[Alteration in resistance of Plasmodium falciparum to chloroquine after cessation of chloroquine medication for twelve years]

In view of the fact the resistance of Plasmodium falciparum to chloroquine occurred extensively in Hainan, a decision was made in 1979 that the use of chloroquine should be quit in the whole province. A longitudinal survey on chloroquine-sensitivity of P. falciparum was carried out during 1981-1991 to observe the variation in resistance of the parasite after the cessation of the chloroquine medication for every 2-3 years. A tendency of progressive decline of resistance was revealed. By using in vitro test, the rate of chloroquine-resistant P. falciparum dropped from 97.9% in 1981 to 60.9% in 1991 (P0.001). The mean dosage of chloroquine for complete inhibition of schizont formation declined from 10.46 pmol/microliters in 1981 to 3.02 pmol/microliters in 1991 (P0.001). The percentage of population requiring larger dosage (6.4 pmol/microliters to completely inhibit schizont formation declined from 83.3% in 1981 to 17.4% in 1991 (P0.001); whereas those requiring small dosage (1.6 pmol/microliters), increased from 4.2% in 1981 to 60.8% in 1991 (P0.001). In in vivo test, the rate of chloroquine-resistant P. falciparum decreased from 84.2% in 1981 to 40% in 1991 (P0.001). The proportion of RII plus RIII cases of the total resistant cases dropped from 59.4% in 1981 to 37.5% in 1991 (0.02P0.01).