Susceptibility of Plasmodium falciparum to chloroquine in northern Liberia after 20 years of chemosuppression and therapy.

The in vivo and in vitro susceptibility of Plasmodium falciparum to chloroquine was investigated in northern Liberia after 20 years of continuous chemosuppression and therapy with 4-aminoquinolines. In all patients studied (n = 53) parasitaemias were cleared within four days. There were no recrudescences in 16 patients followed-up for 28 days. All isolates of P. falciparum tested in vitro (n = 26) showed sensitive patterns. Schizont maturation was inhibited by a chloroquine concentration of between 0.25 and 0.75 mumol-1. In this area of Liberia no resistance to chloroquine was found in spite of extensive use of 4-aminoquinolines. This may support the view that importation of at least partially resistant strains, rather than local mutation of P. falciparum, precedes selection of resistant strains. Hence, we conclude that regular intake of chloroquine by groups at risk is justified if it is combined with regular monitoring of drug susceptibility.