Plasmodium falciparum field isolates commonly use erythrocyte invasion pathways that are independent of sialic acid residues of glycophorin A.

Erythrocyte invasion by malaria parasites is mediated by specific molecular interactions. Sialic acid residues of glycophorin A are used as invasion receptors by Plasmodium falciparum. In vitro invasion studies have demonstrated that some cloned P. falciparum lines can use alternate receptors independent of sialic acid residues of glycophorin A. It is not known if invasion by alternate pathways occurs commonly in the field. In this study, we used in vitro growth assays and erythrocyte invasion assays to determine the invasion phenotypes of 15 P. falciparum field isolates. Of the 15 field isolates tested, 5 multiply in both neuraminidase and trypsin-treated erythrocytes, 3 multiply in neuraminidase-treated but not trypsin-treated erythrocytes, and 4 multiply in trypsin-treated but not neuraminidase-treated erythrocytes; 12 of the 15 field isolates tested use alternate invasion pathways that are not dependent on sialic acid residues of glycophorin A. Alternate invasion pathways are thus commonly used by P. falciparum field isolates. Typing based on two polymorphic markers, MSP-1 and MSP-2, and two microsatellite markers suggests that only 1 of the 15 field isolates tested contains multiple parasite genotypes. Individual P. falciparum lines can thus use multiple invasion pathways in the field. These observations have important implications for malaria vaccine development efforts based on EBA-175, the P. falciparum protein that binds sialic acid residues of glycophorin A during invasion. It may be necessary to target parasite ligands responsible for the alternate invasion pathways in addition to EBA-175 to effectively block erythrocyte invasion by P. falciparum.

Responses of multidrug-resistant Plasmodium falciparum parasites to mefloquine in Nigerian children.

Thirty-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 base as a single oral dose. They were followed-up using the modified 28-day WHO 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 31 remained so throughout the period of observation. Two patients who were parasitaemic on days 16 and 28 were successfully treated with a sulphadoxine/pyrimethamine combination. Parasitological and clinical responses were well correlated. The mean parasite clearance time was 65 +/- 10.2 hours. A mefloquine concentration of 64 pmol/well inhibited schizont growth and the EC50 and EC99 were 5.5 and 5.4 pmol/well (1.1 and 10.8 mumol/l blood) respectively. This indicates reduced parasite susceptibility to the drug in vitro.

Sensitivity of Plasmodium falciparum to reduced dose of mefloquine in pregnant women in Nigeria.

Mefloquine base, (12.5 mg/kg body weight), was administered as a single oral dose to 34 pregnant women with Plasmodium falciparum parasitaemia. They were followed up in vivo using the modified 28-day WHO extended field test. The sensitivity of P. falciparum isolates obtained from these women to mefloquine (MQ) was evaluated in vitro. All women were parasite negative by day 4 and remained aparasitaemic throughout the 28-day period of observation. Parasitological and clinical responses were well correlated in all the patients. Minimal side effects, after drug intake, were reported by these women, but they all resolved spontaneously. The determined Mean Parasite Clearance Time (MPCT) was 57.7 +/- 14 hours. Seventeen parasite isolates were cultured in vitro; 9 (53%) grew satisfactorily. Schizont growth inhibitions was obtained at mefloquine concentration of 32 pmol/well (6.4 pmol/mu L). The effective drug concentration that gave 99% parasite growth inhibition (EC99) was 25.6 pmol/well (5.1 pmol/mu L); which indicates high parasite susceptibility to the drug in vitro. However, low dose of MQ may be ineffective in clearing parasitaemia in areas with mefloquine resistant parasite strains.

Efficacy of a 3-day oral regimen of quinine in an area of northern Nigeria with low-grade resistance of Plasmodium falciparum to chloroquine and sulphadoxine-pyrimethamine.

The efficacy in vivo of a 3-day oral regimen of quinine (30 mg/kg/day) was assessed in 34 children with falciparum malaria in an area of northern Nigeria with previously documented low-grade parasite resistance to chloroquine and sulphadoxine-pyrimethamine (SDX/PYR). By day 4, all 34 children were free of parasites. Mean parasite clearance time and fever clearance time were 2.7 and 1.7 days, respectively. However, on day 14, 5 (14.7%) children were again parasitaemic and 4 of them were clinically ill. They were again treated successfully with a standard course of oral chloroquine. No adverse drug effects were recorded. Of the 34 children, 9 parasite isolates were successfully cultured in vitro. EC50 and EC99 were 14.0 and 126.0 pmol per well respectively, indicating decreased parasite sensitivity but no resistance in vitro. In conclusion, the 3-day course of quinine was found to be an effective alternative to standard chloroquine treatment in the study area.

Sensitivity of Plasmodium falciparum to pyrimethamine in vivo and to sulphadoxine/pyrimethamine combination in vitro in pregnant women of northern Nigeria.

The in-vivo response of Plasmodium falciparum to a single dose of 25 mg pyrimethamine was evaluated in pregnant women in a semi-rural area of Zaria, northern Nigeria. Forty-four women were enrolled in the study and followed up for 14 days. Thirty-one (70%) of the women had cleared their parasitaemia by Day 7 and they remained parasite negative throughout the 14-day period of observation. The MPCT was 2.48 +/- 0.76 days. Thirteen (30%) of the 44 women were parasite positive either on Day 7 (7.16%) or Day 14 (6.14%) and re-treatment with a curative dose of chloroquine (25 mg kg-1 over 3 days) resulted in complete parasite clearance. Of the eight P. falciparum isolates successfully cultured from these women, seven (87.5%) were highly sensitive and one was resistant in vitro to the SDX/PYR combination.

Sensitivity of Plasmodium falciparum to chloroquine in pregnant women in Zaria, northern Nigeria.

Thirty-five pregnant women with Plasmodium falciparum parasitaemia were treated with chloroquine 25 mg/kg body weight over 3 days. They were followed up in vivo by use of a modified 14-day WHO extended field test. In vitro sensitivity of the parasite isolates to chloroquine was assessed by using the WHO microtest method. All pregnant women were aparasitaemic by day 7 and there was no parasite reappearance throughout the 14-day observation period. The mean parasite clearance time (MPCT) was 3.2 +/- 1.4 days. Eleven parasite isolates obtained from the women were highly susceptible to chloroquine in vitro. The effective drug concentration that gave 99% parasite growth inhibition (EC 99) was 0.36 mumol/l.