CHANGES IN THE RESISTANCE OF PLASMODIUM-FALCIPARUM TO CHLOROQUINE IN HAINAN, CHINA

In 1979, in view of the widespread resistance of Plasmodium falciparum to chloroquine in the island of Hainan, China, its use as an antimala rial was suspended throughout the island. A longitudinal survey of the chloroquine-sensitivity of P. falciparum was carried out over the per iod 1981-91 to investigate whether its resistance had changed from the 1979 level. In-vitro assays were carried out every 2-3 years, while i n-vivo tests were performed annually over the period 1981-83 and also in 1941, Resistance to chloroquine declined progressively after its us e had stopped. The in-vitro tests indicated that the rate of chloroqui ne-resistant P. falciparum was 97.9% in 1981, but dropped to 60.9% in 1991 (P <0.001). The mean concentration of chloroquine for complete in hibition of schizont formation was 10.4 pmol/mu l in 1981, but decreas ed to 3.0 pmol/mu l in 1991 (P <0.001). The proportion of samples take n from malaria cases that required high concentrations (>6.4 pmol/mu l ) of chloroquine for complete inhibition of schizont formation was 83. 3% in 1981, but only 17.4% in 1991 (P <0.001); at low concentrations ( <1.6 pmol/mu l), the corresponding proportions increased from 4.2% in 1981 to 60.8% in 1991 (P <0.001). in the 4-week in-vivo test, the rate of chloroquine-resistant P, falciparum decreased from 84.2% in 1981 t o 40% in 1991 (P <0.001). RII + RIII cases represented 59.4% of the to tal resistant cases in 1981, but decreased to 37.5% in 1991 (0.02 > P > 0.01).