High level of sensitivity to chloroquine of 72 Plasmodium falciparum isolates from southern Cameroon in January 1985.

The sensitivity to chloroquine, quinine and mefloquine of Plasmodium falciparum isolates from 3 areas of southwest Cameroon was evaluated using an in vitro microtest with estimation of parasite growth by 3H-hypoxanthine incorporation. Among 2,429 children examined, P. falciparum was found on thin smears in 124 of them, 76 isolates were submitted to in vitro tests and 72 were successful. In the locations studied, some of which are close to the area of Limbe where in vivo resistance has been reported, all 72 isolates were found fully sensitive to low concentrations of chloroquine (mean EC50 5.9 ng/ml or 18.5 nmol/l of medium). In 47 of these isolates simultaneously tested using WHO microtest predosed plates, the sensitivity was identical. Out of 39 tests performed with quinine, 35 were successful. While most strains responded to low concentrations, some showed a decreased sensitivity to the drug, the EC50 of 4 of them being in the range 230-300 nmol/l. Each of the 17 isolates tested with mefloquine was susceptible to very low concentrations of freshly prepared drug solution. While chloroquine-resistant strains may already exist in Cameroon, the present study suggests that they would be restricted to a limited area and are not widespread. Data also suggest that monitoring of the sensitivity of P. falciparum to quinine might soon be necessary.