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SummaryThirty-three children aged 6 months to 7 years from an area with multidrug-resistant Plasmodium falciparum strains were treated with 25 mg/kg body weight of mefloquine hase as a single oral dose. They were followed-up using the modified 28-day W H O extended field test. The parasite isolates from these patients were cultured in vitro with different concentrations of mefloquine. All children were parasite-negative by day 4, and 3 T remained so throughout the period of observation. Two patients who were parasitaemic o n days 1 6 and 2 8 were successfully treated with a sulphadoxine/pyrimethamine combination. Parasitological and clinical responses were well correlated. The mean parasite clearance time was 65 i 50.2 hours. A mefloquine concentration of 64 pmol/well inhibited schizont growth and the EC,, and EC,, were 5.5 and 54 pmol/well (1.1 and 10.8 pmol/l blood) respectively. This indicates reduced parasite susceptibility to the drug in vitro.keywords mefloquine, resistant Plasmodium falciparum strains

The World Health Organization (WHO) extended field test was employed to assess the in vivo sensitivity of Plasmodium falciparum to sulfadoxine-pyrimethamine combination in 44 children in Zaria urban area. 36 children (82%) were fully sensitive to the drug and 8 (18%) were resistant at the RI level. 8 parasite isolates were obtained from the children and successfully cultured in vitro using the WHO microtest (mark II) system. The 8 isolates underwent schizogony at concentrations of 10,000 pmol sulfadoxine/125 pmol pyrimethamine per well, indicating in vitro resistance.

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.