[Fluctuation in the resistance of Plasmodium falciparum to chloroquine in China].

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/microl blood in 1981, decreased to 1.63 +/- 1.47 pmol/microl blood in 1997 (P<0.01). The proportion of samples taken from malaria cases that required high concentration (>6.4 pmol/microl 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/microl blood of chloroquine for complete inhibition of schizont formation were 97.4%, 17.2 +/- 12.6 pmol/microl blood and 58.9% in 1981 respectively, and dropped to 70.4% (P< 0.01), 4.0 +/- 3.3 pmol/microl 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.