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Experimental infections of laboratory-reared anopheline mosquitoes were carried out with 57 Plasmodium falciparum gametocyte carriers from Cameroon. Prevalence of infected mosquitoes and oocyst intensity were determined by two independent methods. Young P. falciparum oocysts were detected on day 2 after feeding using an immunofluorescent assay, and the results were compared with direct microscopic examination of midgut oocysts on day 7 postinfection. The immunofluorescent assay was based on a FITC-labeled anti-25-kDa monoclonal antibody, while the direct microscopy was performed on midguts stained with 2% mercurochrome. Young oocysts were easily detected by their typical and bright green-fluorescing Pfs25 positive coat and their characteristic pattern of pigment granules under transmitted white light examination. The agreement between the results of the two methods was assessed using the Kappa coefficient on prevalences of positive infections and the interclass correlation coefficient on arithmetic mean oocyst load per infected midgut. The results indicated a low agreement between the two methods for the comparison of prevalences of infected mosquitoes. However, this agreement was near perfect for the comparison of mean oocyst intensities. Prevalences of positive infections and the overall number of parasites per positive gut were significantly correlated for both methods. Thus, the immunofluorescent test could be an appropriate tool for early determination of malaria infection in mosquitoes, particularly under laboratory conditions. The possible applications of this immuno-fluorescent technique are discussed.

Postma, N. S., Hermsen, C. C., Zuidema, J., and Eling, W. M. C. 1998. Plasmodium vinckei: Optimization of desferrioxamine B delivery in the treatment of murine malaria. Experimental Parasitology 89, 323–330. Optimization of desferrioxamine B (DFO) delivery for the treatment of malaria was studied in Plasmodium vinckei infected mice. DFO was administered by three different treatment regimens: (1) multiple subcutaneous injections of free DFO, (2) intraperitoneal infusion of free DFO, or (3) multiple subcutaneous injections of liposomal DFO. In a first series of experiments, DFO treatment was started prior to infection. Multiple subcutaneous injections of free DFO before and during infection suppressed parasitemia, whereas injections only prior to infection did not. Suppression of parasitemia and long-term survival (>1 month after infection) of mice were obtained by intraperitoneal infusion starting 1 day before infection (14 days, 130 mg DFO/kg/ day) or by subcutaneous injections of liposomal DFO prior to infection (days − 1 and 0, 400 or 800 mg DFO/kg/day). The efficacy of the antimalarial activity of liposomal DFO was influenced by the drug-to-lipid ratio but was hardly affected by bilayer rigidity. In a second series of experiments, DFO treatment was started at days 6 and 7 after infection. Parasitemia was reduced by all three treatment regimens; however, long-term survival was obtained only by treatment with liposomal DFO (days 7 and 8, 400 mg/kg/day). The present results indicate that continuous exposure of the parasite to low doses of DFO suffice to clear parasitemia, whereas high doses of free DFO administered intermittently do not. A right balance between dose of DFO, time of exposure to DFO, and parasitemia suppresses parasitemia even in the treatment of late-stage malaria. It was shown that liposomes are suitable carrier systems for DFO in experimental malaria therapy when given prior to infection and, moreover, in the treatment of advanced stages of malaria.

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The effect of natural malaria transmission-blocking factors in the blood of Plasmodium falciparum gametocyte carriers was assessed in two types of functional bioassays. In the direct membrane feeding assay (DMFA), a comparison is made between the infectivity of gametocytes from a naturally infected gametocyte carrier in the presence of autologous plasma and the infectivity in the presence of replacement plasma from nonimmune donors. In the standard membrane feeder assay (SMFA), cultured NF54 gametocytes are used to measure the capacity of endemic sera to block transmission. In the DMFA, 18 out of 48 sera (37.5%) from Cameroonian gametocyte carriers reduced transmission significantly, while in the SMFA 22 out of 48 sera (45.8%) produced transmission reduction. There was a positive correlation between both assays (r + 0.41, P < 0.05). Antibodies against epitopes of transmission-blocking target antigens Pfs48/45 and Pfs230 were measured in competition ELISAs and compared with the results of DMFA and SMFA. Serological reactivity in competition ELISAs against three epitopes of Pfs48/45 was significantly higher in the group of transmission-reducing sera in both the DMFA and the SMFA, especially for epitope III. No significant difference was found for Pfs230 antibodies (epitope I). Sensitivity of the serological assays was approximately 60%, with a specificity of around 70%. Serological tests cannot replace the functional bioassay in field situations as yet, but can contribute in the selection of sera for SMFA evaluation.