Abstract: Objective To investigate whether chloroquine-resistance of Plasmodium falciparum had changed after stopping or reducing the use of chloroquine as an antimalarial in Hainan and Yunnan provinces. Methods WHO standard in vitro microtest and 4-week in vivo test were used, assays were carried out in different time after stopping or reducing the use of chloroquine. Results In vitro test in Hainan indicated that the rate of chloroquine-resistant P.falciparum was 97.9% in 1981, and dropped to 26.7% in 1997(P<0.01). The mean concentration of chloroquine for complete inhibition of schizont formation was 10.46±7.14 pmol/μl blood in 1981, decreased to 1.63±1.47 pmol/μl blood in 1997(P<0.01). The proportion of samples taken from malaria cases that required high concentration (>6.4 pmol/μl blood ) of chloroquine for complete inhibition of schizont formation was 83.3% in 1981 and only 6.7% in 1997(P<0.01). In the 4-week in vivo test, the rate of chloroquine-resistant P.falciparum decreased from 84.2% in 1981 to 18.4% in 1997(P<0.01). RⅢ cases accounted for 53.1% of the total resistant cases in 1981, and for 14.3% in 1997(P<0.01). In vitro test in Yunnan revealed that the rate of chloroquine-resistant P.falciparum, the mean concentration of chloroquine for complete inhibition of schizont formation and the proportion of samples taken from malaria cases that required>6.4 pmol/μl blood of chloroquine for complete inhibition of schizont formation were 97.4%, 17.2±12.6 pmol/μl blood and 58.9% in 1981 respectively, and dropped to 70.4%(P<0.01), 4.0±3.3 pmol/μl blood(P<0.01) and 16.6%(P<0.01) in 2002 respectively. Conclusion The resistance of P.falciparum to chloroquine declined progressively after its use had been stopped or reduced in Hainan and Yunnan provinces.

Key words: Plasmodium falciparum, Drug resistance, Chloroquine