmodium falcip arum isolates from 4 sites in Myanmar. Except at Mawlamyine, the site close st to the Thai border, prevalence and degree of resistance to mefloquine we re lower among the Myanmar isolates as compared with those from Thailand. G eometric mean concentration that inhibits 50% (IC50) and 90% (IC90) of Mawl amyine isolates were 51 nM (95% confidence interval [CI], 40-65) and 124 nM (95% Cl, 104-149), respectively. At the nearest Thai site, Maesod, known f or high-level multidrug resistance, the corresponding values for mefloquine IC50 and IC90 were 92 nM (95% Cl, 71-121) and 172 nM (95% CI, 140-211). Me floquine susceptibility of P. falciparum in Myanmar, except for Mawlamyine, was consistent with clinical-parasitological efficacy in semi-immune peopl e. High sensitivity to artemisinin compounds was observed in this geographi cal region. The data suggest that highly mefloquine-resistant P. falciparum is concentrated in a part of the Thai-Myanmar border region.