Chloroquine resistance of Plasmodium falciparum in semi-immune children in Zaria, northern Nigeria.

Fifty-nine children with Plasmodium falciparum malaria were subjected to the World Health Organization (WHO) extended field test to assess the in vivo sensitivity of the parasite to chloroquine in Zaria urban area, Nigeria. The parasites in 53 children (90%) were positive but those in 6 (10%) were resistant at the RI-RII level. 36 isolates from the patients were successfully cultured in vitro for the WHO standard microtest. 13 (37%) of the isolates underwent schizogony at chloroquine concentrations of 1.6 microM/litre and above. Probit analysis showed that the chloroquine concentrations producing 50% (EC50), 90% (EC90) and 99% (EC99) schizont inhibition were 0.4, 1.6 and 4.9 microM/litre, respectively. The results indicate a rapid decline in the sensitivity of P. falciparum to chloroquine in the study area during the past 3 years.